David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa
Now that the first cutting is proceeding, start thinking about scouting regrowth in the coming weeks for potato leaf hopper, which typically appears towards the end of May. Stubble treatments are rarely necessary, as cut alfalfa stimulates adults to seek ‘greener pasture.’ Resistant varieties have glandular trichomes that glue nymphs down and trap them; these varieties may have as much as 75% fewer leafhopper than non-resistant varieties. There are also non-yellowing varieties which still need to be scouted for as they will suffer leafhopper damage even without the visual after-the-fact symptoms. High aphid populations continue to be reported in some fields, in these fields it is best to cut earlier rather than spray.
Field Corn
Begin scouting fields next week for cutworm damage. By the end of next week we will be approaching degree days for 4th instar black cutworm to be active and begin cutting plants.
Soybean
Some of the early planted soybeans are now emerging out of the ground. Keep a sharp eye out for soybean stand reducing pests. This past week, damage was observed from both slugs and seedcorn maggot. SCM is favored by recent incorporation of organic matter such as cover crop or previous crop residue. Feeding injury will show up as runty plants that are well behind and smaller than neighboring plants. Affected cotyledons might not fully emerge out of the soil. Stems often swell and turn a grayish color with poor root development. Slug feeding however will look like regular gouges taken out of the cotyledons and the stems. Unlike SCM, slugs are favored by no-till, high soil surface residue. There are no rescue treatments for SCM. For slugs, rescue treatments of Ferroxx and Deadline are available, but unless pellets are broadcast no later than the first signs of seedling emergence and feeding injury, they may be too late. Baits perform best when several days of dry weather follow, giving the slug time to dehydrate. A third seedling pest to begin scouting for on the earliest planted soybean is bean leaf beetle. Thresholds are high, 2 beetles per plant with greater than 40% defoliation. It is very unusual to see this level of bean leaf beetle activity.
Wheat
Scout for armyworm in wheat, especially in New Castle County to Smyrna. Reminder – all pyrethroids except Mustang have a 30 day pre harvest interval. Thresholds are 1-2 worms per row ft. Armyworm tends to hide during the day under residue. Barley thresholds are a little lower, 1 per row ft. Cereal leaf beetle larvae were observed in a couple of fields in very low numbers.