Spider Mite Resistance Management in Corn

The following article was written by a group of university entomologists working on corn spider mites in the Western Great Plains. There is a concern across the region about the potential for spider mites to develop resistance to the few effective miticides available on corn. This article is written from a regional perspective. A few points should be noted concerning the corn spider mite situation in Nebraska:

1. There are differences between twospotted spider mites and Banks grass mite in susceptibility to corn miticides in Nebraska. Banks grass mites are more susceptible to the available miticides than twospotted spider mites. Dimethoate is still effective against Banks grass mites, but not against twospotted spider mites. This points to the importance of identifying mite species.

2. In areas where spiders mites have been a problem, carefully consider the use of insecticides against other corn pests, such as European corn borer, western bean cutworm or adult corn rootworms. In the case of first generation European corn borer, use of products containing B. t. is an effective option which has the least harmful effects on spider mite natural enemies. In the case of other insect pests, be sure to follow treatment guidelines in deciding whether or not treatment is needed.

3. The article discusses use of pesticide mixtures. We do not recommend use of pesticide mixtures in Nebraska. This is a practice in Texas where spider mites are much more common and more difficult to kill than in Nebraska.

Additional information on corn spider mite management in Nebraska can be found in NebGuide G1167, "Spider Mite Management in Corn and Soybeans".

MANAGING CORN SPIDER MITE RESISTANCE TO MITICIDES ON THE WESTERN HIGH PLAINS

High Plains Corn Spider Mite Working Group:
Tom Archer, Texas A&M University	Phil Mulder, Oklahoma State University
Larry Buschman, Kansas State University	Carl Patrick, Texas A&M University
Ed Bynum, Texas A&M University	Frank Peairs, Colorado State University
Jack Campbell, University of Nebraska	Stan Pilcher, Colorado State University
Greg Cronholm, Texas A&M University	Frank Schweissing, Colorado State University
Gary Hein, University of Nebraska	Ron Seymour, University of Nebraska
Randy Higgins, Kansas State University	Phil Sloderbeck, Kansas State University
Jerry Michels, Texas A&M University	Bob Wright, University of Nebraska
Pat Morrison, Texas A&M University

On the western High Plains, spider mites are considered a major pest of irrigated corn. There are no reliable non-chemical controls for spider mites in corn which places a strong reliance on chemical control. Although control of corn spider mites varies from field to field and from year to year, they are generally more difficult to control in the southern part of the region with control problems gradually decreasing to the north.

Currently, over much of the region, only two miticides are regularly effective against corn spider mites: bifenthrin (Capture 2E), and propargite (Comite II). Availability of limited numbers of miticides is of great concern to growers and consultants, because spider mites are notorious for their ability to develop miticide resistance. In the future, spider mite control could be very difficult if we lose our current products. The situation is also of concern because progress in developing new miticides for corn is not encouraging. There are no obvious alternatives now or in the near future, even for Section 18 registrations, should currently registered miticides become unavailable. Therefore, resistance management is essential to prolong the useful life of current miticides, because there are no new miticides currently being developed for corn.

Resistance develops because individual spider mites differ in their tolerance to a given miticide. Repeated use of any a single miticide removes the susceptible individuals from the population, leaving only the most resistant mites to reproduce. Over time, as each new generation is selected, the percentage of resistant individuals increases. Development of resistance can be delayed when mixing of populations of selected mites in treated fields occurs with unselected mites on nearby, untreated alternate hosts. Over enough time, however, the overall level of resistance in the pest population can be expected to increase to the point where the miticide will no longer be useful. This undesired development of resistance has occurred with several organophosphate miticides which were formerly used in corn on the western High Plains.

The length of time required for resistance to develop is difficult to predict and is affected by many factors: species of spider mite, number of applications per year, number of spider mite generations per year, spider mite movement from fields to alternate hosts, miticide efficacy and mode of action, application dosage, and thoroughness of application coverage. However, we believe that we can reduce the rate of resistance development by using a resistance management strategy. Such a strategy is especially important for corn on the western High Plains where spider mites frequently reach damaging levels. Special precautions need to be taken to prolong the useful life of existing miticides.

The following RESISTANCE MANAGEMENT STRATEGIES are proposed to reduce the rate of development of resistance to corn miticides. These measures are particularly important in areas that regularly need dependable miticides for use in corn.

1. Use cultural practices to reduce spider mite problems. Mite problems often develop in water-stressed areas of the field, so corn should be given adequate water to keep plants healthy. Avoid planting corn next to winter wheat, particularly if the wheat is known to be infested with mites. In general, follow sound agronomic practices to produce the healthiest and most vigorous crop possible.

2. Use corn borer insecticides carefully. Apply insecticides for corn borers and other corn pests only when absolutely essential to avoid serious yield losses. This precaution is necessary because some insecticide applications induce spider mite outbreaks that require follow-up miticide applications. For this reason, planting corn borer resistant corn hybrids could reduce the need to treat for spider mites if the use of these hybrids results in fewer insecticide applications.

3. Use miticides sparingly, but effectively. Miticides should not be used unless mites are threatening based upon treatment thresholds or application guidelines. When applied make sure adequate carrier and appropriate adjuvants are used to assure optimal miticidal activity and avoid making applications when weather conditions are unfavorable.

4. Rotate between different miticides. Avoid using the same miticide in the same fields year after year, since using any single pesticide repeatedly will select for resistance. Consider rotating between Comite II and Capture 2E, or consider using dimethoate or disulfoton if they are effective locally. If a miticide + synergist mixture (see point 7 below) is recommended in your region, the mixture should be considered as a single chemical in the rotation schedule.

5. Always use labelled rates of bifenthrin (Capture 2E). Avoid the use of reduced rates of Capture 2E for corn borer control. Reduced rates can lead to increased exposure of mites to Capture 2E and speed the buildup of resistance. Lower rates of Capture 2E may be effective as corn borer treatments, but this approach selects for resistance in mites and does not provide effective miticidal activity. Several other corn borer insecticides are available and should be used in place of Capture 2E when mites are not a serious threat.

6. Minimize pyrethroid insecticide use, in areas where miticide resistance is a significant concern, to avoid development of cross-resistance. Cross-resistance occurs when high levels of resistance to one pesticide lead to resistance to other pesticides with similar modes of action or target sites in the pest.

   In practice, cross-resistance usually occurs between pesticides that are closely related chemically. Some pyrethroids used for corn borer control, such as lambda-cyhalothrin (Warrior), are closely related chemically to bifenthrin (Capture). Cross-resistance among such similar insecticides is likely. There are effective nonpyrethroid insecticides available for corn borer control that can be used in areas where the development of miticide resistance is a concern.

   The Warrior label claims spider mite suppression, so it probably has enough activity to select for resistance in mites. Thus use of Warrior may lead to cross-resistance to Capture and the loss of miticidal efficacy of one of our two remaining effective corn miticides. Other corn borer pyrethroids, such as permethrin (Pounce & Ambush), are not known to have miticidal activity in the field and are probably of less concern relative to cross-resistance to bifenthrin. These pyrethroids, however, often induce outbreaks of outbreaks (probably by killing mite predators) and thus may indirectly lead to expanded miticide use.

7. Use mixtures only where recommended and be aware of possible problems with mixtures. Use miticide mixtures that are known to be synergistic.Nonsynergistic mixtures actually may be antagonistic, making mite problems worse than they would have been without treatment. Check with yourCooperative Extension entomologist to determine if the use of mixtures is advisable in your area and for a list of effective combinations. If mixtures are recommended, be sure to (1) never use Capture 2E at rates lower than those on the label, and (2) follow a rotation schedule according to theguidelines in point 4 above.

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