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Issue No. 7, Article 1/May 11, 2007

Maintain Vigilance for Early-Season Insect Pests of Field Crops

We already have published several articles about alfalfa weevils, armyworms, and black cutworms, but an update and another reminder to keep a close watch on these pests are in order.

Alfalfa weevils. As I indicated in last week's issue of the Bulletin (no. 6, May 4, 2007), alfalfa weevils have been actively feeding in many alfalfa fields in the southern half of Illinois, and their feeding in the crowns may delay regrowth significantly. Mike Roegge, University of Illinois Extension educator in Quincy, observed lots of alfalfa weevil larvae feeding in a field in Adams County, and the field had not been showing signs of regrowing since the freezing temperatures earlier this spring. He indicated that the field needed to be treated and that other fields in western Illinois already have been treated with insecticides.

Economic thresholds (which should be used with a grain of salt in freeze-damaged alfalfa) and insecticides suggested for alfalfa weevil control were published in issue 2 of the Bulletin (April 6, 2007). Also, remember that you can track alfalfa weevil development in your area by using the degree-day calculator. Based on degree-day accumulations, alfalfa weevil activity should be noticeable throughout the state.

Armyworms. Ron Hines, seed agronomist for Growmark's southern region, has continued to report captures of moths in his "trap line" in southern Illinois, from Pope County to Fayette County, and the numbers increased noticeably during the last two weeks of April. This large flight is a little later than the flights that preceded the outbreak in 2001, but recall that there were some early captures of moths in Illinois in 2007, too. Ron also reported that another source indicated finding some very small armyworm larvae in wheat in Washington County.

Wheat fields, corn fields that had or have grassy weeds, and grass pastures should be watched carefully for armyworms during the next couple of weeks. Many people were surprised by the intensity of the infestations of armyworms in 2001 and consequently responded too late to prevent significant losses. Being aware of the situation in your area, and especially in specific fields, will go a long way toward preparation--just in case.

Black cutworms. Cutworm damage (cutting, not leaf feeding) has been observed in a few early-planted fields in western Illinois. The black cutworm larvae responsible probably are from eggs that were laid by moths that flew into Illinois in March. In at least one situation, the damage was occurring to corn that had been treated with Poncho 250.

This very early report of black cutworm damage could precede more reports of damage because of the more recent intense captures (mid- to late April). We could be in for a long spring, with black cutworm injury occurring throughout the month of May.

In last week's issue of the Bulletin (no. 6, May 4, 2007), I discussed the importance of distinguishing between dingy and black cutworms. However, I failed to mention the claybacked cutworm, an old foe that has received very little press over the past few years. This species, unlike the black cutworm, overwinters as a partially grown larva in Illinois, so cutting damage caused by claybacked cutworms may appear very early in the season. Following is a paragraph written by Mike Gray in response to some frequently asked questions. The information should enable you to distinguish between black and claybacked cutworms:

"The claybacked cutworm is often confused with its close relative, the black cutworm. However, the skin granules of claybacked cutworm larvae are very small, slightly convex, and set contiguously like blocks in pavement. The skin granules of black cutworms vary in size and are more isolated. These differences in the cuticle (skin) create a smoother appearance for claybacked cutworms. Additionally, the dorsal (upper) surface of claybacked cutworms is usually paler (gray to pale orange) than the lateral portions of the body. Claybacked cutworms overwinter as half-grown larvae in the soil. In essence, they get a jump on black cutworms when it comes to cutting each spring. Large infestations of claybacked cutworms can cause economic losses in some cornfields each year. They are most often observed in fields that were planted to clover or alfalfa the preceding year. There are no established thresholds for claybacked cutworms, but the thresholds used for black cutworms are probably reliable. Keep in mind that claybacked cutworms, because of their larger size earlier in the spring, often cause damage to very young corn plants, so a quick diagnosis is important."

Yet another species to keep your eyes on. Insecticides suggested for control of black cutworms are effective against claybacked cutworms, too.--Kevin Steffey

Kevin Steffey

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