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Why Do Insecticides Fail?

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Sometimes a turf manager will apply an insecticide to control a turf insect problem and feel that the application did not work. In these instances, several questions should be asked.

1. Did the material really fail? If the insect population was not sampled before the application was made, there is no way of knowing how many insects were present before the application. Thus, it will be very difficult to know whether this application "failed." As an example, it is not unusual to have 30 to 40 white grubs per square foot in some turf areas in New England. If a person applies a grub material before taking a count and later returns to this area and finds eight grubs per square foot, he or she might think the application did not work very well. But, if there were 40 grubs at the start, the insecticide killed 80% of the grubs - which is very good!

2. Do you have an accurate identification of the insect? Some pest insects resemble others, and some pests resemble beneficial insects. Meanwhile some insecticides are relatively specific and are effective on a few kinds of insects. If you think you have an aphid problem and in fact you have a mite problem, you will almost certainly select a control option that will not work very well.

In addition, several insects look very similar but have different life cycles. For example, there are five or six species of white grubs in New England, each of which has a slightly different life cycle, behaves differently in the soil, and responds differently to insecticides.

3. Have you chosen an appropriate insecticide? There are many insecticides available for use on turf - traditional, biological, organic, or otherwise - with a range of characteristics. Some work well on white grubs because they penetrate thatch readily, but might be less appropriate for surface feeding insects like chinch bugs. Others are bound up in the thatch and do not reach the grubs but are very effective on chinch bugs.

Some insecticides work very quickly and then break down into inactive forms within a few days. Others take several days to begin having an effect on the target insect but normally remain active for a few weeks. Both kinds of materials can be very useful but must be used wisely.

4. Have you timed your application well? Most insects have a few developmental stages (for example, eggs and pupae) which are virtually untouchable. Few, if any materials will affect these stages. However, small larval stages often are quite susceptible to insecticides. Ideally, an application should target pest insects for when they are in those small, sensitive stages.

Not everyone has the luxury of timing every application perfectly. If an application must be made (because of logistics or scheduling difficulties) early in the period when a turf manager expects an insect problem to develop, he or she probably should use one of the slower acting but longer lasting materials. On the other hand, if an application is made after the insect population has gotten established, one of the faster acting materials probably would be a good choice.

Finally, some insects are nocturnal, so late afternoon or evening applications tend to be more effective. For example, the caterpillars of sod webworms and cutworms are active only at night and remain in their burrows during the day, well away from insecticides and weather extremes. If a turf manager can apply a caterpillar control in the evening, the material will be markedly "fresher" and more effective when the caterpillars come to the surface that night to feed.

5. Have you watered the treated area appropriately? Many insecticides should be watered in, at least to some extent, as soon after application as possible. This serves to move the material somewhat closer to the target insect, as well as to get it further out of reach of direct sun-light. However, studies at Ohio State and Cornell have shown that many of these materials do not penetrate very far into the thatch. It turns out that watering after application alters the behavior of some insects, and draws them into the area where the pesticide remains.

For example, white grub materials should be watered in with at least 1/4-inch of irrigation soon after application. This helps to move the material off the surface and into the thatch. At the same time, the water draws the grubs from the soil into the soil/thatch interface, where their natural foraging behavior results in contact with the insecticide.

Applications of insecticides targeted against surface feeders should be watered in lightly, just enough to get this material off the blades and into the thatch where the target insects are active.

Finally, if weather conditions are extremely dry for an extended period and you are planning to apply an insecticide against a soil insect (like white grubs), irrigating the area before application often will improve the effectiveness of the application. For example, white grubs move down deep into the soil profile (several inches) when conditions are very hot and dry. By irrigating the grub-infested area 24 to 36 hours before a planned insecticide application, a turf manager can trick the grubs into sensing that conditions are improving. Once the grubs have moved back up into the root/thatch zone (in response to the irrigation), the insecticide has a much better chance of working.

Final Thoughts

Many insecticides can work well as long as they are used appropriately. Keep in mind that no chemical will eliminate every insect - so be reasonable with your expectations. Choose wisely, water wisely, and manage wisely.

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Written by: Dr. Pat Vittum

Revised: 05/2011

Topics: 
Commercial Horticulture
Commercial Horticulture topics: 
Insects and Mites
Turf