Insect Pests to Keep in Mind: Armyworm, Black Cutworm, Alfalfa Weevil, and Pea Aphids

Posted: May 4, 2017

Insect populations seem more active than usual this year, blame it on the mild winter if you like, or blame me, I can take it!
Figure 1: Large number of pea aphids crowding onto an alfalfa stem in Lancaster County (Photo by Jeff Graybill, Penn State Extension, Lancaster County)

Figure 1: Large number of pea aphids crowding onto an alfalfa stem in Lancaster County (Photo by Jeff Graybill, Penn State Extension, Lancaster County)

I have heard reports from Midwestern states of abundant populations of armyworm moths coming north, indicating that small grain, hay fields and emerging corn fields should be scouted in the coming weeks for armyworms. Unfortunately, I do not have more specific information on the occurrence of this pest species in Pennsylvania, so general vigilance will be needed to find emerging populations of caterpillars. I do, however, have more specific information below on black cutworm, alfalfa weevil, and pea aphids.

As mentioned last month, again this year Penn State Extension and the Department of Entomology are monitoring black cutworm populations with pheromone traps. As moths arrive, if we capture eight or more moths over the course of two nights (a “significant flight”), there is an elevated risk in that particular area of cutting damage by caterpillars later in the spring. States to our west, like Indiana and Illinois, have seen large flights of black cutworm moths, so the risk appears to be elevated this year, and now they are arriving in Pennsylvania.

Over the past two weeks, we have detected four significant flights of moths. The first arrived in Lancaster County (near Manheim), the second in Lycoming County (near Montoursville), the third occurred in Lebanon County (near North Cornwall), and the fourth was in Franklin County near Fayetteville. We continue to trap for others, but will now begin degree-day accumulations to predict the timing of cutting for those areas to inform folks when they should be scouting fields for damage.  Rescue treatments are usually the most efficient and economical tactic for managing black cutworm. For more information, see our factsheet.

As we have mentioned previously, alfalfa weevils remain active. It is time (or even past time) to scout fields for feeding, looking for chewing damage on the younger leaves. Alfalfa weevil does not grow to damaging populations in every field, and in many fields small parasitoid wasps control weevil populations effectively. So get out there and scout to find where populations are and whether populations may need to be controlled later in spring. Our revised factsheet accounts for recent higher prices for hay.

Also in alfalfa, we have encountered large populations of what appear to be pea aphids. This aphid species can be present in small numbers in most alfalfa fields, but can get out of control when indiscriminate insecticide use knocks out natural enemies and allows the aphid populations to grow unchecked. In the fields in Lancaster County visited by Jeff Graybill, some stems have scores of aphids per stem (Figure 1), resulting in some pretty marginally looking alfalfa (Figure 2). The solution to this large populations is spraying the field with a labeled insecticide (preferably a different class than has been used routinely to avoid resistance issues) and then scouting the field regularly to watch for resurgent populations. Hopefully natural enemies will recolonize these fields and restore natural control; in the future, IPM is the best approach, so scout the field and apply the economic threshold (≥ 100 aphids per sweep) if you find aphids.

Alfalfa field with pea aphid damage

Figure 2. Alfalfa field in Lancaster County that is heavily infested by pea aphids (Photo by Jeff Graybill, Penn State Extension, Lancaster County)

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