Managing Insect and Mite Pests of Texas Small Grains

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Integrated pest management (IPM) is the use of multifaceted techniques that are the most economical and ecologically sound to control insect and mite pests efficiently and effectively. IPM includes:

• Cultural practices, such as crop rotation, fertilization, and variety and planting date selection
• Biological control, including the use of pests’ natural enemies
• Chemical control, aiming to use selected insecticides and rates judiciously to keep pest numbers below economically damaging levels

The best pest-control strategy is prevention— using good agronomic practices and cultural methods to keep the plants healthy. Apply insecticides only when the pest population or plant damage reaches levels that can cause crop losses greater than the cost of treatment. These levels are called the economic threshold or action level.

For insect and mite pests of wheat and other small grains, IPM includes sound cultural practices, grazing management, biological control, and, if necessary, insecticide applications.

Cultural Practices

Several cultural practices can reduce pest numbers—choosing resistant varieties, maintaining plant health, rotating crops, and planting later in the season. Reduced tillage has produced mixed results on pest populations, decreasing some pests and increasing others.

Varietal selection

Some wheat cultivars are resistant to pests such as the greenbug, Hessian fly, and Russian wheat aphid (Fig. 1). However, these pests can develop new biotypes that overcome the resistant genes in these cultivars. Plant breeders continually work to develop new lines that are resistant to these new biotypes and to improve yield and grain quality.

The practice of catching seed at harvest and planting it the next season can result in the deterioration of seed quality, plant vigor, and plant resistance. Wheat variety trials at multiple locations over several years have identified the varieties that are best suited for local growing conditions. The varieties have been proven to maximize yield, qual ity, maturity, winter hardiness, and disease and insect resistance in specific regions of Texas.

Plant health

Factors that affect a small grains plant will also affect the insects and mites that feed on it. The healthier, larger, and more vigorously growing

the plant, the more pests it can tolerate without significant loss. Thus, optimum planting date, soil fertility, and irrigation will increase a crop’s pest tolerance. Research has demonstrated this increase for crops attacked by the bird cherry-oat aphid, greenbug, Hessian fly, and winter grain mite.

Crop Rotation

Crop rotation is particularly useful in managing pests that have a limited dispersal range, such as the Hessian fly, white grubs, winter grain mites, and wireworms. To increase the effects of rotation, destroy the crop residue soon after harvest and eliminate volunteer host plants (a practice called summer host termination) and, where possible, avoid planting next to neighboring crops that are hosts to insect pests of small grains.

Planting Date

The planting date will be influenced by the planned use of the small grains—whether for livestock grazing or for producing grain. Small grains are cool-season plants and do not grow well when daytime temperatures are in the upper 90s. When planting early for grazing, producers should understand that the field is at higher risk for disease and arthropod problems if there is a period when both summer host plants and seedling small grains are growing next to each other. This allows arthropods and dis- eases to move from the summer host to the seedling small grains. To minimize this risk, plant later in the season and use resistant varieties. The timing varies within production regions; check with the local Texas A&M AgriLife Extension Service office for the optimum planting dates in your area. Producers can also plant another small grain. For example, oats are not a host of Hessian fly and can be planted where Hessian fly is a threat. In many regions, producers interested only in grain production can delay the planting date until after mid-October to early November. Delayed planting allows wheat and barley to miss at least part of the first generation of Hessian flies. It also leaves less time for greenbugs, Russian wheat aphids, and bird cherry-oat aphids to establish populations. Destroying volunteer small-grain plants before planting the small grains crop can reduce infestations of wheat curl mite.

Tillage

Tillage has long been recognized as an important practice for insect control. It not only destroys host plants, but it also may bury some insects too deep for survival or physically kill shallow soil-borne or overwintering pests. Plowing under stubble and volunteer wheat reduces the Hessian fly and some of the other pests that remain in and around fields.

Reduced tillage leaves more crop residue on the soil surface, reduces soil temperatures, and increases soil moisture. Research shows that reduced tillage may encourage certain diseases and insects, particularly soil dwelling insects and pupae. For example, the wheat curl mite is a particular problem in the Texas Panhandle, where reduced tillage allows it to survive between crops on volunteer wheat, native grasses, and corn. Winter grain and brown wheat mites increase where there is crop residue. However, other research indi- cates that reduced tillage decreases aphid numbers. Intensified pest management may be needed to prevent crop losses in a reduced tillage program.

