Management of Aphids and Thrips on Leafy Vegetables
John C. Palumbo
Research has been conducted at the Yuma Agricultural Center for the past 5 years to gain an understanding of aphid and thrip population dynamics in spring lettuce, spinach and cole crops. This information coupled with insecticide efficacy studies has allowed us to formulate recommendations for managing these serious pests of leafy vegetables. Provided below is information on species composition, sampling and chemical control of aphids and thrips. This paper should provide guidelines for pest control advisors and growers in making management decisions.
As temperatures begin to increase in January and February, aphids rapidly colonize untreated crops and weeds. On leafy vegetables, the aphid complex will consist primarily of green peach aphid, Myzus persicae, potato aphid, Macrosiphum euphorbiae and turnip aphid, Lipaphis erysimim. On cole crops, one can also expect to find cabbage aphids, Brevicoryne brassicae. Because of their ability to contaminate harvestable plant parts, preventing aphids from colonizing plants is critical. Cultural management tactics and natural enemies can reduce the impact of aphids, but control with insecticides is usually required to prevent economic damage in spring crops. Provided below is information summarized from research conducted over the past few years that outlines approaches for managing aphids in leafy vegetables.
Fields should be checked at least twice a week beginning in January. Ideally, a sample of 25 plants in each quadrant of a field should be sampled. Aphids tend to be prevalent along upwind field borders and next to other leafy vegetable crops or weeds, so initial sampling should be focused in these areas. Because aphid populations are generally clumped within fields, each field should be uniformly sampled.
The use of yellow sticky traps can be used to monitor aphid movement into fields. Research conducted in lettuce over the past years has consistently shown that colonies of wingless aphids on plants were observed immediately following sharp increases in the number of winged aphids caught on traps in the spring. Used properly, yellow traps placed within fields near upwind edges, can provide an early indication of when economic colonization by aphids is beginning. However, proper identification of aphid species is important because many aphid species are dispersing to wheat and alfalfa also (pea aphid, blue alfalfa aphid, greenbug, etc.). If necessary, contact an Extension agent or specialist for assistance in identification.
Several predators and parasitoids attack aphids on leafy vegetables. However, natural enemies rarely provide adequate control of high field populations in spring crops. Consequently, control with insecticides is often the only viable alternative to preventing aphids from contaminating harvested products. Below is a summary of management alternatives for aphids in leafy vegetables based on replicated research trials:
Prophylactic Soil Treatment Approach
Responsive Foliar Approach
If a grower decides not to apply an Admire treatment at planting there are options that can provide adequate control of aphids. Studies conducted at the Yuma Agricultural Center over the past 3 years have shown that several foliar insecticides are still effective against aphids on leafy vegetables. Combinations of older products such as Orthene, Endosulfan, Metasystox-R, Dimethoate and pyrethroids can provide suppression of aphid populations on lettuce and cole crops with limited residual. Metasystox-R is particularly effective against cabbage aphids. Repeated applications will probably be necessary, depending on time to harvest and aphid pressure. Provado (foliar formulation of Admire) also provides a foliar alternative to Admire. It can suppress aphid populations for 7-10 days and is very effective when combined with Endosulfan or Metasystox-R. None of these products provides a quick, rapid knockdown of established aphid colonies (ie., phosdrin ) and their reentry intervals and pre harvest intervals vary, depending on rates and crops. Note: Always consult the products label before recommending or applying any insecticide.
Thrips are present season long in leafy vegetables, but are usually most abundant during the spring after temperatures begin to increase. They are most important in head, leaf, romaine and baby mix lettuces, cabbage and spinach because of the cosmetic scarring they cause to leaves and contamination of harvested plant parts. Thrips can build up in weedy areas, and other surrounding crops, moving to lettuce in large numbers when host plants begin to dry down. Further, once adults disperse onto plants, they can readily reproduce and rapidly colonize in high numbers. We are uncertain what the developmental rate of thrips is on leafy vegetables, but field observations suggest that they can complete development from egg to adult in less than 3 weeks when temperatures are near 70°F.
Western flower thrips, Frankliniella occidentalis and Onion thrips, Thrips tabaci are the main thrips species that occur on leafy vegetables. Because western flower thrips are generally more difficult to control with insecticides than onion thrips, it is important to correctly identify species composition. Identifying thrips can be very difficult because of their small size and similarities in color. Adult western flower thrips are about 1/20 in. (1.5 mm) in length and immatures are generally light yellow in color. Western flower thrips have reddish_orange ocellar pigmentation and eight_segmented antennae. Onion thrips are slightly smaller than western flower thrips, being only 1/25 in. (1.2 mm) long and their body is yellow with brown blotches on the thorax and abdominal terga. The legs are yellowish_brown and the antennal segment I and the base of segments III to V are brownish_white, the rest of the antenna is brown. Their ocellar pigment is gray, and they have seven_segmented antennae. If necessary, contact an Extension agent or specialist for assistance in identification.
Like aphids, thrips can disperse onto crops at anytime, thus it is important that fields be monitored regularly. Thrips can generally be found throughout the plant, feeding on the undersides of leaves, but prefer to hide in complex plant parts, flowers and other folded tissue where they are difficult to detect and reach with insecticides. There are several methods for sampling for thrips on leafy vegetables:
Cultural management has only a limited impact on thrips populations because of their ability to rapidly disperse from native vegetation, weeds and crops. Further, there are few natural enemies that feed on them. Consequently, control with insecticides is often the only viable control alternative. The following points should be considered when attempting to chemically manage thrips populations in leafy vegetables:
This is a part of publication
AZ1101: "1998 Vegetable Report," College of Agriculture, The University of
Arizona, Tucson, Arizona, 85721.