SMALL GRAINS INSECT PEST MANAGEMENT

MANAGING THE CEREAL LEAF BEETLE IN SMALL GRAINS AND CORN

John Van Duyn, Steve Bambara, NCSU Extension Entomologists
Robert Ihrig, NCSU Graduate Research Assistant
Ames Herbert, Virginia Tech Extension Entomologist
Glenn Chappell, VCE IPM Area Agent
David Moore, VCE Agent, Middlesex, Co.

The cereal leaf beetle has become a significant pest of North Carolina and Virginia small grain in recent years. Cereal leaf beetle is native to Europe and was first detected in Michigan in 1962. Since that time it has spread throughout most of the eastern United States, including most of North Carolina and Virginia. The insect can become very numerous in small grain fields and the larvae are capable of reducing grain yield by eating the green leaf tissue.

Cereal Leaf Beetle Description and Life Cycle

Description -- The adult cereal leaf beetle is about 3/16 inch long and has metallic, bluish-black head and wing covers. The legs and front segment of the thorax are rust-red. Eggs are elliptical, about 1/32 of an inch long, and colored yellow to burnt orangish yellow. Most often the eggs are laid singly or end-to-end in short chains on the upper leaf surface between, and aligned with, the leaf mid veins. Larvae are very small when newly hatched and grow to a maximum size that is slightly longer than the adult. Larvae are slug-like in shape and resemble small Colorado potato beetle grubs, except for their coloration. The head and legs are brownish-black and the body is orangish yellow. However, body coloration is usually obscured by a black globule of mucus and fecal matter held on the body, giving the larva a shiny black, wet appearance. This liquid substance wipes off easily and after walking in an infested field, a person's shoes and trouser legs will be soiled.

Life Cycle -- Adult cereal leaf beetles overwinter in fallen leaves, ground litter, or other debris along hedge-rows, within wooded areas, or other protected sites in the vicinity of last seasons grain fields. In the spring adults colonize and lay eggs in small grain fields during March and April, although it can be earlier or later depending on spring temperatures. Most egg laying occurs during late March and through mid-April with adults preferring late-planted and thinly sown fields. Eggs hatch in about five days. Larvae develop in about 10-12 days under ideal temperatures, however, development times vary considerably during the spring. Peak larval populations usually occur in mid-April to early-May. Small larvae eat a very small amount but when full grown, larvae have voracious appetites. Upon reaching full size, the larvae dig into the ground and pupate. After a short period in the soil a new summer generation of adult beetles emerges in late-May and June. New beetles move from small grain fields and feed on grass plants for a short period but then remain inactive through most of the summer. Because these adults need to feed before becoming inactive, they often congregate and feed in corn fields adjacent to the small grains from which they emerged. Adult feeding on corn appears like many line-etchings on the blades and can cause concern to the farmer. However, damage is usually cosmetic rather than yield reducing (see section on corn at the end of this publication). Cereal leaf beetle does not lay eggs in corn. In the fall, beetles move to wooded areas, hedge-rows, and ditch-banks to overwinter. There is only one generation per year.

Damage to Small Grains

Damage to wheat, oats, and barley is caused by the larvae feeding on leaves during April and May. Although adults will feed on young small grains plants, their feeding does not affect the plant's performance. However, cereal leaf beetle larvae can reach very high numbers in small grains and larger larvae can defoliate wheat plants. The larvae eat long strips of green tissue from between leaf veins and may skeletonize entire leaves, leaving only the transparent lower leaf surface tissue. Severely defoliated fields can take on a white "frosted" cast when lots of green tissue is lost on the upper leaves. Often cereal leaf beetle populations will be somewhat localized and damage will be confined to spots or sections of fields or farms. Damage can build-up quickly, often in as little as five days, after larvae become large and warm temperatures make the insects very active. Leaf feeding reduces the plant's ability to make its food and limits reproductive growth, particularly if the upper leaves are destroyed. Yield reduction to 45% has been observed when defoliation was near 100% and the damage occurred early in the heading period. Later damage, late in the head-fill period does not have a great impact. Yield reductions of 10% to 20% are typical in infested commercial fields. Recent research has indicated that the three stem leaves (flag leaf and two leaves below) all contribute to filling grain heads and achieving favorable grain test weight. This research suggests that the two stem leaves are about as important as the flaf leaf. Also, damaged leaves will not be replaced by the plant.

Management of Cereal Leaf Beetle In Small Grains

The North Carolina Department of Agriculture (NCDA) has released several species of exotic parasites throughout the state. Up until a few years ago, the Virginia Department of Agriculture and Consumer Services (VDACS) also reared and released parasites. These parasites develop within cereal leaf beetle eggs and larvae and have the potential to keep populations below an economic level. Parasite release programs have worked well in several other states but have had limited success in our states up to the current time. Efforts to introduce new and better adapted parasites for the Virginia-North Carolina area are continuing by USDA and NCDA.

Native biological control agents, especially lady beetles, appear to consume cereal leaf beetle eggs, and perhaps, young larvae in early spring. Whereas farmers can do little to stimulate the development of these biological control agents, unnecessary or excessively early insecticide application may cause cereal leaf beetle problems by removing these valuable predators. Therefore, insecticides should only be applied on the basis of a demonstrated need as determined by proper scouting and threshold use. Avoid using insecticide with top dressed nitrogen application as this has been demonstrated to be too early in relation to cereal leaf beetle movement into grain fields. Also, these early treatments can reduce predators and therefore enhance beetle populations later in the season.

