David Owens, Extension Entomologist, owensd@udel.edu
Sweet Corn
Scout whorl stage sweet corn NOW for fall armyworm infestation. Multiple reports of fall armyworm came in this week from small worms just hatching and only causing window-paning to larger worms working in the whorls. Sweet corn thresholds are 15% early and late whorl infestation and 30% mid-whorl stage. We have several options available for fall armyworm control. The key is to treat early infestations. By the time large holes are noticeable in leaves, the worms are either very deep in the whorl and will be hard to reach with an insecticide or they may have left. I prefer not to burn one of our 2-4 shots of Elevest or Besiege on fall armyworm because whorl stage sweet corn will be silking when moth pressure comes up in early August. We will want all the chlorantraniliprole that we can use for silk protection. If caught early enough, pyrethroids may do enough on them; other options include Rimon, Intrepid, Intrepid Edge, Avaunt, and the spinosyn class.
A report came in this week of spider mite stippling on husk flag leaves. Agri-Mek has a 7-day PHI, Oberon has a 5 day PHI, and Zeal has a 21 day PHI. A high rate of a pyrethroid or a pyrethroid + Lannate might knock them back just enough to reduce them for harvest, but in my experience, pyrethroids only annoy spider mites and they quickly come back with a vengeance. Oberon sometimes is not fast enough to suppress them in time for a harvest if the sweet corn is going to be harvested inside of Agri-Mek’s 7-day PHI.
Corn earworm moth counts are generally very low across the state but with a couple of exceptions. Some trap sites even indicate a 5-day spray schedule, but this just makes me nervous.
Thursday trap counts are as follows:
| Trap Location |
BLT CEW |
Pheromone CEW |
| 3 nights total catch | ||
| Dover | 1 | 83 |
| Harrington | 1 | 5 |
| Milford/Canterbury | 0 | 3 |
| Rising Sun | 1 | 19 |
| Wyoming | 0 | 18 |
| Bridgeville/Redden | 0 | 5 |
| Concord | 0 | 5 |
| Georgetown | 1 | 19 |
| Woodenhawk | 0 | 4 |
| Laurel | 1 | 55 |
| Lewes | ||
Cucurbits
Striped cucumber beetles have just started emerging from the soil. One easy way to monitor for their presence in watermelon is to look at flower feeding. That will at least tell you they are there. This generation will best be treated with acetamiprid or cyclaniliprole. Carbaryl can be very effective, but it is devastating to pollinators and can flare up mites. Pyrethroids can flare mites as well and have not been effective in recent bioassays or spray trials.
Mites are becoming an increasing concern with the hot weather we have been experiencing. It does not take long for their populations to increase 5-10x. You may see hotspots with heavily stippled leaves that are yellowing and senescing. Mites will primarily impact the plant’s ability to set and mature late sets, but as far as I can tell, they do not have a great impact on the first or second set. Of the short PHI materials, Acramite/ Banter and Portal have been consistent. Kanemite has some translaminar activity, and Magister has powdery mildew activity. On translaminar materials such as Kanemite, Zeal, and AgriMek, remember not to mix them with sticker adjuvants or fungicides that have a lot of stickers such as Bravo.
Squash bugs are very active right now. Thresholds are 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 bug.
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. Eggplant is a spider mite magnet, but mites tend not to show feeding signs until heavy populations have built up. Scout eggplant for defoliation as well, Japanese beetle and potato beetles can cause heavy defoliation. Stink bugs are hard to scout for. Rutgers IPM program (Kris Holmstrom) wrote recently: This is the time of the season when native brown stink bugs become active in tomato fields. These true bugs, move into irrigated tomato fields as forage in the surrounding environment dries out. Feeding results in “cloudy spot”. Increases in stink bug injury are often found by crews picking the fruit. Growers should consider 1-2 insecticide applications to limit fruit injury if this damage is increasing in harvested fruit. If actively scouting fields, insecticides should be considered if stink bug adults, nymphs or new fruit injury is found in 2 or more sample sites in a 10-site sample. “
Snap Beans
Scout for spider mites and potato leafhopper. Worm damage potential is very low right now.
Potatoes
Colorado potato beetle adults have emerged and are laying eggs again. Scout for larvae and for defoliation, especially on later maturing plantings or varieties.