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Watch for Alfalfa Weevil and Clover Leaf Weevil Activity

Based on growing degree days, alfalfa weevil larvae should be hatching from eggs statewide, and small pinholes may be visible in the new alfalfa growth near the tips of the plants. While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists. While things are very busy as row crops are to be planted, those of you who are growing high quality alfalfa hay should take the time to monitor fields for weevils over the next few weeks.

Clover leaf weevils (CLW) are occasionally a problem but are very vulnerable to fungus disease and so haven't been pests since the late 80's early 90's when spring rains were rare. The dry conditions over the past several years may have aided them in building their populations, although rain over the last week could have knocked populations down. Clover leaf weevil larvae will be in the debris around the crowns during day. Scratching in the soil around the crowns and counting the number of larvae found per crown will help give a better idea of clover leaf weevil infestation. Their brown heads will help distinguish them from the black-headed alfalfa weevil. Table 1 will compare the alfalfa weevil and the clover leaf weevil.

Both and alfalfa weevil and clover leaf weevils feed on first cutting alfalfa as larvae, and regrowth of the first cutting as adults. While research conducted in northeast Nebraska has shown that clover leaf weevil larva feeding does not cause yield reduction to first cutting alfalfa, alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting.

Table 1. Comparison of Alfalfa Weevil to Clover Leaf Weevil.

It is essential that fields be monitored for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are a small (1/16 to 3/8 inch in length), pale yellowish green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back. The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed.

Once the alfalfa is about 4-6 inches or so in height, take a bucket, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of the bucket. Average the number of weevil larvae per stem. Use the following charts to aid you in making a decision on whether to control alfalfa weevils. Each chart has been developed for a different alfalfa value. To treat or re-sample depends on the average number of weevils per stem, the stem length, and the value of the alfalfa. When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than to treat. Insecticides registered to control alfalfa weevil larvae include Ambush, Baythroid, Cythion, Furadan, Guthion, Imidan, Lannate, Lorsban, Mustang Max, Penncap M, Proaxis, Pounce, Sevin, and Warrior. Check the labels or see the Entomology Website at: http:/ /entomology.unl.edu/instabls/instabls.htm to find the use rates.

Scout Emerging Corn for Cutworms

Cutworms and other insects may hinder emerging corn plants this spring even if seed was treated with insecticides or Bt corn hybrids were used. High populations of insects may overwhelm the protection provided by controls, such as insecticides applied at planting time whether liquid, granular or seed treatment or whether it was a Bt corn hybrid. Also in some cases products are not labeled for the full spectrum of insects we may encounter in Nebraska . For example, Herculex I Bt corn hybrids list black cutworm on the label, but not other soil cutworm species.

Cutworms can cause serious damage to corn in the first couple weeks after emergence so it is important to scout fields for damage. Several species of cutworms attack corn. The severity and the area affected will vary greatly, depending on species involved, previous crop history and weather conditions. The black cutworm does not overwinter in Nebraska , and infestations depend on moth movement in southerly winds in the spring. Fields with winter annual weeds, or abundant crop residue are more attractive to the egg-laying moths in the spring. Other cutworm species (dingy, army, sandhills) overwinter as partly grown caterpillars. Remember that early detection of a problem is essential because most of the cutting occurs within seven days of plant emergence.

Generally, a postemergence "rescue” treatment should be considered if cutting is observed on 5 percent or more of plants and the worms are one inch or less in length.

Rescue treatments are effective in controlling soil cutworms. Ambush 2E, Asana XL, Baythroid, Lorsban 4E, Mustang Max, Warrior, Proaxis, Pounce 3.2EC, or other insecticides with similar active ingredients will give satisfactory control as post-emergence sprays. If soil is dry or crusted, rotary hoeing immediately before or after Lorsban application may enhance control. The other insecticides are pyrethroids and should not be incorporated.

For more information about insecticide products and rates, visit UNL's Department of Entomology's Web site at http://entomology.unl.edu. For more information about managing cutworms, consult UNL Extension NebGuide G1154, Corn Cutworms, available at local extension offices or online at http://www.ianrpubs.unl.edu/sendIt/g1153.pdf.

