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The tephritid Cecidochares connexa has been used to good effect as a biological control agent on the Asian/West African biotype of the invasive alien shrub Chromolaena odorata, but does not develop well on the different, southern African C. odorata biotype. A stem-galling tephritid fly, Polymorphomyia basilica, from the northern Caribbean islands, was considered as a potential biological control agent for C. odorata in South Africa. Life history traits and host range on 32 asteraceous plants were investigated in single-choice tests and using single pairs of adults in no-choice tests, under laboratory conditions. Positive biological characteristics of P. basilica include a high rate of increase, the production of several generations per year, long-lived and mobile adults, the ability of females to produce viable offspring without repeated mating, and the ability of adults to eclose from galls on dry stems. Use of a single pair of adults for no-choice tests proved to be efficient. Oviposition and larval development through to adulthood occurred on three other South American and on two South African species; for the latter, one was in the tribe Eupatorieae, closely related to C. odorata, and the other in the Astereae, less closely related to the weed, but both at a lower and slower rate. Females tended to retain their eggs under no-choice conditions in the presence of an unsuitable host, and to compensate by ovipositing at a higher rate when presented later with a C. odorata plant. The poor offspring survival on non-target plants tested in this study demonstrates the suitability of P. basilica for release in South Africa.

Increasing evidence suggests that the responses of insect herbivores to environment-mediated changes in the phenotypic and phytochemical traits of their host plants are more complex than previously thought. Here, we examined the effects of habitat conditions (shaded versus full-sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) (Asteraceae). We also determined neonate larval preference of the specialist herbivore, Pareuchaetes pseudoinsulata Rego Barros (Lepidoptera: Erebidae) (a biological control agent) for shaded versus full-sun leaves. The study further evaluated the performance of the moth on C. odorata leaves obtained from both shaded and full-sun habitats. Leaves of C. odorata plants growing in the shaded habitat had higher water and nitrogen contents compared with full-sun leaves. Plants growing in shade had longer leaves but full-sun plants were taller and had greater aboveground biomass compared with shaded plants. Although neonate larvae of P. pseudoinsulata preferred to feed on full-sun foliage, development was faster when reared on shaded foliage. However, survival, pupal mass, growth rate, and Maw’s host suitability index of the moth did not significantly differ between full-sun and shaded foliage. Our inability to demonstrate significant differences in key insect performance metrics in P. pseudoinsulata between shaded and full-sun foliage, despite neonate larval preference for one of the foliage types, suggests that neither of the foliage types can be considered a superior host, and reiterate the fact that relationships between host plant quality (modulated by light intensity) and phytophagous insect performance are not simple.

Despite the release of about 1.9 million individuals of Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae) in KwaZulu-Natal for the biological control of Chromolaena odorata (L.) King and Robinson (Asteraceae) in South Africa, the moth probably only established at one of the 30 release sites and its population level is generally low in the field. To determine whether climate incompatibility in South Africa is responsible for the poor performance of P. insulata, the effects of temperature on life-history traits were investigated under several constant temperatures. Although a degree-day model estimated between 3.9 and 10.0 generations of the moth per year in the weed’s invaded range, survival and fecundity declined while development time was prolonged at constant temperatures below 25 °C, indicating that both direct and indirect negative impacts of low winter temperatures, such as increased mortality, slow development and reduced fecundity as well as exposure to natural enemies, may partly explain the poor performance of P. insulata in South Africa.