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Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment-mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full-sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata(L.)King&Robinson(Asteraceae).Inaddition,theperformancewasevaluatedintwogenerations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full-sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non-structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but aboveground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full-sun foliage. Although immaturesurvival and adultlongevity did not differbetweenhabitats, Maw’shost suitabilityindex indicatedthat shaded leaves were moresuitablefor the growthandreproductionofP. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light-mediated changes in plant traits and leaf characteristics can affect insect folivore performance.

The effect of host plant dissimilarity on insect preference and performance was tested using two morphological forms of Chromolaena odorata (L.) King & Robinson (Asteraceae) (one from Florida, USA, another from South Africa), and a specialist herbivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae) from Florida, USA, that was introduced as a biological control agent in South Africa. Although this insect did establish at one site, of some 21 sites at which over 800 000 individuals were released, its population level in the field has remained low after an initial outbreak in 2006. To explain the poor performance of P. insulata, we hypothesised that P. insulata larvae prefer Floridian C. odorata to the southern African C. odorata, which is morphologically and probably genetically distinct, and that larvae reared on Floridian C. odorata should have higher fitness and performance. We tested this by comparing insect performance metrics on each of the two plant forms in laboratory experiments. Apart from pupal mass, which was significantly greater on southern African C. odorata, P. insulata performance metrics were similar on both plant forms; there were no significant differences in total leaf area consumed, egg and larval development, immature survival rates, feeding index, host suitability index, growth index, and fecundity between the Floridian and southern African C. odorata plants. In sum, we could not demonstrate that differences in plant forms in C. odorata are responsible for the poor performance of P. insulata in South Africa.