THIS MONTH’S GUIDE concentrates on Myzus persicae (green peach aphid, or GPA). Each month, we bring you a different crop protection issue, ranging from weeds and diseases to insects and even wildlife.
We’ve reached out to pathologists to learn how to spot and treat this pest. This month, our contributors are Ricardo Bessin, University of Kentucky; Thomas P. Kuhar, Virginia Tech University; and John C. Palumbo, University of Arizona. Although we originally looked at focusing on aphids in general, we learned that of the many aphid species, GPA is one of the most important. It tolerates older insecticide chemistries and its population levels rapidly increase. As a result, our contributors say GPA can wreak havoc on vegetable crops more effectively than many other aphid species.
Scientific name: Myzus persicae
Common name: Green peach aphid (GPA)
Geographical Range: GPA originally hails from Asia and can be found in the desert wherever vegetables are grown. Distributed widely across the U.S., GPA travels long distances, carried by wind and storms. GPA adapts better to colder climates than melon aphid, another species causing issues for vegetable growers in warmer climates.
Crops affected: Entomologist John C. Palumbo, University of Arizona, says GPA can develop on hundreds of host plants in more than 40 plant families. It attacks the following desert vegetable crops: artichoke, broccoli, cabbage, carrot, cauliflower, cantaloupe, celery, cucumber, fennel, kale, kohlrabi, lettuce, mustard, parsley, parsnip, pepper, potato, radish, spinach, squash, tomato, turnip, and watermelon.
GPA not only attacks vegetable crops. It also causes problems for ornamental and landscape crops.
Kuhar: In temperate regions, aphids overwinter in the egg stage on host trees. Then in the summer months, GPA flies to vegetables and other plants. They may feed, mature, and reproduce parthenogenetically (asexually) on these hosts all summer. Or they may produce winged females that disperse to other host plants and form new colonies.
This can go on all season long. Under high-density conditions, deterioration of the host plant, or in the fall, production of winged forms predominate. With their ability to reproduce asexually, populations can build up quickly on plants.
GPA on vegetables has an elevated significance because growers frequently apply synthetic pyrethroids. While these broad-spectrum insecticide sprays control a number of important insect pests, they are also highly toxic to beneficial insects, many of which feast on aphids. This loss of natural enemies can intensify secondary pest outbreaks. That’s particularly true if the pest insect has developed resistance to the insecticides used. GPA is resistant to pyrethroids. Multiple pyrethroids applications routinely lead to aphid outbreaks.
Palumbo: Excessive feeding by heavy GPA infestations can stunt plant growth, and GPA can vector viruses that are pathogenic to most leafy vegetables. However, it is the contamination of harvestable plant material (e.g., lettuce heads, celery hearts, spinach leaves) by the aphids themselves that makes them economically important. Contamination of leafy vegetables with just a few aphids can often downgrade quality, or even render the product unmarketable. Once aphids begin colonizing plants, they can be problematic, particularly close to harvest.
Bessin: We also have issues not with colonizing aphids, but the winged aphids that serve as vectors of viruses. As they move through fields and probe with their piercing sucking mouthparts, they can transmit some types of plant viruses. For us in Kentucky, this can happen at any time of the year, but is more common in late summer and fall.
Colonies of aphids on leaves and stems also will produce significant amounts of honeydew, which can promote growth of sooty mold and reduce the appearance and quality of otherwise sound produce.
Kuhar: As large GPA populations feed on leaves, they excrete large amounts of honeydew, which causes sticky fruit and a decrease in marketability. This also leads to the growth of black sooty mold fungus on the fruit and the leaves.
Palumbo: The most effective treatment is selective insecticides that are easy on natural enemies.
There are two approaches to GPA control in the desert southwest. The first management approach involves the soil application of a systemic neonicotinoid insecticide imidacloprid (Admire Pro). Growers can achieve long residual control of GPA in leafy vegetables and cole crops by a single, at-planting soil application. Through root uptake, the application provides significant reduction of aphid colonization on winter crops for up to 75 days. An industry standard since 1993, this prophylactic approach is applied on as much as 80% of the head and leaf lettuce acreage planted annually in the Arizona and California deserts.
The second approach to aphid management in desert-growing areas is a preventative foliar approach. Growers routinely treat fields not planted with imidacloprid with foliar insecticides when they detect an aphid colonization (see above). Apply foliar sprays for aphid control based on a nominal action threshold — when an average of 10% of plants have aphid colonies (with four or more immature aphids) present. Sample plants five to seven days following sprays and retreat if the threshold is exceeded again.
Bessin: Recently there have been a number of more selective insecticides registered for vegetable crops that effectively control aphids. This includes insecticides in the IRAC 4A, 4C, 4D, 9B, 9C, and 29 classes. Availability will depend on the vegetables needing treatment. With some of these products on some vegetables, systemic treatments can provide extended control, but producers need to follow pre-harvest intervals carefully.
Cultural Practices: Growers can sometimes avoid aphid infestation problems with effective cultural practices like sanitation, including removing crop residues immediately after harvest. Aphids can also be abundant on weeds, so proper weed control in and around crops may help prevent aphid buildup and dispersal onto adjacent crops.
“Reflective plastic mulches can help to suppress aphid populations under open-field conditions,” Kuhar adds. “Light-colored mulches (e.g., silver, white) reflect short-wave light, repelling flying aphids, resulting in fewer aphids landing on and colonizing crop plants. Research has shown that aphid densities and incidence of virus diseases transmitted by aphids is lower, and crop yields are higher, where reflective mulch is used.”
Natural/Biological Control: Natural enemies including common lady beetles, lacewings, syrphid flies, and many parasitic wasps can help suppress aphid populations infesting leafy vegetables. Cultural practices such as planting companion crops (i.e., sweet alyssum) that conserve and enhance natural enemy populations can also be helpful in keeping aphids from colonizing crops.
“Horticultural crop oil has been shown to suppress aphid populations and has minimal impact on predators,” Kuhar says.
Monitoring/Sampling: Scout fields thoroughly once stands are established. Pay close attention to field margins nearest the direction of prevailing winds.
Aphids often begin colonizing plants with the movement of a few winged females that give birth to live nymphs in the fields. Many of these offspring will become mature, wingless aphids that in turn will deposit more live nymphs. It is typically these wingless nymphs that cause the most problems with the contamination of leafy vegetables.
Infestations can develop quickly when weather conditions are favorable, and fields should be scouted frequently — at least two to three times per week.