Several effective insecticides are registered for use on small grains (see sections on scouting and insecticides). In addition, where cereal leaf beetles are present and the Hessian fly has not been a problem, avoiding late planting may be beneficial. Late-planted fields are more attractive to cereal leaf beetles for egg laying in the spring. Additionally, thick planted/tillered wheat fields are less subject to develop high cereal leaf beetle populations. In general, following sound agronomic practices for high yield small grain production reduces the impact of cereal leaf beetle.

Scouting For Cereal Leaf Beetle In Small Grains

Scouting Method -- Scouting should be done after peak egg laying has occurred and up to 50% of eggs have hatched, usually in early- to late-April. Development will occur earlier in the Coastal Plain versus the Piedmont and Valley Regions; earlier in south, later in north. On warm springs scouting should be done earlier versus on cooler springs. Scouting should be done when both eggs and mostly small larvae are in the field (counts should include both forms). If the population is mainly made-up of eggs, then scouting should be at a later date, when a minimum of 50% is in the larval stage.

Eggs are elliptical, about 1/32 of an inch long, and colored yellow to burnt orangish yellow. Most often the eggs are laid singly or in small scattered groups (often end-on-end) on the upper leaf surface between, and aligned with, the leaf mid veins.

Samples should be taken at a minimum of 10 random sites in the interior of each field (avoid the edges). At each site, 10 tillers (stems) should be examined for cereal leaf beetle eggs and larvae. This will result in 100 tillers (stems) per field being examined. Eggs may be on the leaves near the ground. Record the number of eggs and larvae counted at each sample site. After leaving the field, calculate the total number of eggs + larvae found. Alternately, stems can be examined at random while walking through the major portion of the field; again 100 stems per field should be examined.

Because cereal leaf beetle is often unevenly distributed in the field, it is often necessary to determine if a portion of a field is above threshold. If the random sampling indicates an uneven distribution (lots in some samples but few in others), it may be necessary to subdivide the field into two or more parts and sample each part as an individual field.

In instances of very high counts the sampling can be abbreviated after the samples have exceeded the threshold. For instance, after examining 30 tillers the scout has found 35 eggs+larvae which exceeds the threshold for 100 stems. However, if this is done the scout should realize that the portion of the field not scouted may not have high populations.

Threshold for Egg/Larval Counts -- 25 eggs and/or larvae total per 100 tillers (this is an average of one per each four tillers or 0.25 eggs and/or larvae per tiller).

Scouting Frequency -- Once egg laying has reached a peak, many fields will need only a single scouting for eggs and larvae. If the proportion of eggs in the sample is 50% or greater then sample again in 5-7 days.

Insecticidal Control Of Cereal Leaf Beetle In Wheat

The cereal leaf beetle is easily killed with low rates of several insecticides. When selecting an insecticide for use against cereal leaf beetle, consideration should be given to the presence of aphids and armyworm since certain insecticides are better choices for unique pest combinations. Choices are presented below in Table 1. Cereal leaf beetle has only one generation per year and if insecticide is applied correctly one application will give adequate control.

Precautions -- It is not advisable to add an insecticide to top dress nitrogen. If insecticide is applied very early it will likely fail to control cereal leaf beetle and can actually increase numbers by removing predators.

Table 1. Recommended Insecticides For Cereal Leaf Beetle (amounts/acre)* PRODUCT CEREAL LEAF BTL ARMYWORM APHIDS Malathion 57% 1.5 pt NO 1.5 pt Karate 1 EC 2.6 oz 2.6 oz 2.6 oz Sevin XLR PLUS 2 pt 2 - 3 pt NO Lannate 2.4 LV 1 pt 1 - 1.5 pt 1 pt Furadan 4F ** 0.5 pt NO NO * Follow all label directions. ** Apply Furadan before heads emerge from boot, this may before CLB requires treatment; use appropriate precaution to avoid health risk when spraying Furadan.

Cereal Leaf Beetle In Field Corn

Cereal leaf beetle is a pest of wheat, oats, and other small grains. In early spring larvae build-up in small grains and then change to adults in May. Adults emerge as the small grain crop is drying and migrate to other areas to find suitable food. Often, corn fields bordering small grain fields are attractive to the migrating beetles and these adults colonize the corn, especially along the edges nearest small grain. Cereal leaf beetles eat the leaf surface tissue on whorl stage corn plants. Narrow streaks are eaten between the leaf veins, usually on the surface but sometimes completely through the leaf. If beetle populations are very high, defoliation can reduce yield, but leaf feeding is usually cosmetic. Damage is often alarming to farmers even if it poses no economic threat. Beetles do not reproduce or remain for a very long time in corn fields and, therefore, damage is a single, short-term event that rapidly developing corn plants soon out-grow.

Insecticides are occasionally necessary to prevent serious defoliation to corn fields or portions of fields. To evaluate the need for treatment, two factors must be considered: (1) assessing the % defoliation and (2) determining if beetles are still present in the field at high numbers (is the defoliation going to get worse?). Corn fields should be scouted for defoliation (estimate total defoliation on at least 10 plants/field). Defoliation estimates should be compared to Table 2 to determine potential yield loss. A 5 bu/acre potential yield loss and a large beetle population will justify insecticide treatment in most cases. If treatment is necessary, consult a current edition of the NC Agricultural Chemicals Manual for suggested insecticides to use in corn.

Table 2. Estimated corn yield loss in bushels/acre due to defoliation by leaf feeding insects

http://ipm-www.ento.vt.edu:8000/nipmn/VA-IPM/cereal_leaf_beetle/clbindex.html

Last modified: June 3, 1997

Department of Entomology
Virginia Tech