David Owens, Extension Entomologist, firstname.lastname@example.org
Corn earworm numbers continue to increase. With the cooler weather and looking at next week, a 2 to 3-day spray schedule or a 3-day spray schedule with high pressure may be warranted. As always, rely on the closest trap to your sweet corn as possible. Our traps are stationary, we have seen trap catch vary quite a bit depending on exactly how close the trap is to sweet corn. Look at the statewide trend as well.
Fall armyworm is active throughout the state. It lays egg masses anywhere on foliage beginning at early whorl stage. Thresholds are 30% infested plants at mid-whorl and 15% at tassel push. Scout often, by the time large holes are noticeable, the worm is large and deep in the whorl where efficacy is going to be much reduced or has just left. The big concern with fall armyworm is that, unlike earworm, you can have worms working on the ear before silking. Scout husk leaves for signs of windowpaning. If fall armyworm is present, a pre-silk application may be justified. In UD spray trials in which fall armyworm is NOT present, there has been NO benefit to a pre-silking insecticide application. I like to start my spray program about 1-2 days after the first silk. By this time, about 25-33% or so ears have silks present. Besiege and Elevest are the best worm materials. Apply between late morning and mid-afternoon to avoid honeybees. Honeybees love sweet corn pollen, and there is another little burst of pollen in the evening. By dusk, honeybees are no longer active in sweet corn. Depending on the rates chosen, Besiege can be applied 3-4 times. Both Besiege and Elevest have chlorantraniliprole active ingredient restrictions, so pay attention to the label. Elevest is also more restricted in how much it can be applied due to active ingredient maximum use restrictions on bifenthrin. Pay attention to those, especially if you were planning to apply a Brigade or Sniper or other bifenthrin pyrethroid product. A follow up pyrethroid application is most often tank mixed with Lannate. Do not apply Lannate by itself. If you do not wish to tank mix with Lannate and are using lambda-cyhalothrin, tank mixtures of high rates of Radiant or Intrepid Edge have yielded excellent results in UD spray trials; lower rates have not yet been tested and analyzed. We have seen consistent differences among pyrethroid products in the last several years of spray trials. I do not know if those differences mean that a 2 day spray interval under hot weather high pressure conditions is still necessary. I lean towards erring on caution and advise a 2 day interval, but it is possible that that recommendation is conservative enough should something prevent a 2 day spray interval. Tight pyrethroid spray intervals probably only need to be considered during the first 10 days of silking. After that point, the sweet corn gradually becomes less attractive to moths laying eggs.
At 10 days after silk, scout for aphids. We have received several reports of unusually high aphid activity in sweet corn this season from both corn leaf aphids and from melon aphids. If you look at the commercial vegetable production guide, there is an error in the aphid section for sweet corn. Labeled products for aphids include Lannate, Assail (there may be generics available, check labels), Sivanto Prime and Transform. Complaints have been received even with Lannate in the rotation. Assail has two different rates for aphids and the pre-harvest interval varies accordingly. Sivanto and Transform both have a 7 day PHI. Transform needs to have an adjuvant mixed. It stands to reason that pyrethroids are flaring aphid problems by removing natural enemies, thus a 10-14 day after silk scouting should pick up a growing aphid population and potential problem in time for the most effective products (Sivanto and Transform) to be applied and have some additional follow up scouting to check for efficacy. These are expensive products that should not be applied prophylactically but only based on scouting report. They do not have worm efficacy. I have heard anecdotes of mixed results with acetamiprid. Irrigation should also help with reducing honeydew on ears prior to and at harvest.
Thursday trap captures are as follows:
|Trap Location||BLT – CEW||Pheromone CEW|
|3 nights total catch|
|Lewes||—||187 (5 nights)|
If you haven’t already, scout for stink bugs, tarnished plant bugs, earworms and loopers. Loopers are beginning to make their way into our area and can be difficult to control with pyrethroids. Good worm products include Radiant, Blackhawk, Intrepid, Avaunt eVo, Coragen, Exirel, Vantacor and Minecto Pro. Of these products, we have had the best control of soybean looper with avaunt’s active ingredient indoxacarb, followed by Radiant. Note that none of these products will control stink bugs or tarnished plant bugs. Dimethoate is labeled for tarnished plant bugs, and if combined with a pyrethroid should provide excellent earworm control BUT is extremely hazardous to bees and could set the field up for a looper invasion.
Pay attention to the Bee toxicity ratings in the vegetable guide when considering a pesticide application to lima beans. Beekeepers often place hives at lima bean fields to take advantage of their sometimes abundant nectar.
New this year is an edamame production chapter in the vegetable guide. It can be downloaded here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/BFP_2022-2023_Edamame.pdf. Please note that stink bug and earworm thresholds are much lower than grain soybean production.
The risk of corn earworm damage is increasing with increasing moth capture in pheromone traps. Continue also scouting for mites. The threshold for mites is 4 mites per upper canopy leaflet. Use high water rates and high pressure for good coverage.
Continue scouting for beet armyworm. The caterpillars are green with a small black spot above the second true leg behind the head. They are resistant to pyrethroids.
Continue scouting for rindworm. Second generation cucumber beetles may be emerging so it is possible to have an uptick in beetle activity late in the season. The best thing is to scout for rind injury.
Continue scouting for diamondback moth, harlequin bug, aphids, and cabbage loopers. All other worm pests are present, but these have special considerations. Diamondback moth population genetics tend to be localized, meaning that previous efficacy history should be an important consideration in a given locale when selecting an insecticide. For instance, some areas of Virginia, Delaware, and New Jersey have reported unsatisfactory diamide control, and are likely to again. However, diamides work very well in Georgetown, DE. The best products for harlequin bugs are pyrethroids and neonicotinoids, but pyrethroids generally perform poorly on diamondback moth and risk flaring aphids. Aphids can be controlled by many products, including some good worm materials such as Torac, Exirel, and Harvanta (melon aphids). You need to scout for cabbage loopers because Torac is not labeled for that particular worm species but it has very good efficacy on other worms and aphids.