Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Sweet Corn
Fall armyworm is active in the area. Treatment thresholds are 12-15% infested plants. Corn earworm moth counts are very high for this time of year. Complicating the picture is that there is a wide degree of variation among various states in terms spray schedules based on pheromone trap counts. Our pheromone trap count of 13 or more moths per night triggers a 3-day spray schedule. Some states recommend a 2-day spray schedule when moth counts are at 20 per night and the temperature is above 80 °F. Last year, we compared a pyrethroid sprayed at a 2-day interval with a pyrethroid sprayed at a 3-day interval in August-silking sweet corn. There was an 11 point difference between the two spray intervals in terms of perfectly worm-free ears, however, there was no difference in percentage of clean + tip damage ears. If relying on a straight pyrethroid, tighter spray schedules are advised. Moth susceptibility testing indicates a potential 19% survivorship rate when exposed to pyrethroids in vials. We will be testing moths again in the next several days.

Diamides (Coragen, Harvanta, Besiege) will give excellent earworm control and have translaminar activity. Recently questions have been raised regarding other labeled products for sweet corn because we do not want to rely heavily on just two modes of action. Intrepid (methoxyfenozide) is labeled for sweet corn but it is a growth regulator. It kills larvae slowly during the molting process and it is possible that worms will still infest tips before dying. Obviously, this would not be acceptable for fresh market sweet corn. Avaunt (indoxacarb) is not labeled for silking sweet corn, but is an excellent worm product for pre-silking worm applications. Radiant and Blackhawk (spinosyn class) are good worm products in a rotational scheme. Last year we rotated it with daimide applications, and the last two applications were pyrethroids and it was effective. One to two applications with a diamide should provide excellent control and could allow a spray schedule some flexibility. However, a straight spinosyn application results in poor control. Under heavy moth flights, a more conservative approach would be to include a pyrethroid with a spinosyn. Lannate (methomyl) tank mixed with a pyrethroid is another excellent treatment to include in a rotation. Bottom line: incorporate multiple, effective modes of action (we have 4) to preserve earworm insecticide susceptibility.

A couple of European corn borer have been intercepted this week in black light traps, 1 in Milford and 2 near Trap Pond.

Sweet corn pheromone and blacklight traps are checked twice weekly on Mondays and Thursdays. By Tuesday and Friday morning, data is uploaded to our website https://agdev.anr.udel.edu/trap/trap.php. Moth counts from Thursday are as follows:

Watermelons
Inclement weather earlier this week has prevented us from checking as many melon fields as usual. Several fields have been treated for mites, be sure to check fields and be prepared for a follow up spray. This is especially true of contact miticides. Also make sure that mites are in fact alive and active. Use a hand lens to determine if the mites are still moving and there are new, fresh looking eggs. Some miticides kill mites slowly, and dead mites will stick to the webbing for some time. Reports of worm activity have been increasing in the area. This includes corn earworm. While earworms feed mostly on the blossoms, they should be considered part of the ‘rindworm’ complex. They are not as aggressive as armyworm or leafrollers but can still scar up fruit.