Oulema melanopus (L.)
Cereal leaf beetle is an occasionally severe pest of wheat and oats in Pennsylvania, but it also feeds on barley, rye, and other grasses. This insect species is native to Europe and Asia and was first detected in the United States in 1962, but quickly spread to most of the wheat-growing areas of the eastern United States. This pest has been known to catch small-grain growers off guard because it tends not to be a perennial problem; however, it can be extremely damaging if populations build and infestations are not detected early. It is more of a problem in thin or poorly established stands, so it can often be controlled effectively just by using good crop management practices. In many parts of its range, it is controlled by introduced species of parasitic wasps, but these wasps may not be well established in Pennsylvania and some states to our south.
Adult cereal leaf beetles (Fig. 1) are about 5-mm long (3/16 inch) and have metallic bluish-black heads and elytra (i.e., wing covers). Their thorax and legs are orange to reddish brown. Eggs are about 1-mm (< 1/16 inch) long and yellow and typically laid singly or in pairs on the upper surface of grass leaves. Eggs darken and turn black as hatching approaches. Larvae are yellowish-orange, but this color is usually obscured by a layer of feces and mucus, giving them a shiny appearance (Fig. 2). This layer protects them from natural enemies and drying out and has been known to ruin a favorite pair of trousers as fields with heavy infestations are walked.
Figure 1. Adult cereal leaf beetle.
Figure 2. Cereal leaf beetle larvae feeding on wheat in Bucks County, PA. Photo by Mike Fournier, Penn State
Cereal leaf beetle has one generation per year. Adults pass the winter in leaf litter in wood lots, hedgerows, and similar areas. In March and mid-April as temperatures increase, adults fly to grain fields to lay eggs, which hatch and young larvae begin to feed. Young larvae do not inflict much damage, but older larvae have voracious appetites and can cause extensive damage (Fig. 3). The larval stage passes in about two weeks depending on spring temperatures. Mature larvae burrow into the ground to pupate. Adults emerge about two weeks later, and will feed in small-grain and corn fields for a short time before remaining inactive for most of the summer. In fall, adults fly to wooded areas seeking shelter for winter.
Older larvae do the great majority of damage, stripping off green tissue between leaf veins and skeletonizing leaves. Large populations can cause fields to turn white; indeed, this "frosted" appearance is often what tips growers off to the presence of damaging populations in their fields (see Fig. 2 & 3). Adult feeding in small grains and corn has not been shown to be economically significant and targeting adults for control has not been shown to be effective.
Figure 3. A "frosted" wheat field resulting from heavy infestation of cereal leaf beetle. Photo by Mike Fournier, Penn State
This pest is most easily managed earlier in spring when eggs or young larvae are present; therefore, in-depth, localized scouting is necessary to detect these life stages. Keep in mind that populations can be quite spotty so checking individual fields is often necessary. Once populations reach one or more larvae per stem, treatment is probably warranted, especially if larvae are feeding on the flag leaf prior to head emergence. Damage later in head filling does not appear to be that significant and if adults are seen late in spring, it is likely too late to manage this pest. For insecticide options, please consult Penn State's Agronomy Guide.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: John Tooker, Assistant Professor of Entomology