Warming reduces parasitoid success and narrows their diet breadth

Warming reduces parasitoid success and narrows their diet breadth | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Ecology Letters This is a preprint and has not been peer reviewed. Data may be preliminary. 24 July 2025 V1 Latest version Share on Warming reduces parasitoid success and narrows their diet breadth Authors : Chia-Hua Lue , Melanie Thierry , Leonardo Jorge 0000-0003-4518-4328 , Nicholas Pardikes 0000-0002-9175-4494 , Megan Higgie , and Jan Hrcek 0000-0003-0711-6447 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175336625.58859972/v1 Published Ecology Letters Version of record Peer review timeline 409 views 240 downloads Contents Abstract Abstract Introduction Materials and methods Results Discussion Acknowledgments Conflict of Interest References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract A significant area of current research is the impact of warming on food webs. However, few interactions per web are typically studied, which limits generalization and precludes evaluation of impact on consumer diet breadth and redundancy of top-down control. Here we show that experimental warming strongly decreased the success of parasitoid development across 28 Drosophila -parasitoid interactions from a tropical rainforest food web. Parasitoids responded consistently despite deep evolutionary divergence. Moreover, warming strongly narrowed the diversity of hosts that the parasitoids could utilize. Host developmental success was much less affected. In contrast, experimental cooling had only a mild effect on parasitoids and hosts. Our findings suggest that the top-down control exerted by parasitoids is likely to weaken due to warming. The range of hosts that parasitoids can use will become more limited, potentially threatening the sustainability of parasitoid populations and changing the balance between trophic levels. Chia-Hua Lue 1,2 , Mélanie Thierry 1,3,4 , Leonardo Ré Jorge 1,3 , Nicholas A. Pardikes 1,5 , Megan Higgie 6 & Jan Hrček 1,3,* 1 Biology Centre of the Czech Academy of Sciences, Institute of Entomology, České Budějovice, Czech Republic 2 Department of Biology, Hood College, Frederick, MD, 21701 3 University of South Bohemia, Faculty of Science, České Budějovice, Czech Republic 4 Centre de Recherche sur la Biodiversité et l’Environnement (CRBE), UMR5300 CNRS-IRD-TINP-UT3 Université Toulouse III – Paul Sabatier, Toulouse, France 5 Department of Biology, Utah State University, Logan, Utah, USA 6 College of Science & Engineering, James Cook University, Townsville, Queensland, Australia * Corresponding author, Biology Centre of the Czech Academy of Sciences, Institute of Entomology, Branišovská 31, České Budějovice, 37005, Czech Republic [email protected] , tel.: +420387775336 Email addresses: [email protected] , [email protected] , [email protected] , [email protected] , [email protected] , [email protected] Running title: Warming impacts trophic interactions Keywords: top-down control, food web, community , climate change, specificity, redundancy Number of words in the abstract : 145 , Number of words in the main text : 4824 Number of references : 47 Number of figures : 3 , tables : 0 , and text boxes : 0 Statement of authorship : CL conceived the project; MT, NAP and JH contributed to the experimental design; CL, MT, and NAP collected the data; LRJ, MT, NAP and JH analyzed the data. MH contributed to obtaining and establishing the experimental lines. JH led writing of the manuscript and all authors contributed critically to the drafts and gave final approval for publication. Data accessibility statement : All data and R scripts used for this study were archived in Zenodo https://doi.org/10.5281/zenodo.16317124. Abstract A significant area of current research is the impact of warming on food webs. However, few interactions per web are typically studied, which limits generalization and precludes evaluation of impact on consumer diet breadth and redundancy of top-down control. Here we show that experimental warming strongly decreased the success of parasitoid development across 28 Drosophila- parasitoid interactions from a tropical rainforest food web. Parasitoids responded consistently despite deep evolutionary divergence. Moreover, warming strongly narrowed the diversity of hosts that the parasitoids could utilize . Host developmental success was much less affected. In contrast, experimental cooling had only a mild effect on parasitoids and hosts. Our findings suggest that the top-down control exerted by parasitoids is likely to weaken due to warming. The range of hosts that parasitoids can use will become more limited, potentially threatening the sustainability of parasitoid populations and changing the balance between trophic levels. Introduction Species interactions play key roles in food webs, providing functions such as top-down control through predation and parasitism. The impact of environmental change on food