Heating up parasitoid-host interactions: high temperature increased mortality of late-instar braconid larvae and reduced ladybird recovery rate

With ongoing climate change, temperature-dependent outcomes of host-parasitoid interactions can affect ecosystem functioning and key ecosystem services such as biological control. However, most studies addressing the impacts of temperature on host-parasitoid systems are biased toward immature host stages and agricultural pests, specifically Lepidoptera. Moreover, despite their relevance for population dynamics, important life-history traits such as host recovery (i.e., restoring feeding and mating behaviour after parasitism) remain largely neglected.

In this study we tested the effects of low (18°C), optimal (24°C), and high (30°C) temperatures on the development, survival, and body size of the braconid parasitoid Dinocampus coccinellae and longevity and recovery probability of its adult host, the invasive ladybird Harmonia axyridis.

At low temperature, susceptible ladybirds, in which parasitoid larvae successfully developed, lived significantly shorter than resistant ladybirds that likely eliminated parasitoid, and control individuals lived significantly longer than both types of infected ladybirds. At optimal temperature, host survival was mainly determined by parasitoid development, as resistant and control ladybirds lived similarly long and susceptible individuals lived significantly shorter. At high temperatures all ladybirds exposed to the parasitoid lived significantly shorter than control ones.

Temperature significantly affected stage-specific mortality of parasitoids, with significantly higher proportion of late larval instar dying at high temperature. This together with increased pupal mortality resulted in inability of Dinocampus coccinellae to complete development at 30°C. Ladybird recovery was significantly affected by temperature, host sex and their interaction, showing highest values at optimal temperature for both sexes, but strongly decreased male recovery at low temperature. In addition, host sex influenced parasitoid body size: female ladybirds produced larger adult wasps.

The combined effects of high temperatures and parasitism negatively affected host life-history traits, with extreme temperatures also mediating cascading effects on its braconid endoparasitoid. We demonstrate how temperature- and stage-specific parasitoid mortality can influence host longevity and the understudied recovery probability, focusing on an adult beetle host, providing novel information for climate change ecology of host-parasitoid interactions.

Methods
Experimental animals

Harmonia axyridis ladybirds were collected during autumn 2022 in Prague and successfully overwintered under semi-natural conditions, i.e., groups of ladybirds in glass jars with perforated lids were placed under shrubby vegetation in the university campus. In mid-April 2023 ladybirds were sorted into males and females in the laboratory. After several days, we established 10 parental pairs to produce offspring for the laboratory experiment. Other ladybirds formed single-sex groups (8-12 individuals per Petri dish). All ladybirds were reared at 24°C, 16L:8D photoperiod and provided with food (Ephestia kuehniella eggs) and water ad libitum for three weeks. Offspring (larvae) produced by parental pairs were also reared in the same conditions as their parents. By rearing the ladybirds from eggs in the laboratory, we ensured that the individuals used in the subsequent laboratory experiment were free of infection by Dinocampus coccinellae.

Single-sex groups were reared with the goal of gathering D. coccinellae wasps, which frequently emerge from overwintering ladybirds during spring. Wasps obtained from the original hosts were allowed to infect novel ladybird hosts in the laboratory to get the next generation of parasitoids that will emerge in early June. Original overwintered H. axyridis females were used as novel hosts for this purpose. In total, we acquired over 60 Dinocampus wasps and 50 of them were used for our laboratory experiment. Each wasp was kept separately in a Petri dish and provided with honey dissolved in water to maximize parasitoid longevity and fecundity.

Laboratory experiment

The sex of each offspring ladybird was determined ca. 24 hours after eclosion from pupae, and ladybirds were accommodated individually in Petri dishes and provided with ad libitum food as described above. When experimental ladybirds were ca. 14 days old (early June 2023), a subset was exposed to Dinocampus wasps, resulting in either a development of parasitoid larvae (‘infected with larvae’ treatment) or unsuccessful larval development (‘infected without larvae’ treatment). The remaining subset, unexposed to Dinocampus wasps, were considered control individuals (‘control’ treatment, see details below). Each ladybird from infected treatments was exposed to 3-5 different wasps in a single day to maximize infection success. The same wasps were re-utilized for several days to infect multiple ladybirds. Immediately after infection (or during that day for control individuals), ladybirds were placed to three temperature regimes: 18°C (low), 24°C (optimal) and 30°C (high), resulting in nine groups in total (infected with larvae, infected without larvae and control treatments at each temperature).

During the following days, we recorded the presence of D. coccinellae pupae, the emergence of adult parasitoids and the survival of ladybirds in each Petri dish daily. For ladybirds from which D. coccinellae pupa emerged and stayed paralyzed we recorded if they recovered from the paralysis, i.e., whether they started to move and feed. Every second day, we provided all alive ladybirds with ad libitum food and water. We cleaned all Petri dishes when needed (at least once per week). Dead ladybirds were stored in a freezer at -20°C and dissected at the end of the experiment to check the potential presence of D. coccinellae larvae and to determine the developmental stage of the parasitoid in the case of unsuccessful development. Finally, we categorized three host statuses: infected with larvae, for ladybirds exposed to D. coccinellae that developed a larva (either because a D. coccinellae larvae was found during dissection or because we observed a larvae emerging from the hosts’ body); infected without larvae, for ladybirds exposed to D. coccinellae, but no larvae were found during dissection (possibly related to defense mechanisms eliminating the parasite during the egg stage); and control, for ladybirds not exposed to D. coccinellae at all. It should be noted that when we dissected the ladybirds, we found different larval stages, so from now on we will refer as early stage (L1-L2): small, yellow 1st and 2nd instar larvae with mandibles (given the morphological similarities and the low number of observations, we merged these two first instars into one category for further analysis), 3rd instar (L3): large yellowish larvae without mandibles, and brownish 3rd instar (L3b): large brownish larvae without mandibles. L3b could be either the final stage (prepupa) that was unable to exit the host or dead L3 that started to be melanized due to host’s immune system response. For all D. coccinellae adults produced in our laboratory experiment, we measured their structural body size, i.e., the width and height of the left wing and the length of the right hind tibia.