An Economic Threshold for Thrips in California Timothy

Timothy is grown as a high value forage crop in California. Thrips (Anaphothrips obscurus Müller) have recently been implicated causing damage by increasing the quantity of brown leaves and lowering the value of this crop. Minimal studies have explored the relation of thrips levels to yield and damage in timothy and management options for this pest are extremely limited. We performed studies in 2006-2008, manipulating thrips levels with disruptive insecticides to establish a hybrid economic injury level (EIL) and provide information on the level of thrips necessary to cause economic loss. Thrips levels were documented and yield and damage was assessed. We also collected bales with varying levels of brown leaves and surveyed growers on the price that they would receive for the bales if it were first or second cutting hay. In all our manipulative experiments, there was a range of thrips levels among treatments and Tetranychid mite levels were flared with cyfluthrin in some experiments. We found no significant impact of thrips or mite levels on yield, but were able to isolate the effect of thrips alone (without mites) on damage in two experiments. The information from these two experiments was combined with the survey data to provide a measure of economic loss based on brown leaf and to establish hybrid EILs. We created EILs for first and second cutting hay prices for methidathion and cyfluthrin. EILs ranged from 19-225 accumulated thrips days/tiller. We then created economic thresholds at 75% the level of the lowest EIL calculated. Economic thresholds ranged from 2-24 thrips/tiller/week. In summary, we found that cyfluthrin can significantly flare mites and we created economic thresholds for thrips in timothy that can be used with population information at a single point in time or can be combined with population information over time.