David Owens, Extension Entomologist, owensd@udel.edu
Sweet Corn
Spider mites occasionally cause problems in sweet corn, particularly if large numbers are developing on the ear leaf at the beginning of silking or earlier. There are no hard and fast thresholds for sweet corn. There are three options, the first is Zeal but it has a 21-day PHI. Oberon is another option with a shorter PHI, but it tends to be less effective against large populations. The third option is two applications of bifenthrin. Spider mites tend to be flared up by other pyrethroids, so caution is advised. At any rate, it is going to have very short residual and only work on mobiles, thus two applications would be necessary if it is going to blunt mites.
With frequent pyrethroid applications, watch out for aphids flaring up. Three species may be present. Other states are reporting high populations of corn leaf aphid, for which the inclusion of Lannate in a spray program can be helpful. CLA is rectangular and blue green. Bird cherry oat aphid is the second species that builds up in sweet corn, and while it is relatively easy to control in other corps, can be problematic in sweet corn. Melon aphids tend to be yellow and are much less susceptible to Lannate. Assail can be applied at low rates with a 1 d PHI or at its full sap beetle rate with a longer PHI. I personally recommend the full rate targeting sap beetles.
Watch out for fall armyworm in whorl stage and tassel push sweet corn. We recently evaluated a non-replicated strip trial 3 days after application. We examined up to 25 damaged whorls for armyworm presence, live worms, and dead worms. Assuming the percentage of damaged whorls was equal across all treatments at the time of application, Intrepid Edge and Avaunt were the best at reducing Fall armyworm, followed by Besiege, and last by Lannate + Warrior. Intrepid Edge can be used in silk sprays for corn earworm, but tank mix a pyrethroid with it!
Corn earworm populations in pheromone traps appear to be slightly lower than last week, but blacklight trap counts are a bit higher. One of our sites began catching high numbers in pheromone traps this week. With high temperatures, and an increase in pyrethroid challenge survivorship, consider tightening spray schedules a day following a pyrethroid application or following a heavy rainfall event. I do not think this tightening is necessary following a Besiege or an Elevest treatment. Over the last 5 seasons, Hero at its high rates have outperformed all other pyrethroids applied alone, followed by beta cyfluthrin and bifenthrin, with lambda cyhalothrin being the least effective pyrethroid (3-day spray schedules). You can read a short summary of spray trials and recommendations here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/pest-management/Sweet_Corn_Ear_Insect_Protection_July_2023.pdf.
A final note: locations with higher brown marmorated stink bug catches may be at risk for some stink bug injury. If you have experienced stink bug damage before or are near a location with higher brown marmorated stink bug injury, add bifenthrin in your spray rotation. I am including the blacklight stink bug captures below to illustrate this point, but these counts can be found on our website at
Thursday trap captures are as follows: http://agdev.anr.udel.edu/trapsb/trap.php.
| Location | Blacklight Trap | Pheromone Trap | Green Stink Bug | Brown Stink Bug | Brown Marmorated |
| Dover | 2 | 32 | 3 | 0 | 0 |
| Harrington | 1 | 54 | 12 | 1 | 0 |
| Milford | 2 | 14 | 5 | 11 | 6 |
| Rising Sun | 3 | 43 | 2 | 2 | 1 |
| Wyoming | 2 | 18 | 3 | 2 | 0 |
| Bridgeville | 2 | 10 | 9 | 5 | 19 |
| Concord | 2 | 32 (M-W) | 8 | 38 | 14 |
| Georgetown | 3 | 38 | 25 | 8 | |
| Greenwood | 1 | 56 | 141 | 65 | 22 |
| Laurel | 5 | 56 | 22 | 11 | 4 |
| Whaleyville | — | 66 | |||
| Lewes | 164 |
| Newark | 34 |
Cucurbits
Spider mites are still a threat given the hot weather. For crops expected to finish up by Labor Day, there are three options: do nothing, move bees off the field and treat with Agri-Mek, or use a bee safe miticide. If populations are low and plants have a good canopy, they should be able to set and size up the last fruits. For crops going into later September or even early October, you may want to keep mite maintenance up.
Rindworm is still a threat. Beet armyworm populations are increasing in the area, among other species. Examine fruit for smooth, even, wide rind scars.
Begin treating young summer and winter squash for squash vine borer again. We have another flight period that should be active now. A regular pyrethroid application to the vines will protect plants from damage. Watch out for flaring aphids though! Aphid populations tend to increase greatly in August.
Finally, squash bugs are very active right now. Include a pyrethroid or Assail into a maintenance program should clear them up.
Fruiting Vegetables
Scout for beet armyworm defoliation in peppers and for stink bug injury to developing fruit. Concentrate fruit scouting to areas near wood lines or field corn where stink bug populations are likely to be greater. Continue scouting for Spider mite damage in tomato and eggplant. Heavy mite populations were observed in eggplant this week. Thresholds in eggplant are 4-8 mites per leaf. Continue scouting for Colorado potato beetle, especially in eggplant.