Comparison of Glyphosate-Based Herbicides in Nebraska

The proliferation of glyphosate-based products into the glyphosate-resistant crop market is unprecedented. Currently, there are more than 40 glyphosate-based herbicides registered for use in Nebraska . The influx of the generic glyphosate-based herbicides has also resulted in their price reduction as distributors attempt to remain competitive. Producers are interested in getting the best weed control for the lowest cost creating an interest in the efficacy of these products. Therefore, we compared efficacy of a variety of glyphosate-based products (generic and brand names) on weed control over 3 years at 6 locations in Nebraska . Weed species composition in our studies included: velvetleaf, common waterhemp, sunflower, kochia, Russian thistle, lambsquarters, and a mix of foxtail species. Depending on the year or location, we tested these glyphosate-based products at two rates (label rate and half-rate): Roundup Ultra, Roundup UltraDRY, Roundup UltraMAX, Roundup WeatherMAX, Touchdown w/IQ, Cornerstone, Clearout 41 Plus, GlyphoMAX, Glyfos Xtra, and Glyphomax Plus.

All herbicides provided excellent weed control (> 90%) regardless of the rate or brand name. For example, there was no significant difference in the level of weed control for a brand name Roundup Ultra Dry when compared to the generic product such is Clearout41 Plus. Another example, the level of weed control was not significantly different for a brand name product such is Roundup WeatherMax when compared to the generic product such is Clearout41 Plus, or any other herbicide tested.

These findings were similar to the results reported by our colleagues from other states. Therefore, generic glyphosate-based products can provide a valuable tool for weed control in glyphosate-tolerant crops, especially to those producers who are interested in lowering crop production inputs by reducing the weed control costs. Perhaps, more important decision by producers should be to select the appropriate herbicide rate for the weeds present, observe environmental factors, and the herbicide costs, rather than choosing a glyphosate trade name.

In addition, with the growing popularity of the relatively inexpensive generic glyphosate- based products there is an even grater need for their proper use. Their value can be preserved only by proper management, and reduced overuse. This becomes even more important when other Roundup-Ready crops become more readily available (e.g. Roundup-Ready corn and Roundup-Ready alfalfa). It is easy to fall into a trap of overusing glyphosate when one glyphosate-resistant crop is grown after another. Therefore, proper use of glyphosate-based technology, as a component of integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about their use, or misuse. For more details about proper use of herbicide-tolerant crops see our NebGuide -G02-1484-A titled: Use of herbicide tolerant crops as a component of an integrated weed management program.

Hoary Vervain Control in Pasture

Hoary vervain (Verbena stricta), also known as wooly verbena or tall vervain, is a commonly found native weed in northeastern Nebraska on over-grazed rangeland, prairies and disturbed sites in all soil types. There are several other types of vervain in Nebraska (prostrate, white, and blue) that have similar growth forms and habits as hoary vervain. Hoary vervain is a perennial forb from the vervain family (Verbenaceae) that reproduces by seeds. The taproot (perennial structure) produces individual erect plants. The stem is nearly round, simple or branched above and can be up to 5 ft tall, covered with soft white hairs. The leaves are opposite, leaf blades are ovate with many teeth. The lower surface is pubescent with highly visible veins. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. Hoary vervain, in Nebraska , can flower from May to September, with blue or purple flowers positioned on the top of the main stem and branches and producing a two seeded fruit.

Hoary vervain provides forage for deer while seeds are important food source for small mammals and upland birds. Native Americans also made a tea from the leaves to treat stomachache. Hoary vervain has no value to livestock because of its low palatability.

This weed can be controlled by various means. Mowing the plants when they are 3-5 inches tall can reduce vervain population considerably for the season. Mowing can be done one or two times per season depending on the amount of rainfall during the season. One mowing done in mid June can be effective (>75% control) if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August. Herbicides can be also very effective in providing a season long control. Herbicide application should be conducted when vervain plants are 3-5 inches tall, which is usually in early part of June. The list of effective herbicides, their rates and cost per acre includes: Salvo (12 oz/acre, $4), Grazon P+D (32 oz/acre, $8), Weedmaster (32 oz/acre, $6), Ally (0.25 oz/acre, $8), and Vista (22 oz/acre, $8).

David P. Shelton
Extension Agricultural Engineer

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