Abstract
Selecting optimal host plants is critical for herbivorous insects, such as fall armyworm (Spodoptera frugiperda), an important maize pest in the Americas and Africa. Fall armyworm larvae are presumed to have limited mobility, hence female moths are presumed to be largely responsible for selecting hosts. We addressed host selection by fall armyworm moths and neonate and older (3rd-instar) larvae, as mediated by resistance and herbivory in maize plants. Thus, we compared discrimination among three maize cultivars with varying degrees of resistance to fall armyworm, and between plants subjected or not to two types of herbivory. The cultivars were: (i) susceptible, and deficient in jasmonic acid (JA) production and green leaf volatiles (GLV) emissions (inbred line B73-lox10); (ii) modestly resistant (B73), and; (iii) highly resistant (Mp708). The herbivory types were: (i) ongoing (= fall armyworm larvae present), and; (ii) future (= fall armyworm eggs present). In choice tests, moths laid more eggs on the highly resistant cultivar, and least on the susceptible cultivar, though on those cultivars larvae performed poorest and best, respectively. In the context of herbivory, moths laid more eggs: (i) on plants subject to versus free of future herbivory, regardless of whether plants were deficient or not in JA and GLV production; (ii) on plants subject versus free of ongoing herbivory, and; (iii) on plants not deficient in compared to deficient in JA and GLV production. Neonate larvae dispersed aerially from host plants (i.e. ballooned), and most larvae colonized the modestly resistant cultivar, and fewest the highly resistant cultivar, suggesting quasi-directional, directed aerial descent. Finally, dispersing older larvae did not discriminate among the three maize cultivars, nor between maize plants and (plastic) model maize plants, suggesting random, visually-oriented dispersal. Our results were used to assemble a model of host selection by fall armyworm moths and larvae, including recommendations for future research.
Highlights
Host plant selection by female herbivorous insects fundamentally affects their offspring’s survival and reproduction, and is relevant to neonate offspring because they are especially susceptible to plant defenses, and must cope with a variety of obstacles to successfully colonize a host [1,2]
Here we addressed whether: (i) ovipositing females discriminate among maize genotypes that differ in their suitability for larvae; (ii) any discrimination among maize genotypes by females is mediated by herbivory and its timing, future or present, as well as by emission of herbivory-induced green leaf volatiles, and; (iii) dispersing neonate and older larvae forage directionally and discriminate among maize genotypes
These results indicated that fall armyworm females discriminated among plant types, and suggested that, contrary to our expectations, their preferences were negatively correlated with the performance of 3rdinstar larvae on the different plant types
Summary
Host plant selection by female herbivorous insects fundamentally affects their offspring’s survival and reproduction, and is relevant to neonate offspring because they are especially susceptible to plant defenses, and must cope with a variety of obstacles to successfully colonize a host [1,2]. An environmental variable relevant to host selection is herbivory, which may be differently important depending on its timing, whether prior, present or future, and whether it induces defensive responses in host plants. Prior herbivory may affect host quality by mediating the amount of food available to a female’s offspring, while present and future herbivory may do so by mediating plant chemical responses to herbivory. Other studies showed that present and future herbivory, such as by insect larvae or eggs, respectively, may affect host selection by mediating the deployment of inducible direct and indirect chemical defenses against an herbivore’s offspring (e.g., [8,9,10])
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