webs and their functioning will thus largely depend on how the interactions are affected (Boukal et al. 2019; Gilman et al. 2010; Sentis et al. 2017; Tylianakis et al. 2008). Food webs typically encompass many species interactions (König et al. 2022; Rasmann et al. 2014) but experimental studies typically explore only a few interactions at a time due to feasibility (Malinski et al. 2024). However, identifying the impact of environmental changes on a diversity of interactions is critical for understanding how general the response is within the community. Moreover, the impact on key aspects of trophic interactions such as consumer diet breadth and redundancy of top-down control can only be evaluated from larger sets of interactions and is therefore poorly understood (Hallam & Harris 2023). Ectotherms, such as insects, are strongly affected by global temperature changes (González‐Tokman et al. 2020; Harvey et al. 2020). Their thermal performance curves are known to be left-skewed, and the impact of warming is therefore expected to be more severe than the impact of cooling (González‐Tokman et al. 2020). Existing studies suggest the possibility of thermal adaptation is likely to be limited, particularly in tropical species (Kellermann et al. 2012). However, it remains unclear whether consumers are affected by temperature changes differently than resource species. There are indications that higher trophic levels may be especially vulnerable to environmental changes, and their role in top-down control may be compromised (Derocles et al. 2018; Tylianakis & Binzer 2014; Voigt et al. 2003). However, data are scarce, particularly for the interacting life stages of consumer and resource species (Derocles et al. 2018). The specialization of interactions plays a crucial role in the dynamics of food webs. Broad diet breadth allows consumers to switch between resource species based on availability, thereby supporting the persistence of consumer populations (Valdovinos et al. 2010). However, environmental change can modify diet breadth (Bestion et al. 2019; Hallam & Harris 2023). Some resource species can become unavailable (Hu et al. 2024), or the consumer may not be able to utilize a resource in the new environment, for example, because of increased metabolic cost (Jeffs & Lewis 2013). Such narrowing of diet breadth can reduce population persistence. When observing interactions in nature (Rasmann et al. 2014), it is often difficult to identify when changes in diet breadth are due to shifted preferences, or inability to utilize a resource in the given conditions. Experiments revealing the fundamental ability to utilize resource species are therefore necessary. In addition to the diet breadth of individual species, the distribution of interactions throughout the food web significantly impacts resilience. Ecological networks that include functionally redundant species typically demonstrate greater resilience to environmental disturbances (Biggs et al. 2020). In food webs with more specialized interactions, energy flows across fewer pathways, which reduces stability and robustness to environmental changes and extinctions (Rooney et al. 2006). Food webs with more specialized interactions may thus face greater risks in a changing world. Here, we investigated how success of consumer and resource species development, consumer diet breadth and redundancy of top-down control are impacted by experimental warming and cooling. We focused on parasitoids, a group of natural enemies that exercise strong top-down control of host populations (Derocles et al. 2018; de Sassi & Tylianakis 2012). The primary interest of environmental change studies is usually warming, but some places may get cooler (Cohen et al. 2021). We measured the outcomes of 28 interactions involving Drosophila species and their parasitoids within a tropical rainforest food web at ambient (24°C), warming (28°C), and cooling (20°C) temperatures. The interactions involved seven Drosophila host species, three larval parasitoids, and one pupal parasitoid. Materials and methods The focal Drosophila -parasitoid food web represents a module of closely interacting species within the complex network of species interactions in the tropical rainforest of North Queensland, Australia (Jeffs et al. 2021). The food web usually consists of (Jeffs et al. 2021). We focused on the seven most common Drosophila (listed in Fig. 1) and four parasitoid species from the food web. The parasitoid species belong to evolutionary lineages with diverse life histories that diverged more than 225 million years ago (Blaimer et al. 2023; Prevost 2009). Three of the parasitoids were larval parasitoids: Asobara sp. (Braconidae: Alysiinae; strain KH B4, reference voucher no. USNMENT01557097, reference sequence BOLD process ID:DROP043-21), Leptopilina sp. (Figitidae: Eucolinae; strain KH 111F, reference voucher no. USNMENT01557117, reference sequence BOLD process ID:DROP053-21), and Ganaspis sp. (Figitidae: Eucolinae; strain