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 When a product doesn’t work and the lawn care operator is sure that the label instructions have been followed accurately, the first thoughts are those of pest resistance. Resistance and resistance management is very important when dealing with fungicides and certain insect/mite pests that often require multiple applications in a season or during the growing of a crop.
But when it comes to the insects and mites that attack ornamental plants and turf, resistance is actually a fairly rare event.
Many of the newer insecticides and miticides tout long residual action, so the applicator may not pay as much attention to his application timing. He might be mixing two or more products that actually counteract one another. Or, he might be spraying the right stuff, but in the wrong place or on top of a thick layer of thatch.
Here are seven important areas to watch.
1. Timing is still important
While it is true that many of our newer insecticides have very long residual action, optimal timing of an application to get the maximum amount of pesticide to the target pest is extremely important.
When dinotefuran first hit the market for scale control, entomologists were saying that you could apply it almost any time in the season and expect excellent scale control. However, field usage demonstrated that timing was still important. The product is rapidly taken up by the plants, but it also degrades relatively rapidly. If you make the application in the summer, when some scales stop feeding, the residues may be gone by the time that the scales resume feeding in the fall.
2. Application technique is important
When I ask lawn care folks, “When was the last time you calibrated your fancy ride-on spreader-sprayer,” I often get silence or blank looks. While you can cover large areas in a short period of time with these spreaders, the area covered isn’t always evenly covered. Like all spreaders and sprayers, they have to be periodically calibrated. This issue is also important for the older hand-pushed spreaders and hand-walked sprayers. Technicians often walk briskly early in the morning and right after lunch, but they can be fairly slow before lunch and late in the day. These applications can vary as much as 20 percent, sometimes more.
Some insecticides have active surface residues and others have translaminar systemic action. If the pesticide is only active on the surface onto which it is sprayed, a spray on the upper leaf surface won’t kill aphids or lace bugs on the lower leaf surface. You will actually have to spray up to hit the lower leaf surfaces. If the insecticide is absorbed into the leaf tissues (translaminar systemicity), only the upper leaf surface needs to be hit, as the aphids and lace bugs will ingest the pesticide as they feed.
3. Tank incompatibility is important
There are two main types of tank incompatibility: chemical and formulation. Chemical incompatibility means that the chemistries of the materials being mixed either degrade or deactivate one or more of the ingredients. Formulation incompatibility means that the special surfactants, suspending agents or dispersants reacted to cause one or more of the chemicals to drop out of suspension (resulting in settling residue or surface layers) or grease-ball (glob together into gunky messes that foul the spray system).
4. What happens after an application is important
Insecticides intended to do their work in the soil-thatch zone of turf need to be watered into this zone. On the other hand, surface and translaminar insecticides need time to dry on leaf surfaces or penetrate the underlying tissues. Providing sufficient time for these events to occur is important and the applicator may need to turn off irrigation systems or wait until after expected rainfall events.
5. Formulation and shelf life are important
As more and more of our insecticides and miticides go off patent, we see more generic formulations. Be sure to verify the efficacy of the formulation you buy – granular products shouldn’t blow away and liquids should suspend easily. And consider the technical support behind the product you’re purchasing. If it doesn’t work, who can you call for help?
When purchasing liquid formulations, ask to see a container of old product. If it appears to have considerable settled residue in the bottom, shake it three to four times. If the stuff in the bottom doesn’t go into suspension rapidly, then you know that this formulation has a relatively poor shelf life.
Ask your vendor for shelf life information on products you are considering. Some common grub insecticide granules have a shelf life of about 18 months. Old product that has no active ingredient, no matter what the price, is no bargain. It’s better to try and purchase sufficient product for a single season than having lots of products resting on the shelves through the winter.
6. Pest pressure can overwhelm the best insecticides
Remember that no pesticide will be 100 percent effective 100 percent of the time. Consider a grub insecticide that gets 90 percent control. With populations of 50 grubs per square foot hot spots, if you kill 90 percent, you will still have five grubs per square foot. While this is not enough grubs to cause visible turf damage, it is sufficient to encourage skunks, raccoons or armadillos to forage. In areas where you have very mobile pests, like the fall webworm on trees, you can treat the first generation and have excellent control, but the second generation of moths can fly in from surrounding areas after your effective residues have disappeared. In turf, this is a common event with sod webworms, armyworms, cutworms, chinch bugs and the annual bluegrass weevil. All these pests have multiple generations and they can be very mobile during the season.
7. Thatch is important
Thatch is that somewhat forgotten sponge that has plagued turf managers for years. Where conditions are right (e.g., turf species, heavy clay soils, no earthworms, heavy fertilizer, etc.), thatch can rapidly build up on top of the soil and this organic layer can act as a blotter paper to stop any pesticide from getting to the soil-thatch interface. Loose thatch, like that found in dense St. Augustine grass, can also serve as a hiding place for southern chinch bugs and hunting billbugs. This kind of thatch also binds insecticides, making them relatively inactive.
Where pests are resistant
With all that said, we do have some turf and ornamental pests that are known to be resistant. There are areas in Florida and Texas where the southern chinch bug has demonstrated resistance to pyrethroid insecticides and they have overcome the genetic resistance of some St. Augustine grass cultivars. Why? It appears that some lawn care companies were applying the same insecticide five to seven times per season. Multiple generations of a pest exposed to the same insecticide will soon result in resistance.
Entomologists are strongly recommending resistance management techniques – alternation of insecticide modes of action and reducing the numbers of applications in a single season.
We also have whitefly and twospotted spider mite resistance in ornamental trees and shrubs. Why? These are the same pests that attack greenhouse plants where they are commonly subjected to the same pesticide, repeatedly, without rotation of chemistries. When these resistant insects and mites are shipped out on bedding plants or other nursery stock, they can establish in our landscapes. Basically, they come to our landscapes already resistant. In this case, it’s not the fault of the landscape manager. On the other hand, if you try a pesticide on these kinds of pests and you don’t get control, don’t go back with the same material. You will need to switch to a different chemistry and see if that is effective.
The author is professor of urban landscape entomology at The Ohio State University.
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