Skip to main content
Dryad logo

Data from: Differing thermal sensitivities in a host-parasitoid interaction: high, fluctuating developmental temperatures produce dead wasps and giant caterpillars

Citation

Moore, M. Elizabeth; Hill, Christina A.; Kingsolver, Joel G. (2020), Data from: Differing thermal sensitivities in a host-parasitoid interaction: high, fluctuating developmental temperatures produce dead wasps and giant caterpillars, Dryad, Dataset, https://doi.org/10.5061/dryad.sj3tx963n

Abstract

1. Insect parasitoids, and the arthropod hosts they consume during development, are important ecological players in almost all environments across the globe. As ectothermic organisms, both parasitoid and host are strongly impacted by environmental temperature. If thermal tolerances differ between host insect and parasitoid, then the outcome of their interaction will be determined by the ambient temperature. As mean temperatures continue to rise and extreme temperatures become more frequent, we must determine the effect of high temperature stress on host-parasitoid systems to predict how they will fare in the face of climate change.

2. The majority of studies conducted on host-parasitoid systems focus on either performance under constant temperature, or a fixed metric of thermal tolerance (CTmax) for individual organisms. However, performance at constant temperatures is not predictive of performance under ecologically relevant, fluctuating temperatures, and measurements of thermal thresholds provide little information regarding the effects of temperature throughout development. We address this by testing the effects of increasing mean temperature in both constant and fluctuating (±10°C) environments throughout development on the performance of the parasitoid wasp Cotesia congregata and its lepidopteran larval host, Manduca sexta.

3. The growth of M. sexta was influenced by mean temperature, diurnal fluctuations, and parasitization status. Caterpillar growth rate increased with increasing mean temperature, but decreased in response to diurnal fluctuations and parasitization by C. congregata wasps.

4. Wasp survival decreased with increasing mean temperature, and diurnal fluctuations decreased wasp survival, especially at higher mean temperatures. Diurnal fluctuations at our highest mean temperature treatment (30°C±10°C) resulted in complete wasp mortality, and parasitized hosts displayed abnormal physiology, wherein they failed to exhibit wasp emergence, did not enter the prepupal stage, continued to feed, and grew up to two-fold larger than a normal, unparasitized caterpillar.

5. Our results indicate hosts and parasitoids in this system have different thermal tolerances during development; the parasitoid wasp suffered complete mortality at a temperature regime that is mildly stressful for the unparasitized caterpillar host species. Our findings suggest C. congregata will suffer more severely under increasing temperatures than M. sexta, with cascading trophic and ecological effects.

Methods

Our experiment consisted of three mean temperatures (25°C, 28°C, 30°C), two diurnal fluctuation treatments (±0°C, ±10°C), and two parasitization treatments (parasitized or unparasitized controls). Sample sizes ranged from n = 24 to n = 47 (mean = 35) for each treatment group (see Table S1). For logistical reasons, the 28°C mean treatments were conducted in March—May 2017, the 25°C mean treatments were conducted in June—August 2017, and the 30°C mean treatments took place in January—March 2020. While our mean temperature treatments did not occur concurrently, analyses using data for the 25°C and 30°C constant temperature treatments from a previous study (Moore et al., 2019) did not alter the qualitative outcomes of our analyses and conclusions (see SOM for comparisons). Organisms were housed in climate control chambers (Percival Scientific 36VL) under 14L/10D hour light cycle. Fluctuating treatments ramped continuously between 2-hour soaks at the high and low temperatures. Humidity was not monitored, but an open container of water was placed in each chamber to prevent desiccation of organisms or artificial diet. 

Manduca sexta eggs were collected from the UNC colony, maintained at 25°C for 3 days, and then placed on artificial diet. Eggs were placed into one of 3 constant temperatures (25°C, 28°C, 30°C) and checked every 24 hours for hatching. Newly hatched 1st instars were transferred to new dishes with fresh diet, and either divided into fluctuation treatments (at 25°C and 28°C) or returned to the constant temperature (30°C). To avoid high hatching mortality at high and fluctuating temperatures, larvae at 30°C were kept at a constant temperature until the molt to the 3rd instar. At molt to 3rd, caterpillars entered the experiment, were given a unique ID number, and weighed. On day 0 of the 3rd instar, caterpillars were divided into parasitized or unparasitized control treatments. After allowing the M. sexta to acclimate to room temperature, caterpillars were parasitized by exposing them to adult wasps. After successful oviposition was observed (oviposition lasting >2-3 seconds), caterpillars were removed from the wasp enclosure. The number of ovipositions for each caterpillar was recorded.

After molting to the 3rd instar, M. sexta were housed individually in petri dishes. Containers were checked every 24 hours for molting, and food was provided ad libitum. Manduca sexta mass and development time were tracked at each instar until wandering (unparasitized controls), wasp emergence (parasitized), or until 3 weeks past the molt to the 5th instar (parasitized without wasp emergence—WOWE; see below). At wandering, control caterpillars were weighed and euthanized. A small number of parasitized individuals displayed wandering behavior (see SOM); these were assumed to be the result of failed ovipositions and were excluded from analyses. At wasp emergence, parasitized caterpillars and wasp larvae were transferred to a clean petri dish and left undisturbed for 48 hours. At this time the total number of emerged wasp larvae was recorded, as well as the number of larvae that succeeded or failed to form cocoons. Caterpillar mass was also recorded at this time. Cocoons were transferred to a small plastic cup and returned to the climate chamber until adult eclosion. The hosts were frozen for later dissection to determine total parasitoid load. Cocoons were monitored daily until eclosion. Twenty-four hours after eclosion, wasps were frozen and the number of eclosed adults was counted

Some hosts (specifically in the 30°C±10°C treatment) failed to exhibit wasp emergence. These hosts without wasp emergence (WOWE) were fed as usual, weighed weekly, and monitored daily for signs of wandering, illness (discoloration, flaccid cuticle), or death. At the 3rd week after the molt to 5th instar, WOWE hosts were weighed a final time, culled, and a subset was frozen for future dissection. A small subset of individuals (n = 8) exhibited wandering behavior, but none successfully pupated.

Total parasitoid load was determined by dissection in hosts with wasp emergence. Dissecting scissors were used to cut the dorsal cuticle longitudinally along the body. The cuticle was pinned to the side and the gut removed. Wasp larvae were removed from host hemocoel and tissues, counted and recorded. The number of unemerged wasp larvae was added to the number that emerged to compute total parasitoid load. Individuals with total load > 300 were excluded from analyses (n = 1).

 

The data has been processed to convert dates to Julian day, dead individuals have been removed, erroneous values have been removed, and various metrics of development time and survival have been calculated.

Usage Notes

This data does contain missing values (coded as NA). Some missing values are context dependent (unparasitized caterpillars will have NA in parasitoid load columns). ReadMe files are available for data and are upoloaded here.

Funding

National Science Foundation, Award: IOS 155559