KH 69B, reference voucher no. USNMENT01557100, USNMENT01557297 reference sequence BOLD process ID:DROP164-21), and one was a pupal parasitoid Trichopria sp. (Diapriidae; strain 66LD, reference voucher no. USNMENT01557254, reference sequence BOLD process ID: DROP096-21). The larval parasitoids are koinobionts meaning the host continues to feed and develop after parasitation, and the pupal parasitoid is an idiobiont meaning the host is paralyzed during parasitation. For more details on the parasitoid strains used see (Lue et al. 2021). We established Drosophila and parasitoid cultures between 2017 and 2018 from Paluma (S18° 59.031’ E146° 14.096’) and Kirrama (S18° 12.134’ E145° 53.102’) sites (70-800m a.s.l.), identified them using morphology and DNA barcoding, and shipped them to the Czech Republic under permit no. PWS2016-AU-002018 from Australian Government, Department of the Environment. We maintained all cultures at 24°C on a 12:12 hour light and dark cycle at the Biology Centre, Czech Academy of Sciences for dozens of generations before the experiments. We kept Drosophila isofemale lines on standard Drosophila food medium (corn flour, yeast, sugar, agar, and methyl-4-hydroxybenzoate). We combined five lines from each Drosophila species into a mass-bred population to revive genetic variation before the start of the experiment. Parasitoid lines were maintained on Drosophila melanogaster and this host was not used in the experiment to avoid potential bias due to maternal effects. Experimental design Our aim was to reveal the impact of warming and cooling on the development of parasitoids and their hosts. In nature, parasitoids and their hosts undergo linked population cycles during which their abundances and relative proportions change widely. To standardize our comparison, we established fixed starting host density and a parasitoid-to-host ratio in the experiment. Because we were interested in the feasibility of the interactions at different temperatures, we selected density and ratio that are generally favorable for development. As a result, hosts experienced only mild competition. Number of parasitoids was such that all exposed hosts were parasitized, but competition between parasitoids was also mild. We abbreviate the success of parasitoid development to “parasitism success” in this paper. Parasitism success thus incorporates fitness components from fertile parasitoid females being presented with hosts (finding a host patch) and attacking them, to the emergence of adult parasitoids. Realized diet breadth in nature is likely to vary widely in time depending on immediate availability of host species. We therefore measure impact of warming and cooling on fundamental diet breadth, i.e. the diversity of hosts a parasitoid is able to utilize in a given temperature. Three identical temperature control rooms simulated different thermal situations: cooling (20°C), ambient temperature (24°C), and warming (28°C) on a 12:12 hour light cycle. The 24°C represents mean annual temperature in rainforest understory at low elevations in this host-parasitoid community (Jeffs et al. 2021). The 20°C and 28°C are close to the lowest and highest daily average temperature of the natural environment. In a climate change model, a 1–5.7 °C increase in temperatures by 2100 is predicted (IPCC 2023), thus 28°C represents a general warming scenario in tropical Queensland, Australia. We exposed each of the seven host species to all four parasitoid species (while maintaining unparasitized controls to assess host developmental success) at the three temperatures in a fully factorial design, replicated in 7-10 vials per treatment (depending on egg availability). We collected Drosophila eggs following an egg-wash protocol adapted from (Nouhaud et al. 2018). The day before egg-wash, we introduced two Petri dishes with agar gel topped with yeast paste in each population cage for one week old flies to lay eggs overnight. We then transferred 50 host eggs into 90mm high and 28mm diameter glass vial with 10mL of food media. Altogether we collected a total of 49,950 eggs. After 48 hours for larval parasitoids and 120 hours for the pupal parasitoid, we introduced three three-to-six-day-old mated female parasitoids to each vial (except control vials). We removed the parasitoids 24h later. Based on preliminary assays, this combination of the number of wasps and exposure length is sufficient for every host larva present to be oviposited into, and the host stage is the most suitable for parasitoid development. We visually confirmed that oviposition was happening in all species and temperature combinations. We checked vials daily for adult emergence for up to 60 days. To avoid confounding counts with possible second generation, we stopped collection from a given vial after five consecutive days without any emergence. A parasitoid attack has three possible outcomes: wasp emerges, fly immune system fights off the infection and adult fly emerges, or both