06-20-12 *CSU Ext Bulletin* Does 2012 have more pest problems in Colorado?

We are seeing more and early infestations of some pests including banks grass mites, European corn borer, grasshoppers, thrips and western corn rootworms.

European corn borer: European corn borer moths continue to fly in some pheromone trap locations in northeastern Colorado. To determine infestation levels, check 50 plants in 4-5 locations in the field.  Be sure to move into the field at least 100 feet from the border to sample.  To determine the number of live larvae, pull the whorl from each symptomatic plant, and carefully unwrap the leaves.

Chemical control of first generation is justified when 25% of the plants have feeding damage and live larvae. Control measures must be taken before the larvae bore into the stalk. A more complex formula which incorporates the number of borers per plant, control cost, expected percent control, and the value of the crop is available in the High Plains Pest Management Guide or can be obtained from the cooperative extension service.

Timely and accurate scouting is the key to managing European corn borer in standard (non-Bt) corn hybrids. Remember that conditions are localized and each field should be scouted to make accurate decisions of its management.

Grasshoppers: We are seeing hatching of grasshoppers in the Front Range areas as well as northeastern Colorado.

Landowners in high risk areas should start monitoring grasshopper populations in rangeland soon after grasshoppers hatch, primarily until end of June. Early scouting is important because treatments are most effective when grasshoppers are small. The goal of scouting is to get an estimate of grasshoppers per square yard, as well as their stage of development.

The simple economic threshold for grasshoppers in rangeland is 15-20 grasshopper nymphs per square yard. This number is equivalent eight to ten adult grasshoppers per square yard.

Treatment options for grasshopper management are based on the Reduced Agent and Area Treatment (RAAT) strategy, which results in untreated swaths and swaths treated with reduced chemical rates. Using lower rates and leaving untreated areas reduces treatment costs by as much as 50% and preserves biological control. Grasshoppers move constantly, insuring that they will enter a treated swath and that levels of control will be similar to complete coverage applications.

Western corn rootworm: We have reports that western corn rootworm larvae have hatched in many fields of northeastern Colorado. The larvae feed on the underground root systems of corn plants. Early scouting of corn fields for this insect is advised in your area. Peak feeding usually occurs from late June to mid July. Lodging (goose necking) of corn plants due to larval root feeding is a typical symptom of damage. Early planted fields will have relatively larger root systems when rootworm feeding starts and this makes them somewhat more tolerant to rootworm damage.

Damage from corn rootworm larvae is most likely in continuous corn. Chemical applications to first year corn are not recommended.

Thrips: Onion thrips are currently reported from various crops including alfalfa, onions, dry beans and corn in Colorado. Large populations are often associated with hot, dry conditions.

Onion thrips and western flower thrips are known to damage alfalfa regionally.  Reliable scouting methods have not been developed for thrips in alfalfa. A combination of sweep net sampling and visual inspections for leaf damage is a good way to determine the presence of thrips in an alfalfa crop.

Research based economic thresholds are not available. If a thrips treatment is contemplated, it is best to cut as soon as possible and treat the regrowth if the infestation persists. Thrips are very difficult to control in alfalfa, excellent coverage is important and two applications may be required for satisfactory results.

In dry beans, onion thrips feed by puncturing the bean leaf and sucking up the plant juices that ‘bleed’ from the plant. A yellow spotting of the leaves will occur at the feeding sites. Extensive thrips feeding results in leaf cupping and distortion that is made more severe by plant stress (low moisture and high temperature). Non-stressed crops often outgrow the damage with little loss in yield. Western flower thrips feed in developing flowers and can cause flower and pod abortion. Pod abortion is worsened by increased plant stress. Both of these thrips can transmit tobacco streak virus (Red Node).

In onions, onion thrips populations can be best determined by counting all the thrips on the plants. The great majority of the thrips are present at the base of the youngest pair of leaves. Thrips nymphs are pale yellow and the winged adults vary from light brown to dark brown.

Knowledge of the relative susceptibility of a variety can be used to modify action thresholds. Since higher numbers of thrips are needed on tolerant varieties to benefit from insecticide treatment the action thresholds will be raised. As an interim recommendation an action threshold of 30 or more thrips/plant or higher is suggested for more thrips tolerant varieties. Action thresholds for varieties that are susceptible would be lower, perhaps in the range of 15-30 thrips.

Banks Grass mites: Early infestations of banks grass mite in corn have been observed in south and northeastern Colorado. Banks grass mites (BGM) are minute, greenish colored arthropods with eight legs and a rounded body. Webbing on leaves and discoloration are often the first signs of an infestation. Initially, BGM are most abundant on the lower third of the plant and density declines as the infestation moves up in the plant. Mites damage corn and small grains by piercing plant cells with their mouthparts and sucking the plant juices.

Banks grass mite builds up on the plant from the bottom up. Treat when there is visible damage in the lower third of the plant and small colonies are present in the middle third of the plant before hard dough stage.

For more pest management information including effective products on each pest problems check the High Plains IPM Guide (http://highplainsipm.org) and www.NoCopestalert.org or contact your county Extension office.

Colorado State University and U.S Department of Agriculture Extension Programs are available to all without discrimination. 

Submitted by: Assefa Gebre-Amlak, Regional Extension Specialist, Colorado State University Extension