Scout for Cereal Leaf Beetle Larvae, But Don't Spray Blindly
Figure 1. Several cereal leaf beetle larvae feeding on the flag leaf of wheat (Photo by Mike Fournier, Penn State Extension).
We have heard reports from south central and southeastern regions of Pennsylvania that cereal leaf beetle larvae are active in wheat and oat fields (barley and rye can be infested occasionally). This spring, however, we have little evidence of widespread infestations. Cereal leaf beetle 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
- is likely to waste money on a product that you do not need.
Similarly, I would strongly recommend avoiding the practice of adding an insecticide to a tank of fungicide, for example, simply because a sprayer is making a pass across the field; unless economically damaging populations of insects are present, there is no point.
Older cereal leaf beetle larvae can be mistaken for slugs because they can look dark and shiny; they cover themselves with their own frass, which is the proper word for insect feces. If you look closely at these shiny animals and see six small legs, you will know you are dealing with cereal leaf beetle larvae and not slugs (Figure 1). Moreover, slugs are nocturnal, so slimy creatures active during the day in small grains have a good chance of being cereal leaf beetle larvae. If young larvae, which are orange, are detected at populations exceeding the economic threshold, cereal leaf beetle is easily controlled with insecticides, but larger larvae are more difficult to control and cause more damage, particularly if they are feeding on the flag leaf. Insecticide treatments are warranted if populations exceed the economic threshold of one larva over an eighth-of-an-inch long per stem over a field or a portion of a field. 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.
Image details: 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)