fly and wasp die. The interaction is characterized by a combination of parasitism success (success of parasitoid development, when wasps emerge) and degree of infestation (host suppression, when wasps emerge or both fly and wasp die). Both measures take host control survival into account to distinguish the effect of parasitoids on host development from the effect of temperature. Parasitism success = P / H C and degree of infestation = 1 - ( H / H C ), where P is the number of parasitoid adults emerging from the parasitized vial, H is the number of adult hosts emerging from the parasitized vial, and H C is the mean number of adult hosts emerging from the unparasitized vials at the given temperature (control). In some cases, H C can be lower than H or P due to stochasticity. In that case we take H C to be the bigger of H or P , considering it the best estimate of the number of fly individuals which could have hatched in the vial if not exposed to parasitoids. We built two sets of models: one including the three larval parasitoids and another including the pupal parasitoid to account for major life history differences. The immature stages of larval parasitoids develop within the host and come into direct contact with the host’s immune system. In contrast, the adults of pupal parasitoid Trichopria paralyze the host pupa and the immature parasitoid develops on the outside of the pupa (but within puparium, last larval skin which surrounds the pupa in Drosophila ). We modeled the effect of temperature on parasitism success and the degree of infestation using Bayesian multilevel regression models. The parasitism success and the degree of infestation were response variables, representing a binary outcome with a binomial distribution. For each host species and temperature, H C was used as the number of trials in the Bayesian models. Then, for parasitism success P was used as the outcome, while for degree of infestation we used H C - H as the outcome. We modeled temperature as a fixed effect. Additionally, host and parasitoid (only in larval parasitoid models) were included as random effects with an intercept and interaction with temperature represented as a categorical random slope, accounting for the potential different responses to temperature of different hosts and parasitoids. Phylogenetic divergence within Drosophila is deceptive. Although classified as a single genus, it represents widely divergent lineages. Specifically, the main split between the lineages studied here happened some 47 million years ago (Suvorov et al. 2022) . Such phylogenetic diversity in insects usually corresponds to subfamily or family taxonomic level (Blaimer et al. 2023). Hosts were therefore included in the models both as a simple categorical random effect, which accounts for non-phylogenetic variability between hosts, and also as a random effect with a correlation structure based on a variance-covariance matrix derived from the Drosophila phylogeny taken and trimmed from a wider phylogeny based on 1000 BUSCO genes in (Kim et al. 2024). The phylogeny is most complete to date and includes all the species studied here. However, we added known approximate dating of nodes from less extensive phylogeny to Fig. 1 (Suvorov et al. 2022). All models were fit using the R package brms (Bürkner 2017), using the default weakly informative priors for all parameters. For each model, eight chains were run in parallel, with 1000 iterations of warmup and 1250 iterations of usable samples, totaling 10000 posterior samples for all models. To assess whether there was any phylogenetic dependence on the results, we also fit alternative models without the additional random effect with phylogenetic correlations, and compared the two models using leave-one-out cross-validation as implemented in the R package loo (Vehtari et al. 2024). We obtained qualitatively the same results whether phylogeny was included or not. We primarily present all results for the models with phylogeny, to account for the non-independence of comparisons between species, but include the non-phylogenetic results in the supplement. Median posterior estimates for parasitism success and degree of infestation were extracted from the models for every temperature/host/parasitoid combination and contrasts of ambient (24°C) against cooling (20°C) and warming (28°C) were calculated as median odds-ratios ± 95% highest posterior density intervals using R package emmeans (Lenth 2024). Host developmental success We analyzed success of development of the Drosophila hosts at different temperatures from control vials not exposed to parasitoids. We used the proportion of emerging adult hosts from the eggs put in the vial as a measure of host developmental success. We used analogous Bayesian analysis with temperature and host as predictors as above, using the number of eggs (50) as the number of trials and number of emerging adults as the outcome. The experimental setup of one-on-one, no-choice assays measures