(News article for May 21, 2022)
The first article in this series, which focuses on prevention of insect pest problems, can be found here.
Last week’s article addressed chewing insects, including caterpillars and beetles, that make holes in plants. This time, I’ll address insects and mites that injure plants by sucking sap from them.
Aphids (“plant lice”), whitefly nymphs, true bugs (such as stink bugs, leaffooted bugs, squash bugs, and tarnished plant bugs), spider mites, broad mites, and thrips have mouthparts commonly described as piercing-sucking or, in the case of thrips, rasping-sucking. Their injury often appears as distortion or stippling (tiny, light-colored spots).
Some aphids, whiteflies, and thrips transmit certain plant viruses. Some – including aphids, whitefly nymphs, soft and felt scales, and mealybugs – excrete the sugary substance honeydew on which sooty mold fungi grow.
As with chewing insects, the effectiveness of different insecticides varies among the different insects and mites, so accurate identification of the culprit is important.
Aphids can often be dislodged with a moderately strong spray of water.
Insecticidal soaps and oils (including both mineral oils and plant-based oils) can be used against some soft-bodied insects like aphids, whitefly nymphs, and spider mites, if you can get good coverage of plants, for direct physical contact with the insects. These pests are often found on the undersides of leaves, so getting good enough coverage for direct contact can be challenging in some cases. These insecticides are safer for beneficial insects than many other products are, because they lack residual activity.
Be sure to observe any label directions about not using oils when temperatures excessively hot or cold and not applying them within a certain amount of time of sulfur application.
Products containing imidacloprid are other options for aphids, whiteflies, and leaf-feeding thrips. A number of imidacloprid-containing products are labeled for trees and shrubs but not vegetables. However, at least a couple of imidacloprid-containing home garden products are labeled for one application to the soil around certain vegetables at the time of transplanting or after seedlings come up. Imidacloprid, unlike most insecticides, can move from roots to aboveground plant tissues.
For thrips, the spinosad-containing products mentioned last week for caterpillars are another option. Horticultural oils are also recommended for thrips.
Effective options are more limited for true bugs. The pyrethroids (active ingredients ending in "-thrin," plus esfenvalerate) typically have efficacy against them. As mentioned last week, these pose more risk to beneficial insects – including predators and parasitoids that help keep pest insects in check, as well as bees – than some of the other mentioned products. To minimize the threat to bees, avoid spraying plants while they’re flowering, and spray at times when bees aren’t active.
Make sure any product you plan to use is labeled for use on the vegetable(s) on which you plan to use it, and read and follow all label instructions.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Aphids on a pepper plant. (Photo by M.H. Ferguson)
Leaffooted bug on a tomato. (Photo by M.H. Ferguson)