Reducing small grains stubble and controlling volunteer wheat plants and summer weeds will help reduce the number of aphids, Hessian flies, and wheat curl mites that can move into wheat fields.

Grazing Management

Growers plant wheat on more than 6 million acres in Texas each year. About 40 percent of the wheat acreage is grazed to some extent, and about 30 percent is used only for forage. Barley, oats, rye, and triticale are also used for livestock forage.

Small-grains pastures are often seeded in late sum- mer or early fall to take advantage of soil moisture and to give the plants time to establish strong root systems that can support heavy grazing by an aver- age freeze date.

Grazing of small grains suppresses infestations of aphids and winter grain mites. However, if the aphid populations rise above the economic thresh- old after the small grains are well established, producers may opt to move livestock on part of the pasture and treat the remainder. After the reentry interval for grazing (specified on the insecticide label), the livestock can be turned into the treated part of the pasture. Once livestock return to the treated area, inspect the untreated area to deter- mine whether aphid control is still needed there.

In a heavily grazed field, aphid infestations are unlikely to be above the economic threshold.

Grazing can reduce aphid numbers 3- to 10-fold over fields not grazed. However, the difference will be less when aphid infestations are very light and many of the aphids are winged. In general, the heavier the grazing, the more an aphid infestation is reduced. When livestock are taken off the pas- tures, the aphids will increase. Livestock are gener- ally removed from wheat pastures just before the wheat begins jointing and rapidly growing. At that point, damaging numbers of aphids are less likely to develop because the plants are large and vigor- ous and the pests’ natural enemies are more active in warm, spring weather.

Biological Control

Factors that often hold insect and mite infesta- tions below damaging levels include weather, inad- equate food, and natural enemies. Recognize the effect of these natural control factors and, where possible, encourage them. Biological control uses living organisms— such as parasites (which live

in or on the host and may kill it), predators (feed on the pests), and pathogens (cause diseases in pests) to reduce pest numbers. The primary natural enemies of insects and mites attacking wheat are several kinds of lacewings, lady beetles, and para- sitic wasps. Other predators include big-eyed bugs, damsel bugs, spiders, and syrphid flies. Pathogenic fungi may control aphids occasionally. For more information on the natural enemies of wheat pests, see the AgriLife Extension publication Biological Control of Insect Pests of Wheat, which is available online at agrilifelearn.tamu.edu.

Biological control is most effective when used with other compatible pest control practices in an integrated pest management program. These prac- tices include cultural control, host plant resistance, and the selective use of insecticides when other practices fail to keep pest numbers below economic levels. Methods of biological control are conserva- tion, importation, and augmentation of natural enemies.

Conservation involves protecting existing populations of natural enemies. To prevent the development of economically damaging pest infestations, avoid using insecticides until they are needed. When you do apply an insecticide, minimize the effects by using those that are selec- tive—that is, more toxic to the target pest than to the natural enemy.

Importation is the release of natural enemies into areas where they do not occur naturally. This method has been effective where an exotic pest has entered Texas without the natural enemies that help control the pest in its native country. For example, several species of natural enemies have been imported into Texas to control the Russian wheat aphid.

Augmentation is the periodic release of natural enemies that occur naturally but not in sufficient numbers to provide pest control. This practice can be effective in greenhouses. However, because little definitive information is available on the effective- ness of releasing natural enemies in wheat (such as when and how many to release), Extension entomologists cannot provide guidelines for augmentation as a management tool in wheat.

Insecticide use

Insecticides are a key component of IPM because they can reduce a pest population rapidly to avoid crop loss. However, they should be used only as a last resort to prevent economic loss from pests. Apply insecticides when pest density or plant dam- age approaches the economic threshold. Inspect the fields once or twice a week to determine crop growth, damage levels, and the pest and beneficial insect population levels. This information is nec- essary to determine if the pest is at the economic threshold or action threshold and if an insecticide treatment is needed to prevent economic loss.

Unfortunately, economic thresholds have not been developed for all pests. When using insecticides, always read and follow the label directions. Because pests are active at different times of the growing season, knowing when specific pests are likely to be present can help you anticipate pest infestations. Figure 2 compares the stages of small grain development. Table 1 shows the periods of plant development when pests are likely to attack. The occurrence and development of different small grains pests are usually related to plant development and various environmental factors.

Although the severity of insect problems cannot be predicted, the pest occurrence profile in Table 1 indicates the insect and mite pests that may attack small grains in various seasons and stages of devel- opment. Inspect the fields carefully to determine the presence and damage potential of each pest.