Jun 29, 2020Learn about aphid management in vegetable crops
Aphids are one of the most common pests across many crops, therefore it’s worth learning a bit about them to be able to improve their management.
Talking about aphids is especially timely because hot summer days favor their quick reproduction and the weather fronts that arrive from the south may bring migratory aphids, dumping them onto crops in the northern U.S.
In Michigan, commonly occurring aphids are the green peach aphid, potato aphid, melon aphid, bean aphid, cabbage aphid and the sunflower aphid. The cabbage aphid feeds only on plants in the cabbage family, but the others can feed on a range of different plants. Since aphids can reproduce without mating, their populations can grow quickly, especially when the temperature is warm. They have both winged and wingless forms, meaning that the same species of aphid can sometimes have wings and other times they are wing-free. When ample food is available during summer, they are usually wingless.
Direct feeding on crops
We often see individual winged aphids early in the season scattered on plants, but these usually don’t lead to aphid outbreaks in the crop. Many of these winged aphids either die or move on from the crop field. However, during late June through August, winged aphids can settle down and start giving birth to wingless daughters that lead to forming aphid colonies. Initially, the aphid colonies cover the back of a leaf, but they can grow and get so large that the plants become deformed due to the feeding of thousands of aphids. These plants are eventually covered entirely in aphids and die.
Colony infestations are usually found scattered throughout a field in hot spots, thus scouting multiple areas and plants in a field is essential to uncover the problem. Looking for deformed plants instead of individual aphids can also help discover colonies in a field.
Another cause for concern regarding aphids is their ability to transmit plant viruses. Since aphids can migrate long distances on wind currents, some of these plant viruses can migrate with the aphids. Many viruses can be picked up during just one brief feeding and transmitted to plants right away. In other instances, it takes a few days for aphids to become infectious, depending on the type of virus-aphid combination.
It is important to know what type of virus the crop is sensitive to in order to find the best management method. For example, virus infection can lead to load rejections in seed potato, therefore measures need to be taken to stop any aphids before they start feeding. In cucumber and squash, there are virus resistant varieties. For other virus-sensitive vegetables, planting as early as possible and using reflective (silver) plastic mulch can delay aphid colonization.
Aphids can usually be controlled with insecticides. However, when broad-spectrum foliar insecticides (e.g., pyrethroids) are used, they kill beneficial insects that eat aphids, such as lady beetles, hover flies etc. Unfortunately, the aphids are then left without any natural controls, which can lead to out-of-control aphid outbreaks. Further, many species of aphids are notorious for their ability to overcome the effects of insecticides. These resistant aphids are not killed by the insecticide.
Resistance can be caused by the frequent application of the same insecticide over and over. One way to avoid this is to rotate insecticide classes—this means changing up the toxin so aphids don’t get used to it. Another course of action is to spray stylet oil weekly or twice weekly, creating a physical barrier on the plant surface instead of a chemical effect on the insect.
There are several naturally occurring beneficial insects that feed on aphids, such as lady beetles, lace wings and hover flies, but these need to be protected from foliar broad-spectrum insecticide applications. While insecticides and beneficial insects often seem incompatible management approaches, there are insecticides available that are less harmful to beneficial insects.
Instead of a foliar application, there are insecticides that can be applied on the seed, soil or roots and the plant takes up the insecticides, thus the beneficial insect is not likely to encounter the insecticides. Some insecticides are more selective in how they kill insects; these are sprayed on the leaves and paralyze the mouthparts of aphids when they start feeding on it, but they leave other insects unharmed. Some biopesticides, such as Grandevo, and Botanigard, have a stronger effect on aphids than on their natural enemies.
The insecticides listed below may not be labeled for all vegetable crops. Visit the Midwest Vegetable Production Guide to learn more.
Soil-applied materials that are systemic in the plant:
- Platinum, thiamethoxam, IRAC 4A
- Verimark, cyantraniliprole, IRAC 28
Foliar-applied materials with some translaminar activity in the plant:
- Agri-Mek, abamectin, IRAC 6
- Exirel, cyantraniliprole, IRAC 28
- Harvanta, cyclaniliprole, IRAC 28
- Movento, spirotetramat, IRAC 23
Materials with some translaminar residual activity in the plant that can be soil or foliar applied:
- Admire Pro, imidacloprid, IRAC 4A
- Belay, clothianidin, IRAC 4A
- Scorpion/Venom, dinotefuran, IRAC 4A
- Sivanto, flupyradifurone, IRAC 4D
Foliar-applied mouth paralyzers that target aphids specifically:
- Beleaf, flonicamind, IRAC 29
- Fulfill, pymetrozine, IRAC 9C
- PQZ, pyrifluquinazon, IRAC 9B
- Inscalis, afidopyropen, IRAC 9D