fundamental diet breadth, describing the diversity of hosts each parasitoid can develop on at a specific temperature. Realized diet breadth could be narrower because of host choice by the parasitoids or competition among parasitoids (see discussion). We characterized the parasitoid diet breadth by calculating the diversity of hosts from which each parasitoid species was successfully reared in a given set of replicates. As there were a minimum of seven replicates per treatment (combination of host, parasitoid and temperature), we used first seven replicates from each treatment to construct the sets. In our main analysis, we grouped all first replicates into one diversity set, all second replicates into another, and continued this pattern until creating seven sets, reflecting the chronological preparation of the replicates. To assess the sensitivity of this approach, we randomized 1000 times the order of the replicates, checking what would be the observed diet breadths if the orders were completely arbitrary. The results are qualitatively the same. Host diversity was calculated using alpha phylogenetic diversity based on Hill numbers (Chao et al. 2014) using package hillR (Li 2018). We set q = 1 which is equivalent to the Shannon diversity index, and we used the same host phylogeny described above to account for host relatedness. An alternative diet breadth metric disregarding host relatedness was also used, by calculating the same index with a star-shaped phylogeny, but results did not qualitatively differ. Observed diet breadth in different sets was modeled by simple linear models, using temperature, parasitoid species and their interaction as predictors, again fitting separate models to larval and pupal parasitoids. Estimated marginal means and contrasts between ambient versus cooling and warming were extracted from the models using the emmeans R package. Results Effect of temperature on host-parasitoid interactions Warming significantly and strongly decreased the parasitism success of larval parasitoids (Fig. 1; Odds ratio (OR) warming/ambient = 0.037 (95% credible interval 0.020–0.056)). An odds ratio of 0.037 means the odds of parasitism success in elevated temperatures is 27x less compared to ambient temperature. In the Bayesian models we used, a 95% credible interval for the odds ratio (OR) that does not include 1 indicates statistical significance. The parasitism success of the pupal parasitoid also significantly decreased with warming but not as strongly (OR warming/ambient = 0.466 (0.192–0.802)). Cooling significantly but weakly decreased the parasitism success of both larval and pupal parasitoids (OR cooling/ambient = 0.828 (0.736–0.934) and 0.530 (0.423–0.633), respectively). There was expected variation in parasitism success between different species pairs at ambient and cooling temperatures (Fig. 1, Fig. S1, Table S1 and S2). However, warming mostly removed this variation in larval parasitoids by strongly and consistently reducing parasitism success. We used degree of infestation index as a measure of host suppression (Fig. 1). Warming significantly but weakly decreased the degree of infestation by larval parasitoids (OR warming/ambient = 0.710 (0.503–0.936)), and weakly increased degree of infestation by the pupal parasitoid (OR warming/ambient = 1.770 (1.016–2.687)). Cooling had an opposite effect and significantly weakly increased degree of infestation by larval parasitoids (OR cooling/ambient = 1.380 (1.259–1.507)), but decreased it in the pupal parasitoid (OR cooling/ambient = 0.415 (0.351–0.483)). For details of contrasts and the model see Fig. S1 and Tables S3 and S4. Effect of temperature on host developmental success Success of host development in absence of parasitoids was significantly decreased by warming but not as strongly and uniformly as the parasitism success (Fig. 2 and S2, Table S5, OR warming/ambient = 0.198 (0.102–0.279)). The success of the development of five of the seven host species studied declined with warming, but the decline was a lot milder compared to parasitism success, which almost universally declined (compare odds ratios of warming on parasitoid and host developmental success (Fig. S1 vs. S2)). Cooling did not have a significant effect on the success of host development without infection (OR cooling/ambient = 0.928 (0.835–1.030)). We quantified diet breadth as the diversity of hosts the parasitoids could utilize ( successfully emerge from). Diversity of hosts the parasitoids could utilize significantly decreased due to warming in all three larval parasitoids (Fig. 3A; Asobara sp. estimate warming/ambient = -0.66 ± 0.18, P < 0.001, Ganaspis sp. estimate warming/ambient = -1.28 ± 0.18, P < 0.001, and Leptopilina sp. estimate warming/ambient = -0.96 ± 0.18, P < 0.001), as well as the pupal parasitoid ( Trichopria sp. estimate warming/ambient = -0.61 ± 0.15, P = 0.001). Cooling did not significantly change the diversity of hosts utilized by larval parasitoids ( Asobara sp. estimate