Pest update in alfalfa, wheat crops

Colorado

Wheat

There is still no sign of leaf rust or stripe rust in Colorado. There are reports of leaf rust in eastern Kansas, but the levels are still very low.

This blast of cold weather might slow the wheat down a bit. Professor Ned Tisserat of Colorado State University says that the wheat is far enough along to be significantly damaged by freezing, but you may see some leaf scorching.

"Color banding' has been identified from wheat samples that were collected from Logan County. This is a physiological problem that is associated with temperature extremes (e.g. going from relatively warm days to light freezes). Affected leaves develop bright yellow bands giving the leaf a sort of zebra effect. Don't confuse this with a mosaic pattern caused by viruses. With recent weather conditions this syndrome may be relatively common.

However, now is the time to look for early symptoms of wheat streak mosaic, high plains, and related viruses.

Regarding insect pests, no Russian wheat aphid infestation has been detected in Colorado so far. However, some Russian wheat aphids being reported from southwest Kansas, the Oklahoma and Texas Panhandles this week. We should probably on the look out for this pest as to determine its population level to use control measures in all susceptible wheat.

We are seeing army cutworm in some wheat fields between Highways 59 and 61 in Washington and Yuma counties.

The army cutworm has one generation per year and spends the winter as a partially grown Caterpillar . It will feed on warmer days throughout the winter. In the spring it feeds more frequently and development proceeds more rapidly. As daytime temperatures rise, the army cutworm is found under soil clods and other debris during the day. In spite of spending so much time in the soil, this is a climbing cutworm that always feeds above ground.

After development is complete a small pupation chamber is built several inches below the soil surface. Moths emerge in May and June and migrate to higher elevations in the Rocky Mountains to escape high summertime temperatures. These moths are the "millers" that become a household nuisance following outbreaks. In late summer and early fall the moths return to the plains to lay their eggs in wheat fields and other cultivated areas. With sufficient moisture eggs hatch and larvae of the next generation start feeding as weather conditions permit.

Plant response and damage

Army cutworms have a very wide host range and will feed on most crops grown. They will feed on just about any green tissue presented to them, although they will show preferences when given a choice. For example, army cutworms have been observed to prefer broadleaf weeds in wheat fields to wheat. When they do damage wheat, they graze in the spring.

Because of the sporadic nature of army cutworm outbreaks, management options are limited to the use of insecticides. Consider treatment if counts are more than four to five per square foot. Pyrethroid insecticides have been the most effective treatments against this pest in university tests. For details of effective products and applications, check the High Plains IPM Guide in the web (http://highplainsipm.org).

Alfalfa

In alfalfa, currently two insects need to be monitored, army cutworm and alfalfa weevil.

Cutworms cause economic damage by cutting plants off at the soil surface (pale western and dingy cutworm species) or by foliar feeding (army cutworm, variegated, and darksided cutworm species). The damaging larval stage occurs in the spring. Young alfalfa seedlings have small reserves of food to regenerate top growth cut off by cutworms. Older established plants are less likely to be killed, but yields can be reduced or growth slowed under high densities of cutworm larvae.

Monitoring

Larvae may be sampled in the spring though this is time consuming and many samples must be taken to compensate for the random field distribution of these pests. Larvae hide in loose soil at the base of the plants or under soil clods during the day and can be detected by sieving soil or visual inspection.

Chemical control may be justifiable when their density reaches three to four per square foot in mature stands. For new stands, two larvae per square foot cause economic damage. Under heavy pressure or high residue conditions, control may be difficult.

Alfalfa Weevil: Larvae feeding in the folded leaves can heavily damage stem terminals, but initial damage is not always clearly visible. The closed, overlapping foliage of the stem terminals should be unfolded to detect feeding damage. Heavily infested stands have a grayish or frostlike appearance due to the dried defoliated leaves.

Damage also may reduce hay quality due to loss of leaf tissue, leaving only the lower quality stems. Damage to regrowth buds may occur when the plants break dormancy and after first cutting. Larval feeding on the regrowth after first cutting may be concentrated in strips coinciding with windrow locations, especially if the first cutting was taken early due to heavy weevil infestation and larvae survived under the windrows. Damage to regrowth may retard plant growth and result in yield reduction and encourage weed establishment.

Timing of sampling: Estimation of the weevil instars present in the field can be calculated using degree-days. Alfalfa weevil development increases at a nearly constant rate as the temperature rises above 48 degrees F (9 degrees C.). The amount of warm weather required for weevil larvae to complete development is measured in units of degree-days.

For the alfalfa weevil, degree-days are accumulated after 1 March for each 24-hour period in which temperatures exceed 48 degrees F (10 degrees C). Using this technique sampling should begin when 148 degree days have accumulated.

In most of locations, accumulated degree days have not reached 148 degree days. For example, degree days accumulated from March 1st to April 9 in Akron (82.95), eastern Adams (116.45), Burlington (52.90), Fort Collins (73.90), Greeley (60.95), Holyoke (74.45), Wray (80.00) and Yuma (92.80). You may check with your local weather stations for degree day accumulation and scouting should be initiated in about a week.

For management information check our IPM guide at http://highplainsipm.org.

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5/7/07

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Date: 4/20/07

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