Managing Cotton Insects in Texas

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• Pest Management Principles
• Insecticide Resistance Management
• Biological Control
• Bt Transgenic Cotton
• Scouting

Texas ranks first in cotton production in the United States, using about half of the country’s cotton acres to produce half of the total crop. Cotton is the leading cash crop in the state and is grown on about 5 million acres annually (Fig. 1).

Cotton production in Texas occurs in the following regions: the Panhandle, South Plains, Permian Basin, Trans-Pecos, Rolling Plains, Blackland Prairies, Winter Garden, Coastal Bend, and Lower Rio Grande Valley (Fig. 2). The South Plains is the largest cotton-producing area with acreage exceeding 3 million in some years. Approaches to cotton production vary from one region to another because of differences in climate, harvest techniques, irrigation requirements, pest pressure, soil types, and variety selection.

For each region, a unique group of insect pests damages cotton, making the crop vulnerable to attack throughout the crop season. Therefore, frequent and careful scouting for insect pests and beneficial insects is critical for successful cotton production.

Pest Management Principles

The term Integrated Pest Management (IPM), a philosophy used in designing disease, insect, mite, and weed pest control programs, helps avoid economic losses from pests, optimize production and environmental sustainability, and minimize risks to human health. Systems based on IPM principles encourage the intelligent integration of compatible and ecologically sound combinations of pest suppression tactics, including:

• Cultural control such as manipulating planting dates and stalk destruction, variety selection, fertilization, and irrigation timing
• Biological control such as conservation of existing natural enemies
• Host plant resistance
• Field scouting as the basis for treatment decisions to keep pest populations below economically damaging levels

Major factors to consider when using insecticides include:

• Efficacy of product
• Protection of natural enemies of cotton pests
• Possible resurgence of primary and secondary pests after applications
• Development of insecticide resistance in pest populations

Using multiple pest suppression tactics allows growers to control pests at a low cost, preserve natural enemies, and slow the development of pest resistance to insecticides and insect control traits.

Applying insecticides at the proper rates and only when necessary, as determined by frequent field inspections and economic thresholds, helps prevent economic losses that pests can cause.

The integrated pest management concept assumes that pests will be present to some degree and, at low levels, do not cause significant economic losses. The first line of defense is to use effective agronomic practices and cultural methods to produce the crop in ways that are unfavorable for pest problems to develop. Use appropriate insecticides only when pest populations reach levels that cause crop damage and losses greater than the cost of the treatment. This potentially injurious population or plant damage level, determined through regular field scouting, is the economic or action threshold. In short, pest management strives to optimize rather than to maximize pest control efforts.

Recent developments have significantly impacted managing pests in cotton. The availability of transgenic, insect-resistant traits in cotton has reduced the incidence of damaging populations of caterpillar pests. Eradication programs have eliminated the boll weevil from all but far South Texas and the pink bollworm throughout Texas. And, neonicotinoid seed treatments have greatly decreased the pest status of thrips and other early-season cotton pests.

Because of these technological breakthroughs, the use of foliar insecticides in Texas cotton is down by two-thirds since 2000 and cotton yields have increased by 50 percent. Predictably, the lower threat level reduced growers’ reliance on field scouting for insects and also affected the frequency that trained scouts have to conduct effective scouting programs at the field level.

Current insect-related threats to the cotton system are pest resistance to insecticides and transgenic technologies, and the potential introduction of invasive pests. Transgenic technologies and neonicotinoid seed treatments have recently become targets of environmental and food safety groups.

Insecticide Resistance Management

The Insecticide Resistance Action Committee (IRAC) defines insecticide resistance as an inherited change in the sensitivity a pest population has to an insecticide, “reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species” (see http://www.irac-online.org/about/resistance/). Reliance on insecticides that act the same way can cause pests to develop resistance to the entire class of insecticides. This phenomenon applies to transgenic traits as well.

