Wimpy or Willful Wasps? The Effect of Heat on a Parasitoid’s Survival

Article: Moore, M.E., Hill, C.A. and Kingsolver, J.G., 2022. Developmental timing of extreme temperature events (heat waves) disrupts host–parasitoid interactions. Ecology and Evolution12(3), p.e8618. Link to article

Featured Image Caption: The tobacco hornworm, Manduca sexta, can be parasitized by a wasp, Cotesia congregata, that lays eggs inside the caterpillar. The young wasps eventually emerge to form cocoons. However, the wasp larvae are vulnerable to heat, and may not survive extreme heatwaves. Photo credit: David Hill License: CC BY 2.0

Never Safe

Insects have a lot to worry about; dangers, both external and internal, can be lethal. There is always the chance that a predator, such as a bird, will simply pluck them for a quick meal. Additionally, entomopathogenic fungi, or fungi that literally consume an insect from the inside out, pose a threat. But fungi aren’t the only organism that take advantage of the nutritious inside of these organisms. Fellow insects, such as a special group of wasps called parasitoids, lay eggs inside another insect with a long, sharp structure called an ovipositor. The eggs hatch inside the host and utilize the insect’s nutrients to grow. Additionally, being inside a host serves as a means of protection from the elements and predators during the vulnerable larval stage. Scientists have been able to track the development of the wasp larvae inside their host, so even though they cannot see the wasp larvae, they can estimate the current stage of wasp development based on how much time has passed since oviposition. The insect ultimately perishes as the adolescent wasps finally burrow up through the cuticle and emerge from the host.

Double Trouble

One example of a parasitoid-host relationship is the Cotesia congregata (parasitoid wasp) and Manduca sexta (caterpillar host). M. sexta caterpillars are pests of tobacco, tomatoes, and potatoes; C. congregata wasps are considered a biological control option. Additionally, there is a third key player, a virus. The virus is known as C. congregata bracovirus or CcBV for short. The female wasp injects the CcBV into the host while she lays her eggs. The CcBV serves to suppress the host immune system and manipulate host hormones so that the wasp pupae have a more favorable environment. In other words, it increases the likelihood that was larvae will survive.

Cotesia congregata is a parasitoid wasp that lays eggs inside its caterpillar host, Manduca sexta. Oviposition is the process of egg-laying; the sharp point at the back of the wasp is the ovipositor. Photo credit: Greta Forbes. License: Public Domain Mark 1.0

However, what if there are other problems the young wasps face, outside of their snug caterpillar home? The virus’ ability to alter some environmental properties in favor of the wasp’s development has its limits. The virus cannot protect wasp larvae from extreme heat, and unfortunately, climate change is making these scenarios far more common. Heatwaves are increasing in both frequency and duration. Just last year, the Pacific Northwest faced a record-shattering heatwave, with temperatures approaching 120°F for several consecutive days. Researchers at the University of North Carolina wanted to investigate the effect of heatwaves on parasitoid wasp development throughout their various life stages prior to emergence from the insect. Namely, they sought to determine if short but intense heatwaves during different stages of wasp development affected parasitoid survival.

Heat Susceptibility when Developing inside a Hot Host Home

To accomplish this, Moore et al. used the M. sexta, C. congregata, and CcBV study system. The researchers designed two separate experiments to tackle their questions. In the first experiment, they exposed caterpillars to the same amount of heat for the same duration. However, the manipulated variable was the life stage of the wasp inside the parasitized caterpillar. In the first treatment (“Early”), caterpillars with freshly laid parasitoid eggs were exposed to the simulated heatwave. In the second (“Middle”) group, caterpillars with young, hatched wasp larvae received the heatwave treatment. Finally, the third group (“Late”) consisted of caterpillars with wasps inside that were just about to emerge. A control group was also established that did not receive heatwave exposure. The researchers then measured how many caterpillars completely lacked live and emerged parasitoid wasps after the heatwave. The acronym they used for this is host WOWE (Without Wasp Emergence). The scientists also measured parasitoid load, or the total number of emerged wasps (if any) for each of the three treatments. Heatwaves that were simulated when the parasitoid wasps were still just eggs largely resulted in hosts WOWE. However, if the heatwave occurred in the Middle or Late stages of wasp development, there were fewer WOWE hosts. In the Middle heatwave, parasitoid load and survival were comparable to controls that were not exposed to heat. Interestingly, the Late heatwave had a detrimental effect on wasp survival, but not load. There was up to 90% mortality of the parasitoid wasps! Dissection of the host revealed wasps that were developmentally fit to emerge but had been killed by the heat.  

Heatwaves can kill the parasitoid wasps during vulnerable life stages. They can perish before they even emerge to form the white cocoons seen on this unfortunate caterpillar. Photo credit: 8thstar License: CC BY-SA 3.0

Cooking Cotesia congregata

The second study investigated the effect of temperature and duration on parasitoid survival. This setup was the opposite of the first study: the life stages were the same when the experiment began; the heat wave started on the day of oviposition. However, in this study, there were two different heatwave temperatures tested (40°C and 42°C) with four different durations (1, 2, 3, or 4 days). Again, a control treatment was reared without heat exposure. In this experiment, results indicated that both temperature and heat duration played a role in parasitoid survival. However, temperature alone after just 1 day was enough to decrease total wasp load by 3 to 5-fold compared to caterpillars at control temperatures. These results were noted at 40°C and 42°C, respectively. In addition, after a 1-day heatwave treatment, 25%-30% of the caterpillars had no parasitoids emerge at all (WOWE). Regardless of the temperature treatment, after 3 to 4 days in either heatwave, 86%-100% of the caterpillars were WOWE hosts. Dissection of the caterpillars revealed more information regarding the fate of the wasps. Remnants of wasp eggs were found, but they perished before they could even hatch.

When M. sexta is parasitized by C. congregata, but the wasps die before emergence, an interesting phenomenon can be observed. Since the virus is still present and continues to manipulate the host, the caterpillar continues to feed instead of pupating, growing to be rather massive in size. Photo credit: Oliver.dodd, License: CC BY 2.0

With climate change contributing to an increase in heatwaves (both temperature and duration), ecosystems may see shifts as a result. Not only that, but the timing of the heatwave is critical in determining parasitoid survival. Parasitoid wasps are key players in population control for insects. This can be especially important for some farmers, as M. sexta feeds on lucrative crops (tobacco). Without its parasitoid to control population levels, we may see higher numbers of these insects, which in turn can threaten crop yield and quality. Perhaps more alarming than the increase in caterpillar numbers is the increase in crop damage. If parasitoid emergence does not destroy the caterpillar, the insect survives, but is still affected by the CcBV virus. Instead of pupating, caterpillars will continue to feed, growing to be very large in size. Costs and benefits of employing parasitoid wasps as potential biological control should be thoroughly analyzed in the context of climate change. Heatwaves affecting parasitoid survival can upset the fragile balance of an ecosystem, leading to disastrous results.

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Maria Marlin

I am a plant pathologist living in Oregon and working for Oregon State University extension. I study soilborne and foliar pathogens that attack ornamental crops, but the vast majority of my time is spent conducting outreach! I train nursery workers in scouting and detecting signs and symptoms of plant disease. I love to write and share my love of science with others! In my free time, I love to horseback ride and adventure through the magical Pacific Northwest that I am so fortunate to call home. Whether it is chasing mountain summits, exploring the rugged coast, or basking in the silence of the mossy, misty, and moody forests, I am my happiest and most inspired when surrounded by nature.

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