Vegetable Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Cucurbits
Striped cucumber beetles continue to be active a bit later than usual. I treated some watermelons a week ago at Carvel with a foliar spray and was a bit surprised to find as many new beetles migrating in this week as I did. The good news is that as vines get larger (vining off the plastic), they become pretty resilient to beetles. Purdue’s threshold is 5 beetles per plant at this stage, and a recent student finished a project indicating that by using this threshold and reducing insecticide applications, there were greater pollinator visits in large experimental plots well stocked by bees, resulting in a significantly greater yield. It will be some time before striped cucumber beetle is a threat for rind-feeding.

Two spot spider mites are building up in fields, still at fairly low levels. We use an action threshold of 1-2 mites per crown leaf or 50% of plants with an infested leaf on young plants, although this threshold has not been rigorously tested. Keep in mind, there may be isolated hot spots that develop in the field or along field edges. Once plants start setting fruit, they divert more of their resources to those fruits and spider mites typically increase at a faster rate.

Another defoliator that we do not have a threshold for is thrips. The underside of a thrips-infested leaf will start silvering and then bronzing. You will also see small black spots in the affected area – those are fecal pellets. Last year, thrips populations built up to large numbers and probably warranted a treatment, at least in my melons. Once there is greater than a moderate amount of thrips feeding, a treatment might be advisable. Such a level may or may not happen this year. Pyrethroids may or may not be effective on them. Assail should be. Some of our best thrips materials, Radiant and Harvanta, are also very good worm products and it might be better to use them targeting Lepidopteran pests. Speaking of worm pests, we are seeing worms developing on foliage, including yellow striped armyworm. Note their presence, but unless plants are being significantly defoliated by them or there are rinds to protect, they should not be a threat yet.

Squash bugs and squash vine borer are active. You can scout for squash bug by looking for coppery egg masses underneath of leaves. A treatment is advised once you find 1 egg mass per plant, and those eggs begin hatching. While inspecting leaves, look also on the leaf petioles for the single, dark brown, cushion-shaped egg of squash vine borer. Alternatively, pheromone traps for squash vine borer are available. Generally, on susceptible squashes, treatments are done prophylactically to protect vines. Be sure to scout squash treated with pyrethroids (and especially those treated multiple times) for aphid flare ups.

Sweet Corn
Traps in our moth trapping network are checked Mondays and Thursdays by Richard Monaco. Within 24 hours, data should be uploaded to the UD Pest Management Insect Trapping website. Thursday trap counts were generally down, possibly due to cooler night temperatures this week. It is possible that counts will increase in the coming days with warmer weather, be sure to check the trapping website Monday afternoon or Tuesday morning to confirm. Pyrethroid survivorship in our vial tests this week is 22%. This year we have added 6 pairs of European corn borer pheromone traps to several of our trapping locations. So far, we have yet to capture a single moth. Thursday counts are as follows: