Chapter 5. Peanut Insect Management

Peanut growers experienced fairly good of conditions during much of the 2001 growing season. However, the dramatic increase in tomato spotted wilt virus was the dominate concern.

Thrips occurred late in peanut fields this year, but the tomato spotted wilt virus they brought created serious problems. Leafhoppers, corn earworms, fall armyworms, and spider mites were relatively light. A few drier areas had some problems with mites. We also saw a few fields with some significant rootworm injury, the first time in 5 years we have seen any problems from this pest.

Thrips and leafhoppers usually are found in peanut fields. An in-furrow systemic insecticide applied at planting is the most common approach used to reduce seedling damage from thrips and reduce leafhopper damage. A number of caterpillars (usually corn earworm) will also attack peanuts during August and September.

Thrips and Tomato Spotted Wilt Virus

Tomato spotted wilt virus incidence increased dramatically in 2001. It is probably safe to assume that this disease will be a problem for the next few years. The virus is found in many weeds and even in winter annuals, such as chickweed and henbit. Therefore, the thrips have plenty of opportunity to pick up the virus each spring.

While the future of tomato spotted wilt virus in North Carolina can’t be predicted, with the help of Drs. David Jordan and Jack Bailey, a coordinated effort was put into place during 2001 to develop management recommendations based upon field research. Our approach was based on successful efforts in Georgia. One point to make up front, however, is that if tomato spotted wilt virus does occur in a field, a foliar treatment at mid-season is not justified. Research in Georgia indicates that follow-up treatments to reduce virus once it is established in the field is like throwing money away. Our research revealed the following trends for managing tomato spotted wilt virus. We saw less virus in VA 98R, NC-V11, and Gregory. Less virus occurred in twin row production and in plots planted at higher seeding rates. At-plant, in-furrow insecticides do help reduce the virus, and Thimet appeared the most effective. Reduced tillage or strip till production also appears to help minimize the level of virus. We do not have a strong indication of the effect of the planting date on the level of virus. Our findings are consistent with the results of testing in Georgia. I am quick to point out, however, that these practices help reduce the incidence of virus; they do not eliminate it.

Use of Systemic Insecticides

Systemic insecticides are an effective production tool. Over 90 percent of the North Carolina peanut acreage is treated annually with phorate (Thimet), aldicarb (Temik), or acephate (Orthene). This eliminates the need for most foliar insecticides unless worms or mites become a problem in August or September. Systemic insecticides are applied in-furrow at planting as a granular formulation. The insecticide is taken up by the roots and transported to the stems and leaves where it provides several weeks' protection against sucking insects, such as thrips, leafhoppers, and aphids.

A spray formulation approach for use in-furrow is also available in the use of Orthene (acephate) 97 as an in-furrow spray. This approach has proven successful and offers an additional option for at-plant thrips management.

When foliar insecticides are used in addition to fungicides, spider mite outbreaks often occur if hot, dry weather, which favors spider mite buildup, persists. The use of systemic insecticides at planting eliminates the need for foliar insecticide treatments for thrips early in the season and for leafhoppers in July, and may decrease the likelihood of mite buildup. However, the early season foliar application probably is not much of a factor in creating late-season mite problems. The systemic insecticides are not effective against worms, and if peanuts are attacked by worms in August and September, foliar sprays may be needed.

"On-Demand" Treatments. IPM focuses on treating only when necessary. At-planting treatments are contrary to that idea. However, the convenience and effectiveness of these in-furrow treatments make most other options less attractive. At-planting treatments provide some peace of mind because growers know that they suppress any potential early season pests (thrips and leafhoppers). However, such treatments assume that these insect pests will be present in economically damaging numbers. At present, this cannot be predicted. Foliar treatments do not seem to have as much of an impact in reducing the levels of tomato spotted wilt virus as do at-plant treatments.

On the other hand, "on-demand" foliar insecticides are used only when the insect populations reach or exceed a predetermined economic threshold. While insect populations are below this level, there is no need to treat; when they exceed the threshold, treatments can be applied to prevent economic damage. During some years, thrips and leafhopper treatments may not be needed, reducing production costs. Such an approach requires a commitment to an effective scouting program.

Foliar Insecticides

There are a number of insecticides labeled for use on peanuts as foliar sprays. Therefore, when selecting an insecticide, consider its potential to cause spider mite outbreaks, its effectiveness, and toxicity, and shop around for the best price. Often, only one insecticide is needed for season-long control of foliar peanut insect pests in North Carolina. Growers should check their fields, know the pest situation, and treat only as needed. Orthene is now available in a 97 percent formulation.

Thrips can be serious pests early in the season if at-planting systemic insecticides were not used, but foliar sprays can be effective. The economic threshold for thrips is 25 percent leaf damage. It is very important to follow this guideline closely. Delaying thrips treatment will still provide control but will not provide any real benefits in plant response.

Potato leafhoppers can also damage peanuts; however, research indicates the economic threshold should be 50 percent leaf damage. Such levels are not commonly seen, but leafhoppers have been more of a problem in recent years, particularly in fields not treated for rootworms.