To delay resistance, growers should use IPM principles and integrate other control tactics into their insect and mite control programs.

One strategy to avoid or delay resistance development in pest populations is to rotate insecticide groups to take advantage of different modes of action. Tank-mixing products from the same insecticide class is not recommended. The combination of insecticide rotations and tank mixtures with insecticides from different IRAC classes reduces the chance of selecting individual pests that are resistant to certain classes of insecticides. These practices may delay the development of resistance, provide better control of target pests, and enhance the long-term sustainability of cotton production systems.

Insecticides with similar chemical structures act on insects in similar ways. For example, pyrethroids, such as bifenthrin, cyfluthrin, and lambda cyhalothrin affect an insect’s nervous system in the same way. Other types of insecticides)organophosphates, such as acephate and dicrotophos, or carbamates, such as thiodicarb)also affect an insect’s nervous system but in a different way than pyrethroids.

IRAC has developed an insecticide mode of action classification system that provides an IRAC number on the insecticide label (see http://www.irac-online.org/). Insecticides with the same number have the same mode of action. This system makes it relatively easy for producers and consultants to determine different modes of action among the insecticides in order to rotate their treatment selection among the insecticide classes and slow the development of resistance in pests.

Resistance to transgenic traits slows when two or more effective traits are incorporated into a cotton variety. This strategy selects for individuals in an insect population that have resistance to more than one transgenic control element at the same time, which is very rare.

Biological Control

Weather, inadequate food sources, and natural enemies can hold insect and mite infestations below damaging levels. Biological control relies on parasites, pathogens, and predators to help control pests. Recognizing the impact of these natural control factors and, where possible, encouraging their action is a key IPM component.

Natural enemies in cotton include assassin bugs, big-eyed bugs, collops beetles, damsel bugs, ground beetles, lacewing larvae, lady beetles (or ladybugs), minute pirate bugs, spiders, syrphid fly larvae, and a variety of tiny wasps that parasitize the eggs, larvae, and pupae of many cotton pests. Avoiding the use of broad-spectrum insecticides until they are needed helps conserve existing populations of natural enemies and prevents the development of economically damaging pest infestations. Selecting insecticides that are more toxic to the target pests than they are to natural enemies minimizes the impact insecticides have on natural enemies.

Bt Transgenic Cotton

Bt cotton is genetically altered by inserting genes from a common soil bacterium, Bacillus thuringiensis, to make proteins that are toxic to specific groups of insects. For example, currently available Bt traits in cotton specifically target caterpillar pests such as beet armyworm, cotton bollworm, and tobacco budworm. Conventional or non-Bt cotton does not produce such insecticidal proteins and is more vulnerable to worm damage.

Since its introduction into US agriculture in 1996, Bt technology has developed from a single-gene trait to multi-gene trait packages. The first-generation Bt cotton (Bollgard) had a single Bt gene that produced (expressed) Cry1Ac. The second-generation Bt technologies, such as Bollgard II, TwinLink, and WideStrike, produce two Bt toxins. The most recent third-generation Bt technologies (WideStrike 3, Bollgard 3, and TwinLink Plus) are three-gene trait products.

Cotton varieties with third-generation Bt technologies have excellent activity against cotton leaf perforators, loopers, pink bollworm, and tobacco budworm, and good activity against beet armyworm, cotton bollworm, fall armyworm, and saltmarsh caterpillar. Some situations may require supplemental insecticide treatment for bollworm and fall armyworm. Recommended economic thresholds used to trigger insecticide applications on Bt cotton are the same as those used for non-Bt cotton but should be based on larvae larger than ¼ inch.

Scouting

Regular field scouting is crucial to any pest management program because it is the only reliable way to determine whether pests have reached the economic threshold. More than just “checking bugs,” scouting determines insect density and damage levels by using stan dardized, repeatable sampling techniques. It also monitors beneficial insect activity, diseases, fruiting, plant growth, and weeds as well as the effects of pest suppression practices.

