David Owens, Extension Entomologist, owensd@udel.edu
Alfalfa
Scout alfalfa fields now for alfalfa weevil. Larvae continue hatching, and we have seen a field or two go above threshold with first and some second instars present. As a reminder, alfalfa weevil thresholds are based on plant height, value of hay, and control costs. Our insect control guide is also based on pure stand alfalfa: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/commercial-field-crop-pest-management/alfalfa/. Thresholds are not applicable in mixed stands with 50% or less alfalfa. There is a similar weevil species that may be found in alfalfa, the clover leaf weevil. It is much less important and typically hatches in the fall and overwinters. It resumes feeding in the spring in the lower canopy. The larvae are very similar but have a BROWN head instead of alfalfa weevil’s BLACK head capsule. Alfalfa weevil larvae are going to concentrate their activity in the terminals.
Corn/Soybean
Gray garden slugs are now active. Reminder: insecticide tank mixes with burndown herbicide applications ahead of planting will not affect slugs, but will kill ground beetles and daddy long legs which feed on slugs. Our early season worm populations are generally pretty low, and if a cover crop is being terminated 2+ weeks in advance of planting, any pests that may be in them will starve. An insecticide tank mix is not recommended.
Small Grains
So far, the only active insect pests in small grains are aphids. Populations are very low in the fields we have been sampling and are well under the 100+/ft thresholds. The natural enemy complex is also active; we have observed lady beetles and parasitoids in fields. At this time, I do not anticipate an aphid population explosion like in previous years.
Seedcorn Maggot
Seedcorn maggot adults are actively laying eggs in soil right now. Any recent organic matter incorporation, such as manure or tilled in cover crop is at high risk for maggot injury. Seed treatments containing a neonicotinoid (thiamethoxam, imidacloprid, or clothianidin) or Lorsban (chlorpyrifos) are highly recommended. There are no rescue options for SCM. In about 2-3 weeks, maggots will be pupating, and the risk lessens somewhat.
Spider Mite Fact Sheet
We have released a very detailed, in depth fact sheet on two spotted spider mite. It should be applicable to most crops grown in our area, both field and vegetable.