Development of guidelines for improved control of gout fly (Chlorops pumilionis) in winter wheat

Abstract

Gout fly (Chlorops pumilionis) is becoming an increasingly serious pest of winter wheat due to earlier sowing of crops, generally milder autumns and winters and the toxic effect of BYDV vector sprays on beneficial gout fly parasitoids. Traditionally a pest of southern England, gout fly has now been recorded on farms over a large part of the UK.

This project was commissioned by HGCA to:

1) establish treatment thresholds and effective spray application windows for gout fly control,
2) determine effects of seed rate and drilling date on gout fly numbers and % plant infestation, 3) identify economic treatments based on cost and crop losses,
4) examine the extent to which gout fly has become a UK-wide problem and
5) monitor the occurrence of gout fly parasitoids in field situations.

Experiments were carried out over two seasons (2002/03 and 2003/04) on two Velcourt commercial farms with a history of gout fly and in fields known to be most at risk, i.e. in sheltered areas with nearby woodlands. Working solely on the autumn generation of gout fly, two main types of experiments were carried out using commercially available insecticide products. Experiment 1 aimed to establish an economic threshold and treatment window for gout fly control and experiment 2 to evaluate crops most at risk in terms of plant population, variety and management practice. The geographic spread of gout fly and the two parasitoid wasp species (Stenomalina micans and Coelinus niger) were investigated through field surveys and questionnaires to the HGCA Agronomists' Alliance.

Early-sown (early Sept.) crops were found to be most at risk with some later-sown crops (October onwards) often free from pest attack. Insecticide seed treatments such as imidacloprid gave a significant level of gout fly control but only when populations were below 40% plants infested. The application of foliar insecticides significantly reduced the percentage of plants infested but application timing was found to be crucial. Spray applications were optimal at GS11-12. Applications after this were not effective and product choice was less important than timing. Despite high levels of gout fly in some cases (50-60%) there was no significant reduction in yield attributable to the infestation. This was even the case in low seed rate, thinner crops and those stressed due to reduced early nitrogen. There was no relationship between the percentage of plants infested (R2 = 0.40) and yield (t/ha). It is likely that in the majority of cases the crop can compensate for the early loss of tillers due to the autumn generation.

The geographical survey results showed that gout fly is now widespread throughout England but that the levels of parasitoids are still very low and do not have a significant impact on gout fly populations.