Insect Pests are Active in Corn, Alfalfa, and Small Grains

As Spring progresses, a variety of pests are active in many crop species; Here I consider black cutworm, alfalfa weevil, and cereal leaf beetle.

As the growing season gains momentum, insect challenges are bubbling up around the state, and I encourage growers to scout their crops for pest activity to assess its potential impact on your crops. I will touch upon black cutworms in corn, alfalfa weevil in alfalfa, and cereal leaf beetles in small grains.

First, as mentioned in previous weeks, in many parts of the state black cutworm  caterpillars are likely to have developed to the point where they will be cutting corn plants (Figure 1). Now is the time to scout your fields now looking for cut plants. Black cutworm caterpillars are nocturnal so you may not see them, but a caterpillar will cut a plant and then consume it, often in an underground burrow; a caterpillar can cut as many as five plants during their larval life. To confirm the culprit, dig in the soil near cut off plants and try to find the caterpillar, but they can be elusive. Sometimes cut corn plants can be caused by birds, but in this case beak marks are often evident on the remaining stems; do your best to identify the cause of cut plants. If you find 2, 3, 5, and 7 caterpillar-cut plants per 100 seedling-, 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. Continue to scout until plants reach V5, when they are rarely troubled by black cutworm.

Second, we have received reports that damage from alfalfa weevil  is evident in some fields, and in some cases the damage is severe. Alfalfa weevil has only one generation per year and is usually only a pest of the first cutting of alfalfa. 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 (Figure 2). If populations exceed economic threshold, which can be found on our fact sheet  , consider cutting the crop or applying an insecticide. If you cut because the damage was heavy, you should scout the stubble to assess the risk of damage to the 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.

Figure 2. Alfalfa weevil larvae with the distinct white line down their backs, and a view of their damage. (Photo by Adriana Murillo-Williams, Penn State Extension)

Lastly, we have heard of reports of cereal leaf beetle  larvae being active in some wheat fields (oats, barley, and rye can also be infested), particularly in eastern Pennsylvania. Cereal leaf beetle larvae are easily identified because the

larvae look slimy and shiny due to their practice of piling their frass, the word for insect feces, on their backs (Figure 3). Infestations are difficult to predict and can be very patchy with some fields heavily infested, while others nearby have 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, may do more harm than good by suppressing beneficial insect populations, and you are likely wasting money on a product that you do not need. 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, there is no point. A Penn State fact sheet provides more details on their lifecycle  and some images of adults, larvae, and their damage.  For insecticide options, please consult Penn State’s Agronomy Guide  .

Figure 3. Several cereal leaf beetle larvae feeding on the flag leaf of wheat. These are not slugs, (which are nocturnal), but beetle larvae covered with mucus and their own feces as a defense against other insects that might want to eat them! (Photo by Mike Fournier, Penn State Extension)