cooling/ambient = -0.05 ± 0.18, P = 0.925, Ganaspis sp. estimate cooling/ambient = -0.02 ± 0.18, P = 0.989, and Leptopilina sp. estimate cooling/ambient = 0.01 ± 0.18, P = 0.997), nor the pupal parasitoid ( Trichopria sp. estimate cooling/ambient = 0.10 ± 0.15, P = 0.738). Sensitivity analysis of distribution of replicates into diversity sets produced qualitatively the same results as the main analysis (Fig. S3). Diversity of parasitoid diet can be driven by diversity of hosts likely to be available (quantified as diversity of hosts which survive in a given temperature when not exposed to parasitism). Indeed, available diversity of surviving hosts significantly decreased due to warming (Fig. 3B; estimate warming/ambient = -0.39 ± 0.04, P < 0.001) but was not affected by cooling (estimate cooling/ambient = 0.018 ± 0.04, P = 0.841). We therefore explored how successfully the parasitoids utilized host diversity in relation to availability. In the warming treatment, Asobara sp. and Ganaspis sp. were successfully utilizing an even narrower range of hosts than what was available (Fig. 3C). For Leptopilina sp. the difference was difficult to assess due to extremely low number of surviving individuals in warming (see Fig. S4). If a parasitoid only rarely survives at a given temperature, measuring its diet breadth is difficult but also less important. The host diversity that the pupal parasitoid Trichopria sp. successfully utilized was very close to available diversity at all temperatures. We repeated all the analyses reported in the results section without including host phylogeny. In case of diet breadth analyses, we replaced phylogenetic diversity by Shannon diversity. The results were qualitatively the same as with phylogeny and we present them in the supplement (Fig. S5-S7, Table S6-S10). Discussion We addressed the impact of warming and cooling on an extensive set of host-parasitoid interactions essential for functioning of a food web. We found that parasitism success was strongly and consistently reduced by experimental warming. Importantly, warming restricted the diversity of hosts the parasitoid could utilize. Host developmental success was less affected by warming. In contrast, cooling had a weak impact on both parasitoids and hosts. Low parasitism success due to warming The sweeping negative impact of warming on parasitism success in 26 of the 28 host-parasitoid interactions studied provides strong evidence that global warming is likely to disrupt these interactions. The two cases with no detected change had no hosts or parasitoids hatched, which means the interaction was not possible in warming (Fig. S4). Hosts were generally also affected by warming, but only three of the seven species strongly, two weakly and two not significantly impacted. The two trophic levels were therefore differentially affected. The evolutionary breadth of the interactions studied is considerable. Parasitoid response is broadly consistent, despite studied on parasitoid species representing deep evolutionary lineages which diverged more than 225 million years ago in the Triassic (Blaimer et al. 2023) and utilizing diverse strategies to suppress the host immunity (Prevost 2009). Studied Drosophila hosts also represent widely divergent lineages that split some 47 million years ago (Kim et al. 2024; Suvorov et al. 2022), and which show large differences in immune function (Salazar-Jaramillo et al. 2014). The negative impact of warming on parasitism has been reported before but on only a few species pairs per study (reviewed by Malinski et al. 2024; Thierry et al. 2019). However, several studies report the opposite trend of warming increasing the parasitism success (reviewed by Thierry et al. 2019). In the Drosophila -parasitoid interactions, parasitoids were more strongly impacted than the Drosophila as parasitism success was reduced more strongly than degree of infestation. In addition to the outcome of interactions in immature stages studied here, the adult parasitoids may be less heat tolerant than adults of their hosts (Furlong & Zalucki 2017; Wenda et al. 2023). Parasitoids will therefore likely be at an increasing disadvantage to their hosts with progressing climate change (Derocles et al. 2018; Tylianakis & Binzer 2014). Narrower parasitoid fundamental diet breadth By measuring a large number of host-parasitoid interactions, we uncovered a significant decrease in diversity of hosts the parasitoids were able to utilize in warming. In Asobara sp. and Ganaspis sp. the narrowing of diet goes beyond a reduction in available host diversity. By assessing fundamental diet breadth, we present strong evidence that parasitoids will be unable to develop on some hosts following warming. Three main factors can modify how this reduction in diet breadth will manifest itself in nature: diet preferences, competition between parasitoids, and host community composition. We measured preference of three of the four parasitoid species studied here