Several types of caterpillars or "worms" may attack the crop later in the season. The most common is the corn earworm. The threshold for treatment varies with the time of year. Generally, earworms occur in August and the threshold for treatment is as soon as the worms reach 4 per row foot. In early September, at least 6 worms per row foot are necessary to cause economic loss, and by mid-September no treatment is justified unless at least 10 worms per row foot are seen. These infestations are very sporadic, so treatment is infrequent. Danitol, an insecticide for corn earworms and spider mites, showed good effectiveness against fall armyworms, as well as earworms, spider mites, and leafhoppers in one test.

Application of Foliar Sprays. Nozzle, size, number of nozzles, ground speed, and pressure influence the rate of chemical output per acre. Calibrate the sprayer accurately to ensure application of the recommended amounts of insecticides. Periodically, check the calibration during the season.

Apply sprays for thrips, leafhoppers, corn earworms, fall armyworms, and other foliar-feeding insects on peanuts with hollow cone or solid-cone nozzles at a minimum of 40 psi and a total of 10 to 15 gallons per acre. Apply aerial sprays for the same insects in a minimum of 3 gallons of water per acre.

Low-volume sprays are ineffective for spider mite control. Apply a minimum of 25 gallons of spray per acre for this pest with adequate pressure for the nozzle setup on the sprayers. Many growers combine spider mite treatments with their leafspot fungicide application. Spray amounts commonly used for fungicide application (12 to 14 gallons per acre), may not be sufficient for good mite control. Change nozzles or slow down if past experience has given poor results. Aerial application for spider mite control has been ineffective.

Use flat fan nozzles to apply a minimum of 20 to 40 gallons of spray per acre directed at the base of the plant for lesser cornstalk borer control. Low gallonage applications for lesser cornstalk borer are an absolute waste of time!

Spider Mites

Spider mites were not a major problem for peanut farmers in 2001, due to the timely rains received in many areas. Remember that problems with spider mites are usually enhanced when certain fungicides and insecticides are used. Suggestions for reducing the threat of spider mites are listed in the control recommendations and in an Insect Note available at the local Extension center. Avoid the use of carbaryl (Sevin) when spider mites are present. The use of Lorsban can also increase the likelihood of spider mite outbreaks. Check peanut fields frequently during late July and August, especially if they are next to cornfields. Options for control of spider mites are limited to two miticides at this time. Therefore, it is important to scout fields and use a spray only when necessary. Spider mites have a great ability to develop resistance and, until new materials are available, we run the risk of resistance developing to our only available miticides. Danitol is labeled for spider mite control in peanuts and gives another product to go along with the previously available product Comite. It is important to remember that like peanut disease problems, spider mites are very much regulated by the weather. Therefore, it is important to look at management of this pest in a manner similar to the way one looks at managing a disease. Unlike caterpillars, for example, that once treated are generally gone for the year, spider mites have the ability to bounce back in hot, dry weather.

It is important to note, however, that controlling mites usually requires two applications about 3 to 5 days apart. Treating one time often will not stop a spider mite problem because Comite or Danitol will not kill the eggs. Unless rain occurs, it is almost certain that the mites will come back with a vengeance in a couple of weeks. Using one spray and taking a "wait and see" approach is often not best unless the problem is caught very early in the season before a lot of eggs are present. With the two-spray technique, the first gets the mites, and the second gets all the mites that have hatched from the eggs present during the first spray.

The southern corn rootworm is one of the most troublesome insects for peanut producers. While pests like caterpillars, thrips, and spider mites can cause severe damage that is often quite obvious above the ground, the rootworms feed below the soil surface.

Beginning in late July and continuing through August, beetles lay eggs in the peanut field. Egg laying and the survival of these eggs depend on the soil being moist. If the soil is hot and dry, many eggs will not hatch, and some of the rootworms that hatch will die. Most of the eggs are laid in the soil near the base of the plant, where the soil stays moister than in the row middles. However, adequate rainfall in late July and early August can result in rootworm infestation. This is why rootworms often damage irrigated fields.

Heavier soils also are more likely to have rootworm problems. The heavier the soil, the better its water-holding ability. This increases the chances that this soil will have the moisture rootworms need for survival. However, this does not mean that sandy soils can't have rootworm problems. If the soil moisture is adequate, rootworms can occur in almost any field.

Management Decisions

The standard management approach for rootworms is an at-pegging granular insecticide application in a band over the row. Dr. Ames Herbert at Virginia Tech and I have developed a southern corn rootworm index and printed it in a brochure that is available at your local Extension center. This index relies on soil type for helping make a good decision about treatment. Knowing soil type is also useful. Heavier soils are more of a risk for reasons already discussed. Any soils referred to as "stiff land" probably should be treated. Fields under irrigation run a greater risk from rootworms because higher soil moisture favors egg laying, egg hatch, and rootworm survival.

