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Scouting For Soil Insect Pests

A back-to-the-basics approach to often overlooked, but costly pests.

By Dr. Ayanava Majumdar “Dr. A,” Extension entomologist and state sustainable agriculture coordinator, Auburn University; and Rudy Yates, regional Extension agent, Alabama Cooperative Extension System print email

The year 2010 was difficult for peanut producers because so many of them were affected by insect pest outbreaks. Although we have new high-producing varieties of peanuts, these varieties are prone to soil insect attack and Aflatoxin contamination – a major cause of concern for the peanut industry. Last year’s drought from June to August caused plant stress that made them more susceptible to pest attack.

Foliar insect pests are more easily seen, but soil pests are cryptic and tend to be overlooked. Lack of insect monitoring and crop scouting also results in late detection of insect outbreaks. Soil pests build up in numbers every few years and outbreaks happen in a cyclical fashion facilitated by inclement weather.

Monitoring for insect pests involves the use of one or more scouting techniques for a reliable estimate of pest population, migration and suppression following management efforts. Scouting is the first step in adopting integrated pest management (IPM), so this article focuses on providing peanut producers a quick overview on scouting methods for soil insect pests.

Scouting Objectives
Scouting should be done with a clear objective in mind, and the procedure must be cost-effective and fast. Scouting may be done to find the following: Absolute insect numbers or relative insect numbers.

For absolute insect numbers, scouting is used to determine the exact number of insects in a predefined area, e.g., insect inside one plant shoot. Absolute estimates can provide valuable information regarding the association of pest populations with crop injury.

The problem with absolute estimates is that it is time-consuming for growers and crop advisors who want to make rapid pest management decisions for numerous acres of peanuts.

Relative insect numbers is the most popular scouting method because of its flexibility. Here, the focus is timely diagnosis of infestation levels so that pest management decisions can be made quickly.

Relative estimates are based on the principle that if a large population of an insect pest is present in an area, then it must be causing a measurable effect on the plants. Besides the direct feeding injury on peanut pods, certain by-products, such as exuviae (molted skin), frass (excrement) and silk tunnels, also provide useful information about infestation and activity.

A drawback of relative estimates is that accuracy depends on the resources invested into scouting and sampling; the greater the number of samples, the more reliable the estimate will be. Improper scouting will generally lead to hasty pest management decisions most likely resulting in the unnecessary use of insecticides.

Scouting Techniques
Monitoring or scouting techniques for estimating relative insect numbers are as follows:

1. Examine the pod for insect injury. This method is technically called population indexing. Although assessment of injury on plant parts can be done rapidly, it is often difficult to correlate actual pest population levels with crop damage.

In peanuts, soil insects can be scouted by carefully looking for evidence such as webbing in the case of lesser cornstalk borer infestation, “pitting” on kernels by burrower bugs and the presence of a fine entry hole on pods made by rootworm larvae. Wireworms will create large feeding holes on one end of the developing peanut pods and contaminate the kernels with soil and other debris.

In a future article in Peanut Grower, we will provide additional details about the lesser cornstalk borer statewide survey in Alabama that has provided some interesting trends over the years. Based on 2010 research, it will suffice to state that peanut growers must constantly take shovel samples throughout the season and examine the soil as well as pegs directly to monitor pest infestation levels.

Crop rotation is one of the most effective ways to counter insect pest buildup and must be carefully planned by peanut growers in high risk areas. Producers in Alabama should use the LCB Day Tabulation and Aflatoxin Risk Index feature on the AWIS Weather Services Web site at www.awis.com/Misc/Peanut/Peanut.

2. Use insect pheromone traps for moths. Traps utilize natural insect behavior to lure specific pest species.

A variety of insect pheromone traps are commercially available to assess the population and activity levels of mobile insect species, for example, the lesser cornstalk borer moths. Traps allow easy visualization of this insect even at low numbers.

The accuracy of pheromone traps depends on lure quality (pheromone release rate and chemical stability), trap placement and maintenance and weather conditions. In general, traps are very effective monitoring tools, but they do not provide information about actual crop injury.

Insect monitoring with pheromones in high risk areas along with direct crop scouting is the ideal way to monitor the insect threat to peanuts. Pheromone traps are inexpensive compared to dollars invested in late-season pest control, which does little to rescue the crop.

3. Use germinating seed baits for wireworms. This is a useful technique for sampling activity of wireworms in soil before and during the season. The germinating seed bait technique has been used successfully in agronomic crops, vegetable crops, pasture and fields under the Conservation Reserve Program because this method is relatively easy to set up and monitor.

The process involves the deployment of pre-soaked seed mixtures (wheat and corn) into six-inch deep soil pits and then covering the seed with black tarp to facilitate seed germination. A handful of seed bait should be put at several locations throughout the peanut acreage; carbon dioxide and heat given off by the seeds will attract wireworms. Excavate each seed bait in 10 to 12 days, and examine it for wireworms. One wireworm or more per trap is an indicator of a high infestation level.

There is no doubt that it is important to scout for below-ground insect pests since populations can build up rapidly and devastate the peanut crop late into the season. Timely detection of soil insects by direct plant study helps in making the right IPM decision and avoiding economic losses in peanuts.

Follow the IPM pest management guidelines in your state, and always read and follow the pesticide label before using a product. PG

For more information, contact Dr. A at 251-331-8416 or by email at bugdoctor@auburn.edu. More information can be found at the Web site www.aces.edu/go/87 and Facebook page: Alabama Peanut IPM Program.

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