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Corn Rootworm - Part 2: Corn Rootworm Management

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Chemical Management

Soil Insecticides

Soil insecticides can be an effective management tool against CRW larvae. Soil insecticides are most often applied at planting or during the first cultivation. They come in a granular or liquid form and must be incorporated during application. Depending upon what insects, besides CRW, need to be controlled the insecticide can be applied in a T-band or in-furrow application method. When insects such as wireworm and white grubs need to be managed, along with CRW, then an in-furrow application is best. However, if CRW and insects such as cutworms need to be controlled then a T-band application should be used (Wright, et al., 1999).

Soil applied insecticides control the CRW at the larval stage.  When the insecticide is applied at planting, the residual activity of the chemical is important. Since the CRW larvae do not emerge immediately after the seed is planted, the insecticide needs to remain active in the soil for many weeks (6-10 weeks) until egg hatch occurs in order to be successful (Wright, et al., 1999). Consideration for equipment calibration and weather at the time of insecticide application is important. Wind can adversely affect the efficiency in which the insecticide is applied. Special equipment is sometimes needed when combating windy conditions at the time of application. Wind-shields can be mounted on either side of the bander on the planter and the bander placed closely to the soil, to reduce the amount of drift.

Soil moisture is also critical to soil insecticide performance.  The solubility of an insecticide is important.  In general, the solubility of most soil-applied insecticides are low, meaning they are difficult to move through the soil profile.  However, moisture is needed for them to move at all.  It is interesting to note that too little moisture keeps the product from moving through the soil to come into contact with the hatching larvae and too much moisture can dissipate the product reducing the longevity of the active ingredient.  Adequate soil moisture is important and crucial to effective soil insecticide performance.

The calibration of the application equipment is equally as important as preventing drift. Correctly calibrating equipment is essential in preventing economic loss when applying insecticide to the field. By applying the correct rate of insecticide given the acreage, the producer will maximize the investment in the insecticide. Improperly calibrated equipment can result in a lower or higher rate of insecticide being applied. A higher application rate is not cost effective, does not always equal better control, not to mention it is illegal to apply more than the prescribed rate of insecticide.

Soil Insecticide Modes of Action

Insecticides are categorized into different groups. For CRW, these groups include contact and stomach poisons. Contact poisons are made up of a large group of insecticides that enter the body when an insect walks, crawls, or eats the treated surface. The other group of insecticides is stomach poisons. These insecticides enter the insect through the gut and are only effective if consumed. Some insecticides are systemic or carried into the plant, where as genetically engineered plants produce a protein that acts as a stomach poison.  The Host Plant Resistance topic in this lesson discusses in more detail the use of genetically engineered plants.

Contact poisons can be broken down further, into the following insecticide families: organophosphates, carbamates, pyrethroids, and others including pyrazoles and chloronicotinyls. Some common soil insecticides that are often applied to corn acres for CRW larvae control are: Force® (tefluthrin), Counter® (terbufos), Lorsban® (chlorpyrifos), Aztec® (tebupirmphos + cyfluthrin), Regent® (fipronil), and Capture® (bifenthrin).

Force and Capture are both pyrethroids. Pyrethroids cause disruptions in the central nervous system of the insect which can eventually cause death. The two main advantages this insecticide group has are: 1) highly effective at low rates and 2) less hazardous than other insecticides for the applicator and the environment.

Counter, Lorsban, and Aztec are all organophosphates (OP). OP’s are one of the most hazardous classes of insecticides due to the effect they may have on the nervous system of mammals. Organophosphates cause a key enzyme in insects, (and mammals and birds) cholinesterase, to be inhibited.  This leads to tremors, convulsions, paralysis, and eventual death in the insect.

Regent is in the pyrazoles class of insecticides. Like pyrethroids, pyrazoles cause disruptions in the central nervous system which can cause the insect to die.

Foliar Insecticides

The purpose of foliar insecticides is to reduce the number of beetles that are capable of laying eggs for the next season. Foliar insecticides cause death by contact, so death can occur from the chemical actually landing on the beetles or from the beetles walking across leaves coated with the insecticide. Foliar insecticides are most commonly applied aerially.

 
 Fig. 2:  Example of an aerial pesticide application (USDA)

The timing of foliar application is also very important in the effectiveness of the insecticide. Since male CRW beetles emerge prior to females it is best to wait two to three weeks after the first beetle is found to apply the foliar insecticide. This will allow the best control of the female beetles that will be laying eggs in the soil to produce next year’s CRW population.

Foliar Insecticide Modes of Action

Some of the common foliar applied insecticides that are used to control CRW beetles include: Penncap-M® (methyl parathion), Warrior® (cyhalothrin), Pounce® (permethrin), Ambush® (permethrin), and Dimethoate. Penncap-M is an organophosphate and Warrior, Pounce, and Ambush are all pyrethroids.

There are some restrictions associated with certain foliar applied insecticides.  Therefore, the label must be carefully read and followed.  Some insecticides restrict the use to only times when the plant is not shedding pollen. Although these insecticides may be very effective in controlling CRW, these restrictions can cause application-timing issues and ultimately reduce the effectiveness of the insecticide.

The timing of foliar application is also very important in the effectiveness of the insecticide. Since male CRW beetles emerge prior to females it is best to wait two to three weeks after the first beetle is found to apply the foliar insecticide. This will allow the best control of the female beetles that will be laying eggs in the soil to produce next year’s CRW population. Some of the common foliar applied insecticides that are used to control CRW beetles include: Penncap-M® (methyl parathion), Warrior® (cyhalothrin), Pounce® (permethrin), Ambush® (permethrin), and Dimethoate. Penncap-M is an organophosphate and Warrior, Pounce, and Ambush are all pyrethroids. There are some restrictions associated with certain foliar applied insecticides.  Therefore, the label must be carefully read and followed.  Some insecticides restrict the use to only times when the plant is not shedding pollen. Although these insecticides may be very effective in controlling CRW, these restrictions can cause application-timing issues and ultimately reduce the effectiveness of the insecticide.

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