Growers or crop consultants should check fields at least once and preferably twice a week to determine what species are present, their density, and the amount of damage. Most pests can be monitored visually by thoroughly checking whole plants or plant terminals.

However, some pests, such as plant bugs (for example, the verde plant bug), are more reliably sampled using a beat bucket, drop cloth, or sweep net (Figs. 3, 4, and 5).

Beat bucket sampling method

• Tilt a 2.5- or 5-gallon white or black plastic bucket toward the plants.
• Grasp the plant stems of two or three representative plants (depending on plant size) and bend them into the bucket.
• Vigorously shake the plants against the side of the inside of the bucket.
• Hit the outside of the bucket several times to knock the bugs to the bottom and quickly inspect the inside of the bucket to count pests.
• Count the adults first because they can fly from the bucket and may be missed during scouting.
• Keep a running total of the number of plants shaken and the adults and nymphs of the insect being monitored.
• Shake a minimum of 40 plants to get an estimate of the number of insects per plant.

Drop cloth sampling method

• Use an off-white or black cloth specific to the row spacing (such as 36 × 42 inches on 40-inch rows).
• Staple a thin strip of wood, approximately 1-inch wide, to the short sides of the cloth
• Select a random site in the field and spread the drop cloth on the ground in the row-middle from one row to the next
• Vigorously shake the plants in 5 feet of row on both sides of the cloth.
• Count the insects that fall into the cloth, including any insects that fall at the base of the plants. This total gives the number of insects per 5 feet of row.
• Repeat the process in at least 16 locations in the field (sampling 60 feet of row).
• If the results show that populations are close to threshold levels, or if the field is very large (more than 100 acres), sample more areas to increase confidence in the results.

Sweep net sampling method

• Use a standard 15-inch canvas sweep net with a handle. One sample should consist of 50 sweeps across a single row of cott However, if you pick up too much plant material in 50 sweeps, reduce the sweeps to 25 or less.
• Walk briskly down the row and swing the net in front of you, perpendicular to the row.
• Strike the plants so that the lower edge of the rim strikes the plants about 10 inches from the top.
• Keep the lower edge tilted slightly ahead of the upper edge.
• Keep the sweeps far enough apart that you do not sweep plants that have already been jostled by the net.
• Keep the net moving to prevent adults from flying out.
• After each set of sweeps, count all the insect stages in the net.
• Go through the sample slowly, counting insects, inspecting each leaf, and watching closely for adults flying from the net.

Sampling predatory insects

Knowing how many predatory insects and spiders are present helps make pest-management decisions, especially for aphids and caterpillar pests. Also, sampling can identify fields at risk of pest outbreaks due to a lack of predators.

The beat bucket method provides the most rapid and accurate estimate of the presence of predators. The sampling method is similar to the steps discussed above:

• Grasp the stem of a single plant near its base and, while holding the bucket at a 45-degree angle, quickly bend the plant into the bucket so that the terminal and as much of the plant as possible are inside the bucket.
• Still holding the stem, rapidly beat the plant against the side of the bucket 12 to 15 times for 3 to 4 seconds so that the predators get shaken from the plants and fall to the bottom of the bucket.
• Remove the plant, take one step, and sample a second plant. Take another step and sample a third plant down the row.
• After sampling the third plant, record the number of damsel bugs, lacewing larvae, lady beetles, pirate bugs, and spiders in the bottom of the bucket.
• Remove and examine leaves and bolls from the bucket to be sure to record all Tap the bottom of the bucket, so predators that are playing dead begin to move and become apparent.

For an accurate estimate, take 34 beat bucket samples per field (3 plants per sample × 34 samples = 102 plants). Divide these samples into 3 to 4 locations per field. Since beat bucket sampling takes more time (about 45 minutes per field), sample when predator information is most useful for making pest management decisions. For example, sampling for predators once at first bloom and again 2 to 3 weeks later yields information for managing mid- and late-season caterpillar pests.