David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa
Alfalfa Weevil have begun hatching! This week we (Morgan Malone, Calista Turman, Danielle Watkins, and myself) scouted fields in Dover, Houston, Greenwood, Galestown, and Hebron. All but one field had above threshold numbers of very small alfalfa weevil larvae, and I suspect the field that had lower numbers will have above threshold level soon. Alfalfa right now is very small, for the most part less than 5 inches. A good ballpark estimate for PURE STAND ALFALFA is around 0.7 weevil larvae per stem, the fields we visited were 4 or more times greater than this. Larvae are currently deep in the terminals, where it is going to be difficult to get enough product to them for control. Larvae will be active for the next several days until about mid-next week. The forecast is currently calling for temperatures in the upper 40s and lows in the low 30s. That will slow them down considerably. It may be best to wait a bit until after the weather warms again and the larvae are a little bit larger and more active before treating them.
In the last two years, we have performed 3 different spray trials on 2 farms for alfalfa weevil and one on aphids. Pyrethroids in 2021 gave us between 60-80% control, Dimethoate alone less than 50%, and Steward at its low rate was near 90% control. In 2022, pyrethroids gave us about 70% control, Dimethoate gave us 70% control, and a mid-rate of Steward 85%. The mid-rate of Steward also prevented weevil numbers from ticking back up 21 days after application. In the 2022 trial, Besiege at 10.0 fl oz gave NO better control than Warrior II at 1.9 fl oz. This is because the chlorantraniliprole component of Besiege does not have weevil activity.
Our third trial at a different farm site targeted larvae that were a little bit larger. Warrior II gave us only about 66% control 5 DAT whereas Steward at a sub-label rate of only 4 ounces resulted in 80%. Based on 3 years of testing, Steward at its low to mid rate range should provide good to excellent control. Follow up scouting is important after a weevil application to ensure that numbers are sufficiently reduced below threshold and to make sure aphids are not increasing. In 2021 we treated aphids that had exploded in one field several weeks after a lambda-cyhalothrin application. Although aphids are on the label of many pyrethroids, a follow up pyrethroid application did not reduce their numbers while Dimethoate, Sefina, and Sivanto gave excellent control. Fortunately, the alfalfa was tall enough and beginning to bud that early harvest was an option.
One product that we have not tested that is still labeled on alfalfa is Imidan, but based on published spray trials I would expect it to be somewhere between pyrethroids and a mid-rate of Steward. We will be testing it in the next couple of weeks.
You can find the UD alfalfa insect control recommendations page here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/commercial-field-crop-pest-management/alfalfa/ . You may want to check this page periodically in the days ahead, we have updated our insect control guide and it will replace the 2020 version very soon. Please also note that the thresholds are for pure stand alfalfa only. For mixed stands, thresholds are higher until alfalfa consists of less than 50% of the hay at which point it is not considered economical to treat for alfalfa weevil.
Small Grains
Reports have come in of scattered near-threshold aphid counts in barley. Based on observations and trial work in 2019, I suspect malting barley varieties may be a little more sensitive to high aphid pressure than wheat. It is probably too late to make an aphid treatment aimed at reducing barley yellow dwarf virus economically, but in cool weather, aphid populations can increase faster than natural enemies control them. In our 2019, aphids in our trial exceeded 300 per row foot. The threshold at this stage is near 100 per row foot. The cereal aphids are all susceptible to pyrethroid application. Aphids will not impact malting barley quality.