The Green Vegetable Bug, Nezara viridula, has recently become a more significant pest of Australian cotton but it is not a problem every season. This project addressed aspects of the multiple host use and movement of N. viridula as they relate to cotton, as well as the genetic relationship between Australian N. viridula and the global populations of this pest.

Samples of N. viridula were collected from northern and eastern Australia and from a variety of weed and crop hosts, with an emphasis on cotton. The abundance of N. viridula was low for the duration of the project but about 800 adult insects were collected overall. Comprehensive phylogenetic and population genetics methods were used to address each of the questions. The methods developed during this study will be made available to other researchers through scientific publications.

The Australia populations of N. viridula come from two different evolutionary lineages, one European and one Asian. The former is distributed across eastern Australia and the other across northern Australia. At some point in the past some individuals of the Asian lineage have mated with individuals of the European lineage in northern Queensland but these events appear to have occurred only rarely. Across the different host plant species there are no genetic differences between N.viridula that would indicate separate host-specific gene pools.

The N. viridula in eastern Australia are more genetically distinct from one another the greater the geographic distance that separates the sampling localities from which they were collected. This slight genetic differentiation over geographic distance is present in insects collected across two years and this indicates that N. viridula populations remain relatively localised in the short term. This result indicates that the host plants available to N. viridula within each cotton growing region will be the most relevant for predicting the abundance of this insect in cotton. Pest pressure from N. viridula was low for the duration of the project and so this pattern may be different during seasons when N. viridula is present in high numbers. In years of high abundance host plants might be found between growing regions, and allow for the recruitment of N. viridula over a wider area.

Future research that addresses the host use of N. viridula should investigate populations from each cotton growing region independently, as local conditions, such as the crop and weed host plants used by N. viridula each season before cotton becomes attractive, will be the most relevant to late season numbers of this insect in cotton. A previous CRDC funded project has already addressed N. viridula host use in central New South Wales. If cotton is grown regularly in northern Australia then it would be prudent to treat the N. viridula population there as a separate entity, as there may be significant differences in their biology which could affect their host use and abundance in cotton. Any differences would therefore influence the development of management strategies.