David Owens, Extension Entomologist, owensd@udel.edu
Sweet Corn
Continue scouting for Fall Armyworm in whorl stage sweet corn. Reports of larger worms came in earlier this week. Fall armyworm does 80% of its feeding during the last 3 days of larval development, so if you are seeing large holes in whorls, make sure that the worms are still present and are not fully grown. By the time a field is treated, they may have left or done most of that 80% feeding. Also, when they are deep in whorls, they are hard to kill. The final instar will be between 0.6 and 1.3 inches long. If you are seeing mid-sized worms or smaller in tassels as they get pushed out, or if you see their egg masses, you may need to spray the field to prevent them from going to the ear prior to silking. As a reminder, sweet corn thresholds are 15% early and late whorl infestation.
Corn earworm moth counts in traps were high on Monday but then decreased a bit in Thursday’s traps. I am concerned that this is the beginning of an upward trend. If so, this would be a few days earlier than last year. Between now and the end of July, trap counts sometimes bounce around. Also, now that our weather pattern is consistently in the high 80’s to low 90’s, spray schedules should be tightened a day to account for faster egg hatch at warmer temperatures. This is the time of year generally when moth resistance levels begin to increase in vial testing. To be on the safe side, use a tighter spray schedule following a pyrethroid. I think an extra day can be granted following a Besiege or Elevest application.
Thursday trap counts are as follows:
| Trap Location | BLT – CEW | Pheromone CEW |
| 3 nights total catch | ||
| Dover | 1 | 47 |
| Harrington | 1 | 25 |
| Milford/Canterbury | 0 | 53 |
| Rising Sun | 0 | 25 |
| Wyoming | 0 | 0 |
| Bridgeville/Redden | 0 | 9 |
| Concord | 1 | 0 |
| Georgetown | 0 | 10 |
| Woodenhawk | 0 | 5 |
| Laurel | 1 | 58 |
| Lewes | — | 1 |
| Middletown | — | 2 |
Cucurbits
Continue scouting for striped cucumber beetle and, MORE IMPORTANTLY, rind feeding injury. Rind damage can also be caused by one of about a half dozen species of Lepidopteran. Cucumber beetle feeding is going to look ‘dirty’, splotchy, and uneven. Lep feeding is going to generally be a smooth, single color. Acetamiprid is our best striped cucumber beetle product but does not have any impact on the Leps. Harvanta is one of our best Lep materials and does have some cucumber beetle efficacy (though not as good as Assail).
Spider mites remain active. Even in wet weather, mites are hard to dislodge from vegetables such as watermelon and tomato. Do not let your guard down on them in these crops.
Squash bugs are very active right now. The threshold is 1 egg mass per plant. You may need to spray twice to clean them up, about 10 days apart. Eggs are almost impossible to kill with insecticides. The pyrethroids, Assail, and Sivanto Prime all do a good job on squash bugs.
Scout for aphids. Aphid populations tend to increase as we approach August. Aphids can be flared up by frequent pyrethroid application.
Tomatoes and Eggplant
This time of year the main tomato threats are from spider mites and stink bugs. Spider mite thresholds are 4 mites per upper canopy leaflet for tomato and 4-8 mites per leaf for eggplant. Although a little early, we may start seeing fruit worms starting in tomato. We performed a spray trial a few years ago looking at various worm products in the Recommendation Guide, applied at weekly intervals for 3 weeks and had no significant differences in worm damaged tomatoes.
Continue scouting eggplant for Colorado potato beetle defoliation, Japanese beetle defoliation, and spider mites.