Damage & Host Plants
The cereal leaf beetle (CLB) has caused crop damage in Malheur and Umatilla counties. Most of the sites are in irrigated areas. However, in 2006 the pest is becoming more visible in dryland regions. Both adults and larvae of the cereal leaf beetle damage grain crops with the larvae stage being the most damaging and the target of control measures. Generally, the newer plant tissue is preferred with feeding occurring on the upper leaf surface causing characteristic elongated slits.
The cereal leaf beetle has a wide host range including cultivated grass hosts: barley, oats, wheat speltz, rye. Adults may feed on corn, sorghum and sudangrass. Beetles may use resident or grass weeds including: wild oats, quackgrass, timothy, canary grass, reed canary grass, annual and perennial ryegrass, foxtail, orchard grass, wild rye, smooth brome and fescues.
Life Cycle and Description
Adults are the overwintering stage, moving into grain fields and feeding for about 10 days on small grain and grass foliage after they become active in the spring. Adults prefer spring grains over winter grains. The adults are about 1/4 inch in length with brightly colored orange-red thorax, yellow legs and metallic blue head and wing covers. It is important to correctly identify the adults since other beetles, common in cropland, resemble the CLB. Eggs are laid end to end singly or in groups of 2 or 3 on the upper leaf surface near the base of the leaf. Newly laid eggs are bright yellow, darkening to orange-brown and finally to black before they hatch. Egg hatch may take from 4 to 23 days depending on temperatures.
The larvae has a yellow body with brown head and legs. The body is protected by a layer of slimy, fecal material which makes them look like a slug. When working or walking in an infested field the slimy covering will rub off on your clothing. Although both adults and larvae cause feeding damage, the larvae is responsible for majority of the damage. They feed on the leaf surface between veins, removing all the green material down to the lower cuticle resulting in an elongated windowpane in the leaf. Severe feeding damage can give a frosted appearance to the field. The larvae has 3 pairs of legs located close to the head end of the immature insect. When larvae have completed their feeding they shed their slimy covering and drop to the ground, hollowing out an earthen cell for pupation. The pupal stage takes from 10 to 14 days to complete. When new adults emerge from pupation they feed briefly on grasses, before leaving the field and finding a suitable protected overwintering site. Adults are strong, active flyers and can move some distance.
The first sign of damage in the spring is due to adult feeding on the plant foliage. While this is the first sign of adult activity, adults are not the target of control. Eggs and larvae are monitored by plant inspection since thresholds are expressed as egg and larvae numbers per plant or per stem. Examine 10 plants per location and select 1 location for every 10 acres of field. Count number of eggs and larvae per plant (small plants) or per stem (larger plants) and get an average number of eggs and larvae, based on the samples you have taken.
Plant growth stage should be noted because the treatment threshold changes with plant growth stage (3 eggs and larvae or more per plant in smaller plants; 1 larvae per flag leaf in larger plants). Both eggs and larvae can be found by examining the upper leaf surface.
Boot stage is a critical point in plant development and impact of cereal leaf beetle feeding damage can be felt on both yield and grain quality. Before boot stage, the threshold is: 3 eggs and larvae or more per plant (including all the tillers present before emergence of the flag leaf). Larvae feeding in early growth stages can have a general impact on plant vigor. When the flag leaf emerges, feeding is generally restricted to the flag leaf which can significantly impact grain yield and quality. The threshold is decreased at the boot stage to: 1 larvae or more per flag leaf.
Research has shown that it is not economic to spray early in the season but insecticides are more effective when CLB is in the larval stage. The larvae appear as dark, slimy, stubby “caterpillars” that cover themselves in their own fecal material and feed up and down the upper leaves. They leave whitish streaks between the leaf veins as they remove the chlorophyll layer but they seldom eat all the way through the leaf, unlike the adults that may chew longitudinal holes in the leaf.
The most effective time to spray is when most of the larvae are about 1/8 inch long. If you spray too early there may still be a lot of unhatched eggs in the field and it may be necessary to reapply insecticide. The eggs appear as individual, tiny (1 mm) orange/brown specks on the upper surface of the leaf at the midrib about 1 inch from the plant stem.
It is tempting to use insecticide on new infestations in hopes of eradicating the population, but this is not likely to happen. Attempts to eliminate the pest were unsuccessful when it was first found in Michigan in the 1960's. Some reports indicate that premature use of insecticides may actually cause an increase in CLB the following season as the chemicals also kill predatory species such as ladybird beetles.