Epsilon Archive for Student Projects

Abstract

Polyphagy presents difficulties to an insect, especially in terms of host choice. The adult insect must choose from a large number of possible host plant species, something which is considered to require extensive neural capacity to process. In order to decrease the amount of information to process, it has been suggested that polyphagous insects could rely on previous larval experience to make a choice. In the polyphagous noctuid moth Spodoptera littoralis it has frequently been found that female oviposition choice is driven by ‘larval memory’, indicating that the adult chooses to oviposit upon the same host upon which it fed as a larva. However, is there a larval density effect on how the host plant is perceived, and does this effect adult behaviour?

In this study the insects were reared in three different treatments: low-density (optimal), highdensity and starvation (both assumed suboptimal), on three different diets (cotton, cabbage, and maize). A development study was carried out whereby it was indeed found that lowdensity was optimal, in terms of fitness (pupal weight) and fecundity (egg load). High-density larvae were found have a higher investment in immune response, which coupled with the reduction in pupal weight and egg load, indicates that high larval density may cause a reallocation of energy and a negative effect on fitness and fecundity. Starvation was highly detrimental, having a high development time and death rate.

It was found that larval density did not have an effect on larval host choice, however, it was clear that the larvae were making an experience-based choice, irrespective of density. Adult oviposition choice, however, displayed a switch from choice in adults deriving from lowdensity treatments to the supposedly random laying seen in the adults originating from the high-density treatment. Furthermore, first choice in oviposition host showed a switch between low- and high-density reared insects, with high-density adults laying eggs on a nonexperienced plant first.