Vegetable Insect Scouting

David Owens, Extension Entomologist,


Striped cucumber beetles remain active and should be scouted for. Treatment thresholds are 2 beetles per plant in watermelon. If honeybee hives are going to be placed soon, the best treatment options are foliar Assail or a foliar neonicotinoid. A local farmer raised an interesting question that I did not know the answer to regarding how neonicotinoids move in the plant after a foliar application. I sought clarification from industry and was advised that a foliar application of say thiamethoxam would not move systemically into flower buds and new flowers but that the chemical would stay localized to where the droplet hit the leaf. I want to do some more digging, but if that is indeed the case, it opens the playbook up a little bit for the period of time before pollinators are placed on fields. As always, read the labels and follow the bee cautions. Assail is much less toxic to bees than the other neonicotinoids. There is also now a liquid formulation of Assail available.

We collected beetles from two different locations this past week and conducted a pyrethroid efficacy bioassay with Besiege, Mustang, and Brigade at their high label rates. Only Brigade resulted in moderate levels of mortality (~ 40%) and apparent repellency from the treated leaf. At this point, I do not recommend using a pyrethroid for targeting high numbers of cucumber beetles, especially because Assail works extremely well. Now, keep in mind Assail will not affect the rindworm complex, so when we have fruit present, it becomes very important to differentiate rind feeding damage caused by leps vs beetles. Generally speaking, beetles won’t cause rind feeding damage until the 2nd week of July (based on when the first generation emerges out of the soil). Cucumber beetle feeding will have a dirty appearance whereas worm feeding is going to be a smooth, flat, single color, and even scar. The diamide Harvanta, is an excellent worm product and does kill a moderate number of beetles outright. Cyantraniliprole (Exirel and Minecto Pro) kill fewer beetles, but both cyantraniliprole and cyclaniliprole will stop beetle feeding on treated plant tissue.

Sweet Corn

Scout for cutworm and for stink bug damage, especially in no-till fields in which a late grass cover crop was used. Thresholds for brown stink bugs in field corn are 13 or more bugs per 100 plants; this threshold is probably equally valid for processing sweet corn. Threshold and sampling advice can be found here: While cutworms are susceptible to pyrethroids, only bifenthrin has significant brown stink bug activity.

Snap Beans

Seedcorn maggot, potato leafhopper, and bean leaf beetle are all active in the area right now. A neonicotinoid seed treatment can effectively suppress all three pests for 3 weeks after planting. Potato leafhopper causes hopper burn – a yellow to necrotic lesion on the leaf edge. Thresholds are 5 adults or nymphs per sweep.

Cole Crops

Continue scouting for worm pests. Diamondback moth is active right now and should be considered when selecting a worm material. Do not rely on broad spectrum pyrethroids, OPs or carbamates as they will seriously damage the natural enemy complex which can, given the right circumstances, take out nearly all diamondback moth larvae (but only after they have reached their final instar). Thresholds at this point are around 20% infested plants.


Colorado potato beetle began moving around towards the end of last week before cool weather set in again. Thresholds are 50 adults per 50 stems or up to 200 small larvae per 50 stems. Potato leafhoppers are present now on Delmarva and should be scouted for. We are approaching 45-60 days after planting in many locations, and it is likely any in-furrow neonicotinoid treatment may be wearing off. Thresholds for potato leafhopper are 1 per sweep or 1 nymph per 10 leaves. Leafhoppers are susceptible to a wide range of insecticides, including Torac which has potato beetle efficacy.