The diet of Spodoptera frugiperda – part1 : the agricultural pest

In this field mission, we were also very interested in identifying the diet of the Fall Armyworm (FAW). Of course, I’m talking about the larval diet here – the adult moths don’t eat plants other than nectar (sugary water). Knowing the larval diet spectrum of the species is a crucial question for several reasons.

One concerns the pest status. FAW is mainly known as an agricultural pest. The main plant targeted by this insect is the corn. Corn also has many other pests in the USA, among them, several Lepidoptera of the Noctuidae and Crambidae families :

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Helicoverpa zea (Corn Earworm)
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Agrotis ipsilon (Black Cutworm)

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Striacosta albicosta (Western Bean Cutworm)

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Eoreuma loftini (Mexican Rice Borer)

 

 

and the invasive Ostrinia nubilalis (European Corn Borer).european_corn_borer03

The damage that those caterpillars do to crops is really substantial and a lot of research and technological development is devoted to the fight against these pests. Not so long ago, chemical pesticides were heavily used all over the world. Transgenic plants such as Bt corn, gradually replaced the use of chemical agents. Other biological control agents include the use of entomopathogenic viruses, bacteria, nematodes and parasitoids. For most of these pests, there is a scientific interest in understanding why some species of moths are pests and others are not. Indeed, most agricultural plants were domesticated by humans only around 10 000 years ago at most and it is only in recent modern times that intensive agriculture transformed the biosphere in many ways. Plant domestication favored the appearance of plants that never existed before (compare maize with its ancestor the theosint),corn-and-teosinte_h1fields used for agriculture are a completely novel artificial biotope at the scale of the planet, and in addition, global exchanges favored the introduction of exotic plants in many areas. All these parameters contributed to the creation of new habitats, new ecological niches. Pests clearly seized this opportunity to not only use the domesticated plants, but also to the point where their damage was clearly significative, being counted in millions of dollars yearly. One component of this new niche is of course the diet-the plant itself that is being eaten.

It is not only about caloric intake. Clearly, energetic value is the main reason why humans domesticated the plants in the first place. Having a high yield of starch in corn makes it more nutritive for us but also for other animals. Indeed we produce amylase that helps breaking down the starch into di- and trisaccharides converted during the digestion into glucose, which is used by the body to produce energy. So obviously, if we bred a type of plant that contains high amount of starch (such as corn, rice, potato etc.), it should also be beneficial for insects.

However, plants and insects go back a long way. And since then, plants learned to defend against voracious herbivorous critters (see also [1]). They can’t outrun their predators but, among other defensive strategies, plants developed their very own versions of chemical pesticides as a mean of pest control. (By the way, that’s one of the reasons that plants have always been a wonderful reservoir of compounds used in medicine). Corn, for example produce phenolic compounds to defend itself. In turn, insects have learned how to overturn these defensive measures. This process is called detoxification. The genomes of insects harbor many enzymes devoted to the neutralization of toxic compounds. So, as plants evolved new defenses, insects developed new ways to overturn them. But when you observe the natural world, you quickly realize that this co-evolution has not happened similarly for everyone. For example, some insects adapted very well to one type of plant that could otherwise be poisonous for anyone else. On the contrary, other insects can eat a large variety of plants and seem to have the means to fend off all chemicals that plants throw at them. These different strategies are called respectively specialized species versus generalist species [2]. As can be expected, there are several gradations in between these two extremes.

At first glance, agricultural pests seem to be specialist (like a corn specialist for example). But if you think about it, that specialization could have happened only at very recent times. So we’re only looking at a specialization event from an ancestral form that had the capacity to adapt to new types of plants, a feature more indicative of generalist insects. It seems fairly easy for a generalist animal to turn into a specialist. If ever one plant provides a niche where the fitness of the animal is way better than on any other plant, it will tend to become more specialized. The trade off however is the following. Anything should go wrong in the evolutionary history of the niche, the insect might not be able to turn to other food sources anymore. A generalist insect, on the other hand, may not have for any given plant the best fitness, but it is probably able to adapt to changing conditions by switching to new host plants. It is thus fair to say that generalists might have the potential to adapt to new niches, thus becoming agricultural pests if the new niche is an agricultural environment.

For this reason, we would really like to assess the native diet of FAW. There is a huge bias in the literature towards the physiology of Spodoptera on agricultural plants but almost nothing on other types of plants. It is clear that FAW is polyphagous and that it has different performances on different plants. But what is its full regimen that conferred him the ability to become a major corn pest ? Which plants led him, during its evolutionary history, to be able to jump on corn ? Is it better specialized to corn than on any other plants ? Are we observing, with agricultural pests, an ongoing specialization process ? And, in fine, what are the molecular predispositions that conferred FAW this nuisance potential ?

[to be continued]

[1] War AR, Paulraj MG, Ahmad T, et al. Mechanisms of plant defense against insect herbivores. Plant Signaling & Behavior. 2012;7(10):1306-1320. doi:10.4161/psb.21663.

[2] Hardy NB, Otto SP. Specialization and generalization in the diversification of phytophagous insects: tests of the musical chairs and oscillation hypotheses. Proceedings of the Royal Society B: Biological Sciences. 2014;281(1795):20132960. doi:10.1098/rspb.2013.2960

 

 

 

corn vs rice strains

specialist vs generalist

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