David Owens, Extension Entomologist, owensd@udel.edu
Cole Crops
Diamond back moth continues to be active in fields. As a reminder, thresholds decrease to 5% as the plants begin to produce harvest structures (cupping to head formation in cabbage, beginning flower heads in broccoli), and kale is pretty much a constant low threshold. Rotate among two modes of action during a generation, which is roughly 30 days long. After the first set of rotations, switch active ingredients to a different mode of action set. Once the crop is harvested, destroy it quickly to deny DBM from using it as a green bridge to any later fall crop.
Cucurbits
Continue scouting for cucumber beetles. Recent cool, cloudy weather may have paused their migration into fields temporarily but that only means we need to be extra vigilant this week. Thresholds on young watermelon plants are 2 beetles per plant. While cucumber beetles do not transmit bacterial wilt to melons, they do open the plant up for infection with other pathogens and they can weaken the stem when feeding directly on it. Be sure to look under leaves and in the planting hole, especially in the afternoon as beetles hide from direct sunlight. If your plants are less than 2 weeks from having honeybees, consider a foliar treatment rather than a drip application. Effective foliar products that are less bee toxic include Assail and diamides such as Exirel, Minecto Pro, and Harvanta. Several other neonicotinoids are labeled for foliar application, and I suspect that this type of application is less likely to result in nectar or pollen residues, but have not seen research to indicate such.
Squash bugs are active. Begin scouting for egg masses. If you see one egg mass per plant, get ready to treat the plants. Wait until egg masses hatch, as early instar nymphs are the most susceptible to a treatment, while eggs are impervious to insecticide penetration.
Finally, begin scouting for spider mites. In 2021, spider mite populations peaked in many of the fields we sampled at the end of June. Pay attention to field and obstacle edges. This is where we saw most of our mites last year and the year before. Fields with pivot points often have weeds around that pivot point that can harbor spider mites and serve as a small point source. In 2021, we found some of our first mites adjacent to a small grave plot in the middle of a field.
Tomatoes and Eggplant
Scout for aphids, mites, and flea beetles. Flea beetle feeding on eggplant can very quickly lead to significant and even complete defoliation of small transplants, especially if they are not quickly growing. Tomatoes are also susceptible to flea beetles.
Snap Bean
Continue scouting for bean leaf beetle defoliation. Thresholds are 20% defoliation.
Sweet Corn
Begin scouting for whorl stage corn earworm. While it is unusual for us to reach the whorl threshold of 15-30% this early, it can happen.