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Alfalfa Weevil and Clover Leaf Weevil in Alfalfa

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It's that time of the year again! Time to gear up for the new crop production season and time to re-subscribe to the Northeast Nebraska IPM-Crop Production Newsletter.

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Please fill out the accompanying form and get it to us as soon as possible so you can receive our weekly update on conditions influencing crop production in northeast Nebraska . (KJ)

Insects to Watch for In Alfalfa

Weather Effects - It is difficult to predict what the last few weeks of below normal temperatures will do to the insect populations across the state. At the very least, insects that had become active before the extreme cold of two weeks ago were stressed by the very cold conditions. Insects that had not come out of their winter dormancy were likely not affected. Development according to growing degree day models for alfalfa weevils came to an abrupt halt and accumulations are just starting again. Without knowing for sure what the effects were, we need to scout our fields to make sure the insects don't “sneak up” on us while we rush to plant our spring crops.

Alfalfa and wheat can be affected in the early season by several insect pests. In wheat, army cutworms and pale western cutworms can cause problems. In alfalfa, army cutworms and alfalfa weevils can cause severe damage.

Army Cutworm (AC) and Pale Western Cutworm (PWC) in Wheat

Damage to wheat can be scouted for any time the wheat is green and growing. Most of the activity from these insects occurs in the western third of the state but army cutworms can be found over the entire state. The AC overwinters as a partially grown larvae and the PWC overwinters in the egg stage. However, PWC eggs hatch in February and March during warm spells, so both of these cutworms should be in the larva stage at this time. AC larvae will be dark and slightly larger than PWC larvae, which are a light color. Look for both of these cutworms in the soil near the base of the plant during the day, since most of their above ground activity occurs at night. The AC tends to feed on the leaves and may be less serious, but the PWC tends to feed lower and may thin stands by killing plants by feeding below the growing point. Injury symptoms may resemble freezing damage so if wheat damaged by the cold temperatures isn't recovering like it should, these insects may be the problem.

Treatment thresholds are 2 - 4 per foot of row in wheat for AC and 1-2 per foot of row for the PWC, depending on stress and moisture conditions.

Baythroid (cyfluthrin), Mustang Max (zeta-cypermethrin), Warrior (lambda cyhalothrin), Proaxis (gamma cyhalothrin), Lorsban (chlorpyrifos), methyl parathion, endosulfan, and many generic products are available for control.

Army Cutworm in Alfalfa

Army cutworms may also prevent green-up of alfalfa by feeding on the new growth. Entire fields can remain brown for a week or more after normal green-up usually occurs. Often patches of a field will green up later than others. Again, these larvae are nocturnal and will be hiding in the soil around the crowns of the alfalfa during the day. The threshold for army cutworms in alfalfa is 2 or more per square foot on seedling alfalfa and 4 or more per square foot for established stands.

Alfalfa Weevil and Clover Leaf Weevil in Alfalfa

The majority of alfalfa weevils overwinter as adults, and then become active and lay eggs. However, some eggs overwinter as well, and hatch earlier. This has resulted in some areas in Nebraska (Panhandle and the northern tier of counties) being hit with 2 flushes of weevils in the spring. In the last few years some areas of the state have received damage to regrowth after the first cutting due to a combination of late larval feeding and adult feeding. This is something to be aware of after the first cutting.

Getting back to the present, 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 as soon as next week. 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 in western Nebraska may have aided them in building their populations, although recent rain 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 after the first cutting as adults (and sometimes larvae). 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.

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.

Controlling Winter Annuals Helps Conserve Moisture

Winter annuals are species that germinate in the fall, overwinter and then continue growing in early spring. The rapid growth of winter annuals starts as soon as the soil temperatures reach around 50-55 F. They also consume large amounts of moisture, which has negative effects on crop seed germination and early growth.

The list of most commonly known winter annuals includes: field pennycress, shepherds purse, henbit, blue mustard and treacle mustard (bushy wallflower). You may want to watch for those species. They make the “green patches” that are commonly seen during late parts of the year (October, November) and early spring (March-April). In addition, dandelion and curly dock, which are not winter annuals, can be also seen in early spring and at the time of corn and soybean planting.

In no-till fields, winter annuals can be effectively controlled with various herbicides. In corn and soybean they can be controlled 0-45 days before crop planting depending on the herbicide or after crop planting but before crop emergence. The same is true for the fields that are planted with herbicide tolerant crops (e.g. Roundup-Ready or Liberty-Link crops). The use of pre-plant or pre-emergence herbicides with residual activity in Roundup Ready crops would also provide a longer comfort zone in which to apply glyphosate-based products. Winter annuals are usually controlled in the first two weeks of May, before they become too large for herbicide efficacy.

Examples of burndown herbicides that can be used in corn and grain sorghum include: Aim (0.3oz/acre), 2-4-D Ester (1.0 pt/acre); 2,4-D Ester + Banvel (0.5 pt+0.5pt); Banvel (Atrazine (2qt); Atrazine + Banvel (2qt + 0.5pt), Atrazine + 2,4-D (2.0 qt+1.0 pt); Field Master (4 qts), Gramoxone Extra (1.5 pt); Gromoxone extra + Atrazine (1.5 pt + 2.0 qt); Roundup Ultra Max (18-26 oz), Touchdown (24-32 oz/acre) and one of many generic glyphosate type products (24-32 oz). The list of pre-emergence herbicides in corn includes: Atrazine, Axiom, Balance, Bladex, and Prowl.

Examples of burndown herbicides in soybean that can be used from 0-30 days before crop planting include: Aim (0.3 oz/acre), Canopy (5-7 oz/acre), Command 3E (1.5 pt/acre); Gramoxone Extra (1.5 pt/acre); Extreme (3 pts/acre), Gauntlet (co-pack, 5.33 + 0.6 oz), Pursuit (4 oz), Pursuit Plus (2.5 pt), Sencor/Lexone DF (16/13 oz), Roundup Ultra Max (18-26 oz), Touchdown (24-32 oz/acre) and one of many generic Glyphosates (24-32 oz). The list of pre-emergence herbicides in soybean includes: Axiom, Pursuit, Prowl, and Sencor/Lexone.

Also note that glyphosate based products (e.g. Roundup, or various generics) used alone may not provide good control of winter annuals if temperatures are cool. Weeds need to grow actively in order to move a systemic product such as glyphosate throughout the plant to get good plant kill. Therefore, during cool spring temperatures avoid using Roundup. Mixtures with atrazine, 2,4-D or other modes of action can improve weed control.

Another thing to watch for is the price you are paying for your glyphosate-based products. Our studies that were conducted over 3 years and 6 locations across Nebraska clearly showed that generic glyphosate provided equal level of weed control when compared the much more expensive Roundup-named products. Why spend more money than what is needed to get the job done?

Also, consult the label directions for additives and for the pre-plant and post-plant intervals. For example, corn should not be planted 5 days before or after application of 2,4-D. Also 2,4-D can be used for burndown in soybean but at least 7 days prior to crop planting. More details on these herbicides and their use rates can be also found in the University of Nebraska Guide for Weed Management. It is a publication that is updated annually, and it can be purchased in every county extension office for the nominal fee of $5.

David P. Shelton
Extension Agricultural Engineer

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