between three Drosophila host species in a different study and found no significant preference (Thierry et al. 2022b). Behavioral preference and its changes due to warming can play a stronger role in more generalized predation systems, such as in spiders (Hu et al. 2024), or lizards (Bestion et al. 2019). Competition between parasitoids is expected to decrease realized diet breadth further. With an additional experiment reported in the supplement, we found that parasitism success was lower in multi-species than in single-species infections in 17 out of 21 interactions tested, implying that competition between parasitoids can have strong negative effects (Fig. S8 and Tables S11-S13). Interestingly, we also revealed an exceptional case of facilitation where Leptopilina sp. achieved higher parasitism success on D. sulfurigaster when co-infecting with other parasitoids than when infecting alone (Fig. S8). Attack of the other parasitoids may have compromised immune response of the host and made it easier for Leptopilina sp. to emerge. The facilitation occurred in cooling and ambient temperatures only, not in warming. We have shown in different studies on a subset of species studied here that competition between hosts could render them suboptimal for parasitoid development and modify the available host community (Pardikes et al. 2022; Thierry et al. 2022a). However, it is unclear whether warming will affect strength of competition between hosts. The negative effects of warming on parasitoids reported here will be made worse by the previously described narrowing of the window of opportunity to attack caused by faster host development (Pardikes et al. 2022). Pardikes et al. (2022) demonstrated that the presence of alternative host species expands the window of availability, but the narrower diet breadth we observe here means that the ability to utilize these alternative hosts will be limited. Possible mechanisms Differences in parasitoid life history can provide insight into the mechanisms behind the response to warming, as larval parasitoids encounter host immunity while pupal parasitoids do not. The impact of warming on the parasitism success was much stronger in larval parasitoids than in the pupal parasitoid. The pupal parasitoid even exerted significantly more pressure on the host in warming (measured by the degree of infestation). Narrowing of diet breadth was also more pronounced in larval parasitoids. In contrast, the pupal parasitoid displayed the broadest range of hosts at each temperature, successfully developing in hosts in proportion to their availability. Low performance in warming, therefore, seems to be connected with living inside the host, rather than being a universal feature of parasitoid thermal tolerance. However, more pupal parasitoids would need to be studied for a general conclusion. The parasitoid larva is either unable to cope with the host immunity when stressed by warming or falls victim to upregulated host immunity. Recent study of behavioral fever in Drosophila melanogaster suggests that increased temperatures upregulate host immunity, especially production of antimicrobial peptides that damage microbiome of the wasp larva (Sheng et al. 2025). Another aspect of immune response to parasitoids is encapsulation. In some Drosophila species it is possible to find melanized capsules of parasitoid egg or larva in emerging adult flies as evidence of successful defense against the parasitoids (Salazar-Jaramillo et al. 2014). We therefore screened hatched flies of all Drosophila species in the experiment for presence of melanised capsules. We found encapsulation only in D. pseudotakahashii . This is consistent with previous findings that visible melanised encapsulation is phylogenetically restricted to species relatively closely related to D. melanogaster (Salazar-Jaramillo et al. 2014). We found no significant difference in encapsulation rate in D. pseudotakahashii between ambient and cooling temperatures (Fig. S9, Table S14 and S15). We could not compare with the warming (28°C) treatment because no fly adults of this species hatched in warming. Encapsulation rate only differed between parasitoid species (Fig. S9). Consequences for food webs The combination of a narrower diet breadth and lower parasitism success due to warming poses a serious threat to parasitoids. We performed a single generation experiment and the dynamic consequences in nature can be complex. While high parasitism success does not necessarily guarantee better parasitoid population persistence, very low parasitism success unequivocally means low top down control and decreased parasitoid population persistence (Pardikes et al. 2022). R estricted diet breadth is likely to further decrease persistence. Our work thus offers a clear prediction of the direction of food web changes. However, confirming the extent of such changes in natural communities is difficult. Field observations in our study system show an increase in parasitism success