Host-parasitoid interactions conjure up rather graphic images of a hoard of small wasps boring through the soft tissue of an unassuming caterpillar. That poor caterpillar. Since I first became aware of this gory dynamic relationship, I always sided with the caterpillar. However, my new-found enthrallment with beneficial insect performing biological control has fostered a new perspective. The host-parasitoid relationship between the caterpillar and wasp maintains ecological balance. Now, we see climate change can completely throw this delicate system out of whack.
In a recent 2021 paper, Moore et al explore the impacts of fluctuating high temperatures on the development of both the lepidopteran larval host Manduca sexta and the parasitoid wasp Cotesia congregata. A previous study with this same host-parasitoid system had found that parasitoids had reduce survival while hosts underwent accelerated growth under constant elevated temperatures (Moore et al, 2019). However, this current study aimed to elucidate the effects of fluctuating elevated temperatures, which are predicted to occur under climate change.
Consistent with previous findings, caterpillar growth rate increased as mean temperature increased but decreased with parasitization. Conversely, wasp survival decreased with increasing mean temperatures. At temperatures that were only mildly stressful for unparasitized caterpillars, the parasitoid wasp experienced complete mortality. In this host parasitoid system, we see a mismatch between thermal tolerances of these two species. The parasitoid wasp cannot survive elevated temperatures.
Both wasp survival and caterpillar growth decreased due to diurnal temperature fluctuations. At the highest mean temperature with diurnal fluctuations (30±10), parasitoid larvae failed to emerge from caterpillar host. Upon dissection of the caterpillar, no wasp larvae were found. However, the parasitized caterpillars without wasp emergence failed complete normal growth. They did not enter the prepupal stage. Instead, they continued to feed and grow. The caterpillars grew to be about twice as larger as unparasitized caterpillars.
Moore et al suggest that repeated exposure to heat stress through fluctuating temperatures may prevent parasitoid establishment in host. During oviposition, the parisoid wasp inject the host larvae with the parasitoids endemic polydnavirus, which suppresses immune function of the host caterpillar. It is possible that high temperatures inhibit the viral activity in the host, preventing parasitoid establishment.
In the face of climate change, this host-parasitoid system is at risk. The parasitoid wasp C. congregata serves as an important biological control mechanism for the host caterpillar M. sexta. The caterpillar, also known as the Tobacco horn worm, is a detrimental agricultural pest. It feeds on the leaves of members of the Solanaceae family: tomatoes and tobacco (Pavuk, 2009). Through parasitism, C. congregata can control tobacco horn worm populations. As we expect temperatures to increase in unstable patterns, we may lose the efficacy of this beneficial insect.
Sources:
Moore, M. E., Hill, C. A., & Kingsolver, J. G. (2021). Differing thermal sensitivities in a host–parasitoid interaction: High, fluctuating developmental temperatures produce dead wasps and giant caterpillars. Functional Ecology, 35(3), 675-685.
Moore, M.E., Kester, K.M. and Kingsolver, J.G. (2020), Rearing temperature and parasitoid load determine host and parasitoid performance in Manduca sexta and Cotesia congregata. Ecol Entomol, 45: 79-89. https://doi.org/10.1111/een.12776
Pavuk, D. (2009). The tomato hornworm and the tobacco hornworm. Michigan State Universtiy,
The caterpillar-parasitoid wasp system is a very interesting, albeit gruesome, relationship, but there is another important player left out here: the plant. Some plants can release volatile compounds when damaged by herbivorous insects to attract parasitoid wasps. These plants include members of the Solanaceae family you mention here (though oddly enough tomato plants release far fewer volatile chemicals compared to its cousins; Wei et al. 2007). This may be the next step in Moore's research, but I think the plant response to warming temperatures is just as important in this system. Do higher temperatures cause plants to release more volatile chemicals (potentially compensating for declining wasp populations), or will it lead to the opposite effect (hitting the plants doubly hard)? It just goes to show how complicated the natural world can be and that climate change can have far-reaching impacts across the board.
ReplyDeleteWei, J., L. Wang, J. Zhu, S. Zhang, O.I. Nandi, and L. Kang. 2007. Plants attract parasitic wasps to defend themselves against insect pests by releasing hexenol. PLoS ONE 2(9): e852.
I thought this was an interesting perspective, especially since- like you- most people tend to favor the caterpillar 'victim'. However, I am interested to look into this a bit further. I tend to dislike the framing of species as pests because they consume agricultural products, although I know this is an important aspect of how people view organisms. I am curious to know if the impacts of this skewed parasitoid-host relationship would be less negative if framed outside of an agricultural perspective.
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