David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa
Alfalfa weevil larvae are active in southern Delaware and Maryland. If you haven’t already done so, NOW is the time to be scouting for weevil larvae. Examine 10 stems for damage and larvae. If you find either, then collect 30 – 50 stems randomly throughout the field and place them upside down in a bucket. After collecting stems, separate them into small bundles and beat them against the inside of the bucket to knock off weevil larvae. Thresholds and early harvest options depend on the stage and height of the alfalfa as well as the cost of control. For alfalfa in the full-bud stage, early harvest is an option, but an insecticide may still be warranted if it is not possible to harvest within 3 days.
We have made significant adjustments to the alfalfa insect control recommendations, which will be posted soon at https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/vegetable-fruit-field/alfalfa/. One addition is the insertion of a table of thresholds based upon alfalfa value and control cost, taken from a fact sheet written by Dr. John Tooker, Penn State: https://extension.psu.edu/alfalfa-weevil.
Dr. Sally Taylor at Virginia Tech has done spray trials for alfalfa weevil and noted less than satisfactory results from pyrethroids. We do not know how extensive reduced pyrethroid efficacy is in the area. Furthermore, this may be the last year that MD alfalfa can be treated with chlorpyrifos. If nothing else, it might be worth thinking about using chlorpyrifos from a resistance management standpoint while it can still be used. Please note that pre-harvest intervals are dependent on use rates and that chlorpyrifos can cause a temporary yellowing of foliage. Other efficacious products include Imidan (same mode of action as chlorpyrifos), Steward, and Carbaryl. There is at least one pre-mix product containing both chlorpyrifos and zeta-cypermethrin, Stallion.
Small Grain
Cereal leaf beetle adults have been observed in fields recently in low numbers. It has been a few years since this pest caused significant defoliation. We are evaluating a degree day model developed by Virginia Tech to predict peak egg lay. We may be close to the target DD in 7 – 10 days. Last year we didn’t start seeing eggs until we hit the target degree day, with peak eggs a fair amount later. However, every year is different.
Aphid populations seem to have decreased throughout much of the area, and a variety of natural enemies are present. However, I received a report of severe greenbug damage in a grass forage crop at Beltsville, MD this past week. Greenbug looks much like English grain aphid, except that the cornicles (aka tailpipes) are green with just a little bit of black at the tip and often with a greenish-blue stripe down the back (English grain aphids can also have this but it tends to be less pronounced).