from high elevations to low elevations (Jeffs et al. 2021). Based on mean annual temperature, this would qualitatively correspond to the increase we found from cooling to ambient temperature. However, predicting the impact of warming beyond existing environmental gradients is very challenging. Interestingly, existing elevation gradient studies from the temperate region show narrower diet breadth at warm end of the gradient (Bonnaffé et al. 2024; König et al. 2022; Rasmann et al. 2014), but extrapolation to even warmer environments is again unclear. Due to lower parasitoid diet breadth and lower parasitism success, the food webs impacted by warming are likely to consist of fewer but more specialized interactions and become less connected and more modular. This implies lower redundancy, when even a localized impact in the food web is likely to lead to loss of function, for example regulation of a particular resource species (Biggs et al. 2020; Sanders et al. 2018). It is possible that other groups of natural enemies, like predators or pathogens, will take over the role of parasitoids (Roslin et al. 2017), but clear evidence is lacking. If parasitoids are not replaced, consumers will be released from top-down regulation until a new balance between the trophic levels in the food web is reached. Herbivore populations, already benefitting from higher primary production (de Sassi & Tylianakis 2012; Tylianakis et al. 2008), are likely to increase further. The effectiveness of biological control in economically important systems could be reduced, as evidence from field trials shows (Derocles et al. 2018; Romo & Tylianakis 2013). Parasitoids are among the most species rich groups of organisms and lower persistence of their populations may lead to a loss of biodiversity. Acknowledgments We thank Anna Mácová, Martin Libra, Joel Brown, Andrea Weberová, Inga Freiberga, Vincent Maicher, Sylvain Delabye, Sara Fernandez Garzon, Vincent Montbel and Varvara Fedorchenko for their help with the experiment. We thank Owen Lewis and Jinlin Chen for comments on the manuscript. The study was funded by the Czech Science Foundation grant no. 20-30690S. Conflict of Interest The authors declare no competing interests. Figure 1. Mean parasitism success and degree of infestation for each host-parasitoid interaction across temperature treatments. Host phylogeny is included, dating of nodes is from (Suvorov et al. 2022). Larval parasitoids A: Asobara sp., L: Leptopilina sp., G: Ganaspis sp. Pupal parasitoid T: Trichopria sp. Significant warming/ambient and cooling/ambient contrasts whose 95% credible intervals are not overlapping with value 1 are marked with a star. The ambient (24°C) treatment always serves as a base for the contrasts and significance compared to 20°C and 28°C, therefore no significance is meaningful at 24°C. Separate model was run for larval and pupal parasitoids. Strength of the contrasts with credible intervals is presented in Fig. S1 and Bayesian model summary in Tables S1-S4. Figure 2. Drosophila host developmental success measured as mean survival from eggs to adults of the control group not subjected to infection in cooling (20°C), ambient (24°C), and warming (28°C) temperatures. Host phylogeny is shown, dating of nodes is from (Suvorov et al. 2022). Significant warming/ambient and cooling/ambient contrasts are marked with a star and strength of the contrasts with confidence intervals is presented in Fig. S2. The ambient (24°C) treatment always serves as a base for the contrasts and significance compared to 20°C and 28°C, therefore no significance is meaningful at 24°C. See also Table S5 for Bayesian model summary. Figure 3. (A) Phylogenetic diversity of hosts successfully utilized by parasitoids decreases due to warming. Significant warming/ambient and cooling/ambient contrasts are marked with a star. (B) Available diversity of surviving hosts at each temperature. 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Collection Ecology Letters Keywords climate change community food web redundancy specificity top-down control Authors Affiliations Chia-Hua Lue Biology Centre Czech Academy of Sciences View all articles by this author Melanie Thierry Biology Centre Czech Academy of Sciences View all articles by this author Leonardo Jorge 0000-0003-4518-4328 Biology Centre Czech Academy of Sciences View all articles by this author Nicholas Pardikes 0000-0002-9175-4494 Biology Centre Czech Academy of Sciences View all articles by this author Megan Higgie James Cook University View all articles by this author Jan Hrcek 0000-0003-0711-6447 [email protected] Biology Centre Czech Academy of Sciences View all articles by this author Metrics & Citations Metrics Article Usage 409 views 240 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Chia-Hua Lue, Melanie Thierry, Leonardo Jorge, et al. Warming reduces parasitoid success and narrows their diet breadth. Authorea . 24 July 2025. 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