Insect saliva: how does the parasitized caterpillars influence plant defenses and growth/fitness?

Ching-Wen Tan & Michelle Peiffer Jared G. Ali, Dawn S. Luthe, and Gary W. Felton

Plants face many environmental stressors that can influence their survival, growth and fitness. Among these, insect are the most diverse, abundant and threating herbivores in nature. As a consequence, plants have developed strategies to protect themselves. When attacked, plants recognize compounds in insect saliva and begin producing defense chemicals. This includes both direct defenses, which interfere with insect digestion and reduce growth and/or increase mortality, and indirect defenses, such as releasing plant volatiles that attract parasitoids to reduce herbivore populations. However there is a cost to the plant; energy spent on defense against insects is energy that cannot be used for growth/reproduction and thus plant fitness may be reduced.

Female parasitoid wasps lay eggs into host caterpillars and the wasp larvae feed on the host caterpillar hemolymph or tissues, ultimately resulting in caterpillar death. Parasitism changes hosts’ physiology in order to make the environment suitable and promote the development of the parasitoid’s offspring. These physiological changes also influence herbivore saliva. We focus on saliva changes after parasitism and evaluate how the changes influence plant defenses and plant fitness.

Parasitism reduces glucose oxidase  activity in corn earworm saliva which results in lower tomato plant defenses compared to plants that receive saliva from non-parasitized caterpillars. Tomato plants produce higher flower numbers and heavier fruit weight when treated with saliva from parasitized caterpillars compared to non-parasitized ones. Moreover, the effects of saliva from parasitized caterpillars also impact the next plant generation: seeds germinate faster and with higher germination percentage when their parental plants were treated with saliva from parasitized caterpillars. These results indicate that plants can distinguish between saliva from parasitized and non-parasitized caterpillars and adjust their defenses accordingly. Thus higher trophic level organisms, parasitoids, can enhance plant fitness by influencing the host caterpillar’s saliva and plant responses.  

Read the paper in full here.

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