Burning dormant timothy: Cultural management for thrips

Grass thrips (Anaphothrips obscurus Müller) are an important pest of timothy (Phleum pratense L.) in Western states. In California, thrips are primarily managed directly through insecticides and indirectly though limited field burning. We evaluated the effect of burning on thrips populations to explore its use as a cultural or physical management tactic. Two plot studies were conducted in the dormant period in 2007 and one in 2008; sampling commenced approximately two weeks after burning. Additionally, burned and unburned fields were sampled to compare thrips populations. In one 2007 plot study, there were 0.32 thrips/tiller in unburned plots and no thrips were found in burned plots. In the other 2007 plot study, there was a significantly higher number of nymph thrips in burned plots compared to unburned plots, but adult thrips numbers were not significantly different. Nymph numbers were extremely low when the 2008 plot studies were sampled compared to those in 2007. However, adult thrips numbers were significantly higher in unburned plots when compared to burned plots. There were no significant differences among thrips numbers between burned and unburned fields that were sampled in either year. In conclusion, burning may reduce thrips numbers, but should be approached with caution as a management tool. Although thrips numbers were successfully manipulated with burning in plot studies, thrips numbers were not significantly different between unburned and burned fields. Finally, we have anecdotal evidence, in addition to evidence in the literature, that burning timothy may reduce yield.