Idris M Barchia

Models for the dispersal in Australia of the arbovirus vector, Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae).

Culicoides brevitarsis is the main biting midge responsible for the transmission of bluetongue and Akabane viruses to livestock in Australia. Models are given for its dispersal after winter from endemic areas at the southern limit of its distribution in New South Wales (NSW); the models might also be applicable elsewhere. Model 1 shows that dispersal can be explained by distance from a key point just outside the endemic area in mid-northern/northern coastal NSW. The model provides probability data for times of first occurrence at sites within regions down the southern coastal plain or up the Hunter Valley towards (but rarely reaching) the western slopes and tablelands. Model 2 shows that the movement depends on temperature and wind speed from northerly and easterly directions. Preliminary data also are given to suggest a relationship between density in the endemic area and the maximum distance that C. brevitarsis can travel in a given year. The models can be linked to other information which in combination can provide probabilities for winter survival outside the endemic area, times of occurrence at sites where it cannot survive winter and times when activity ceases naturally at these sites at the end of the season. This information can be used to predict the potential for virus transmission and indicate zones of seasonal freedom from both vector and virus for the export of livestock.

Phytosanitary irradiation of three species of spider mites (Trombidiformes: Tetranychidae): Phytosanitary irradiation of spider mite

The survival of three species of Tetranychidae mites: Tetranychus evansi (tomato red spider mite), Tetranychus ludeni (bean spider mite) and Tetranychus urticae (two‐spotted mite), exposed to gamma irradiation doses of 200, 300, 400, 500 and 600 Gy was investigated. The aim was to identify an irradiation dose capable of killing or sterilising mites infesting horticultural produce to prevent movement to and colonisation of host plants in non‐infested regions or countries. Eggs were irradiated separately; otherwise, to simulate naturally occurring mite populations, mixed life stages (immatures and adults) were combined and the treatment effects compared to non‐irradiated controls. Irradiated eggs of all species failed to develop and hatch at 200 Gy and above. Tetranychus urticae was the most tolerant species, requiring an irradiation dose of 300 Gy to ensure all adult females were sterilised. Sterilised adults continued to lay eggs, but these eggs failed to develop or hatch. Tetranychus evansi and T. ludeni were sterilised when irradiated at 200 Gy. Based on these results, a dose of 300 Gy is recommended as a phytosanitary measure to prevent the survival of these species of Tetranychidae pests of horticultural produce in non‐infested regions. Combined with data from other studies this study proposes a generic dose for Tetranychidae mites of 375 Gy.

Effects of gamma radiation on the survival of four species of thrips (Thysanoptera: Thripidae): Effects of irradiation on the survival of thrips

The survival of four species of thrips (Frankliniella occidentalis, Frankliniella schultzei, Thrips tabaci and Thrips imaginis) exposed to gamma irradiation at doses of 0, 200, 300, 400 and 500 Gray was investigated. The aim of the work was to identify a generic dose of gamma radiation capable of killing or sterilising thrips infesting horticultural produce to prevent movement to, and colonisation of host plants in noninfested regions or countries. To simulate naturally occurring thrips populations, mixed life‐stages were irradiated, and the effects compared with a nonirradiated control. The eggs of F. occidentalis and T. tabaci were more tolerant to irradiation than those of F. schultzei and Thrips imaginis, but none of the F. occidentalis or T. tabaci eggs developed beyond first stage larvae when irradiated at a target dose of 200 Gy. In mixed life‐stage tests, all species had a similar tolerance to gamma irradiation. At a target dose of 200 Gy, all species were sterilised and no further offspring were produced. Based on these results, a minimum generic dose of 200 Gy would be sufficient to control these species of thrips in fresh horticultural produce destined for export to thrips‐sensitive regions. A target dose of 250 Gy will provide a further margin of safety, until large‐scale confirmatory trials are completed.