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These 3 farm critters are active right now

Protect your beneficials and know the thresholds for applying insecticides.

May 20, 2024

4 Min Read
Green leaves sprouted through soil with small holes
BLACK CUTWORM DAMAGE: Damage from young caterpillars looks like a line of holes across the leaves. Older caterpillars cut plants off at the base and take the plant to an underground burrow to consume it. John Tooker

by John Tooker

Insect challenges are bubbling up around Pennsylvania and other areas.

Now is the time to scout your crops for pest activity to assess its potential impact. Here’s a look at three pests that are currently active in fields: black cutworm, alfalfa weevil and cereal leaf beetle.

Black cutworm

Black cutworm moths have been arriving in Pennsylvania for a few weeks, but the Black Cutworm Moth Trapping Network has detected only three "significant flights" that would indicate an increased risk of cutting damage from caterpillars.

These flights occurred in Blair County on April 15, and two flights occurred in Lebanon County on April 19 and 26. We are accumulating growing degree days for these sites to keep people apprised of the risk. The threat for cutting emerges around 300 degree days, which is likely to occur at these locations this week.

In other parts of Pennsylvania, at least based on trapping results, the risk of cutting does not seem elevated, but black cutworms can still cause damage. Producers are encouraged to scout their fields from emergence to V4.

Damage from young caterpillars looks like a line of holes across the leaves. Older caterpillars cut plants off at the base and take the plant to an underground burrow to consume it.

If you find two, three, five and seven caterpillar-cut plants per 100 seedlings at V2-, V3- and V4-stage plants, respectively, your field will likely benefit from an insecticide application. Keep in mind, however, that if cutting damage is only evident in a portion of the field, you can restrict your insecticide application to that area.

Also, preventative spraying is often useless against black cutworm caterpillars because the window of activity of the insecticide often misses the active period of caterpillars. Continue to scout until V5 when plants are rarely troubled by the pest.

Frank Peairs, Colorado State University, - Close up of alfalfa weevil a larvae on a leaf

Alfalfa weevil

Damage from alfalfa weevil is now evident in some alfalfa fields.

Alfalfa weevil has only one generation per year and is usually only a pest of first-cut alfalfa, unless you live in cooler climates to the west and north, where second cutting can be damaged.

To scout your fields, assess the number of weevil larvae on 30 plants from across the field. Weevil larvae are green with a white stripe down their back. If populations exceed economic threshold, which can be found in this fact sheet, consider cutting the crop or applying an insecticide.

If you cut because the damage was heavy, scout the stubble to assess the risk of damage to regrowth. If you see two or more larvae per crown, then spraying the stubble will likely be warranted. If you treat for alfalfa weevil, it is unlikely that the insecticide will have any benefit for controlling potato leafhopper, which usually arrives in early June.

Ines Carrara/Getty Images - Close up of a cereal leaf beetle

Cereal leaf beetle

This is the time of year cereal leaf beetle larvae are active in wheat, oats, barley and rye, particularly in eastern Pennsylvania.

Cereal leaf beetle larvae are easily identified because the larvae look slimy and shiny because of their practice of piling their frass, the word for insect feces, on their backs. Infestations are difficult to predict and can be patchy, with some fields heavily infested and others nearby having almost none.

Scouting is the key to identifying local populations and populations that are large enough to be managed. If you scout fields and find populations that exceed the economic threshold (one larvae per flag leaf), then using an insecticide is likely to be profitable. If you do not have populations, then an insecticide application is unlikely to be helpful and may do more harm than good by suppressing beneficial insect populations.

Similarly, I would strongly recommend avoiding the practice of adding an insecticide to a tank-mix simply because a sprayer is making a pass across the field, unless economically damaging populations of insects are present.

A Penn State fact sheet provides more details on their life cycle and some images of adults, larvae and their damage.

Tooker is a professor of entomology at Penn State.

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