Seasonal abundance of cotton thrips (Thysanoptera: Thripidae) across crop and non-crop vegetation in an Australian cotton producing region

The spatio-temporal relationships of thrips populations across weeds and cotton (Gossypium hirsutum L. (Malvales: Malvaceae) crops were studied across the agricultural landscape of the Namoi Valley, New South Wales, Australia. In a structured sampling program, the seasonal patterns of host plant use by Thrips tabaci Lindeman, Frankliniella occidentalis Pergrande and F. schultzei Trybom (Thysanoptera: Thripidae) were quantified in relation to their invasion of cotton crops. Plants of 69 species (in 20 families) were sampled; T. tabaci was the most numerous thrips species and was recorded from 31 plant species, F. occidentalis was less numerous but recorded from 35 plant species, and F. schultzei was present at very low densities on 25 plant species. Both T. tabaci and F. occidentalis were mostly collected from weeds flowering in spring and summer, when these plants were most abundant. The seasonal composition of thrips populations on cotton changed from predominately T. tabaci on seedling cotton to F. schultzei and F. occidentalis on mature flowering cotton later in the season. High T. tabaci abundance on early season cotton was attributed to the abundance of T. tabaci on the surrounding weed species, because the weed hosts on which it was recorded were plentiful then. In contrast, the patterns of F. occidentalis and F. schultzei abundance on cotton were not correlated with their abundance on weeds, but rather with the emergence of cotton flowers. The genetic relationships and cryptic species identity of thrips on cotton relative to those on the other host plant species sampled was investigated through the analysis of mitochondrial CO1 gene sequences. Thrips tabaci and F. occidentalis collected from cotton were genetically identical to conspecifics collected from weeds. This is consistent with these insects moving onto cotton plants from nearby source populations on weeds. Frankliniella schultzei is a species complex in Australia, two members of which are present in this region, but only the black species was found in these surveys. This study demonstrates that (i) weeds play an important role in the population ecology of thrips, particularly in T. tabaci infesting early season cotton crops, (ii) the role of weeds warrants consideration in relation to pest management strategies, and (iii) without considering the surrounding landscape (which needs definition relative to each pest species) invasions cannot be readily understood mechanistically.