Monitoring the soil moisture doesn't offer much help for decision making. The critical time for adequate soil moisture to ensure rootworm survival is early August. As discussed in the next section, treatments need to be applied before the grower can determine if soil moisture is going to be adequate. In reality, there is very little that can be done to avoid treatments to fields when a grower isn’t 100 percent sure there is going to be a problem. One point needs to be emphasized concerning preventive treatment. Even if an at-pegging preventive treatment is applied and the conditions that follow do not encourage a rootworm outbreak, some benefits are still derived from the application. These insecticides protect the crop from leafhoppers, offer some white mold suppression, and give some protection should any cornstalk borers and cutworms should they be present. Remember, however, that this is not a blanket recommendation to treat every acre of peanuts with a rootworm insecticide. Treat those fields that are high risk or those in which problems have occurred in the past. The use of rootworm insecticides can increase the likelihood of spider mite outbreaks, which is another good reason to avoid the unnecessary use of such products. It is also important to note that over the last 5 years very few fields have been attacked by rootworms.

Treatment and Applications Options

Rootworm treatments are usually applied after the Fourth of July with ground equipment using properly calibrated hopper boxes to place the insecticide in a 16- to 18-inch band over the row. This provides an important "zone of protection" around the developing pods. Check the height of the bander over the row and make sure that the granules are striking the top of the foliage in at least an 8- to 10-inch band. Granules falling down through the plant should be distributed in a 16- to 18-inch band. Research has shown that light incorporation of the insecticide improves its performance. However, this is often difficult as the row middles may be closed when the materials are applied.

Insecticides can be applied any time from early July to the first of August. Treatments applied after August 5 may be too late to prevent some of the early hatching larvae from feeding on pods. Once the larvae hatch and begin feeding, an insecticide treatment is not effective. The treatment must be applied in advance of the larval feeding. This fact prevents growers from waiting to see if there will be sufficient rainfall to cause a rootworm problem. If growers wait until the end of the first week in August to determine if there was adequate soil moisture to allow a rootworm infestation, it will be getting too late to treat. NC State studies have shown that treatments applied after the first week of August do very little to protect pods from rootworm damage (see section titled "Application after August 1").

Two modified approaches to rootworm control can be used with favorable results, but growers must understand the risks involved. With these two options, growers can save a few dollars or gain additional benefits from rootworm treatment, but they may also increase their risk of crop loss.

Early Application. Many growers have considered the early application of their rootworm insecticides. Lorsban has been promoted most widely for this use. This early application would be at-flowering or approximately mid-June rather than at-pegging (mid-July). There are several possible advantages to this early application. First, the growers begin gaining the benefits of leafhopper control much earlier. In addition, some products offer white mold suppression, and there is speculation that this earlier application might improve this suppression. Earlier application would also mean the middles are more open and fewer vines would be run over with the standard 4-row equipment used for granular application. One final benefit would be that some products might offer some lesser cornstalk borer protection should conditions be extremely dry in late June and July.

Application after August 1. The option for rootworm control other than the standard pegging-time treatment is a delayed treatment. If the application of rootworm insecticides is delayed until after August 1, then the rate can be cut in half. Since the insecticide is applied so close to the time of insect activity, there is little need for long residual activity. As a result, the half rate works well.

There are significant risks with this approach. First, the middles will be more closed, so the use of ground equipment will crush more vines. Most importantly, the insecticide must be applied by August 5. Any delay beyond this date may allow rootworms to begin feeding on pods and do significant damage. Although this delay would not be planned, it can often happen if we get substantial rainfall in the first week in August and the soil is too wet to run ground equipment across the field.

Many things can be done to help prevent damaging insect and mite infestations. Where possible, consider the following suggestions:

1. Do not treat on a schedule or because a neighbor is spraying. Scout fields and treat only as needed.

2. Remove weeds and brush that serve as wild hosts for spider mites from around fields in fall or early spring.

3. Maintain an area clear of weeds and briars around field during the early growing season. Do not mow weeds around fields from late June through early September.

4. Select a peanut variety with multiple insect resistance, such as NC-6, where appropriate.

5. To reduce the probability of spider mite buildup, avoid using foliar insecticides in July and August unless needed to control damaging insect infestations. The fewer insecticide applications used, the lower the probability of creating a pesticide-induced outbreak of mites. Using the leafspot advisory for leafspot applications will help reduce the likelihood of spider mite outbreaks. Avoid unnecessary insecticide applications for rootworms.

6. Avoid moving workers and equipment from mite infested areas to noninfested areas.

7. Avoid following tobacco with peanuts. Tobacco is a preferred host of wireworms, and populations are often established that can attack next year's peanuts. These may attack the seed, resulting in a poor stand.

8. Avoid planting peanuts immediately adjacent to fields of sweet corn. Spider mite populations often disperse into peanuts as the corn matures.

Phorate (Thimet), aldicarb (Temik), and methomyl (Lannate), are extremely toxic to people, animals, and fish. Some other products, such as Asana XL and Karate, are toxic to fish. Always carefully read and observe all safety precautions on the label when handling or applying these materials. Use only insecticides labeled and recommended for peanuts. Follow suggestions on dosage and time of application to avoid residues.
 
 

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.