David Owens, Extension Entomologist, firstname.lastname@example.org
Alfalfa weevil eggs are just beginning to hatch, and by mid to late next week we will be at peak egg hatching thresholds. If you have not already, begin weekly sampling for alfalfa weevil larvae. Examine 10-20 stems on your first visit for the presence of weevil larvae or feeding injury. If you see injury, collect 30 stems from 5-6 locations throughout the field, place them top down in a bucket and beat the stems vigorously to dislodge weevil larvae. Thresholds depend on control cost, plant height, and value of the hay; depending on these factors, they range between 20 and 120 larvae per 30 stems. You can find threshold tables at our University of Delaware alfalfa insect control fact sheet. Labeled products include Baythroid, Warrior, Perm-Up, Mustang Maxx, Steward, Imidan, and Lorsban. Carbaryl 4L is labeled but does not control adults. Dimethoate has performed well in VA trials. Be sure to use high pressure and good water volume to ensure good canopy penetration.
Aphid numbers have been quite low in the fields we are monitoring. Natural enemies (parasitic wasps, lady beetles) are also active. Aphid thresholds are very high in the spring, between 100–300 per row ft, but if natural enemies are present, thresholds are even higher. Wet weather also promotes fungal pathogens of aphids.
We are seeing gray garden slug eggs hatching now. Recent published research from Pennsylvania suggests a corn threshold of 1 per square foot with defoliation injury present and slug-favorable weather conditions.