If a field has cabbage seedpod weevil and diamondback moth feeding, should economic spray thresholds be reduced to account for the combined pressure?
In the case of these two species, the answer is no. Count them separately and stick to the economic control thresholds for each. Why? Because weevils attack flowers and buds, but diamondback moth right now are eating more leaf tissue than flowers.
Where an “additive” effect of multiple species can make sense is when more than one species are feeding on key yield-producing areas — flowers, buds and pods — at the same time. For example, if you have lygus bug and cabbage seedpod weevil in the same field and if they are both at 50% to 60% of their economic spray thresholds, spraying may provide an economic benefit — especially if the crop is already stressed.
There has been no research to support this interpretation of the thresholds, but this is currently the best guess from experienced entomologists.