Schizaphis graminum (Rondani)
Figure 7.7 Greenbug.
Identification / Life Cycle
Greenbug is a pale green aphid approximately 1/16 of an inch long with a darker green stripe down the center of the back. Greenbug colonize wheat fields when winged females migrate from native grasses or other wheat fields. These females produce nymphs which give rise to multiple generations of parthenogenic wingless aphids. As wheat matures, winged females develop and migrate elsewhere. Males are produced in late summer and fall. Sexual reproduction does occur, but in most states, eggs are either infertile or are not subjected to sufficient cold hardening to hatch. Greenbug nymphs produced from eggs are more likely to be found in the northern states.
Plant Damage and Response
Greenbug damages wheat by sucking phloem fluid from the plant and injecting toxins. Plants damaged by greenbug have yellowed leaves that turn brown and necrotic as damage increases. Moderate to heavy greenbug infestations at any time will reduce yield through direct damage to the plant and indirectly in young wheat by impacting root development. With high greenbug density, seedling to mid-sized wheat plants can be killed. Early to mid-season greenbug infestations are often noticed as circles of yellowing or brownish wheat within the field. As the season progresses, these “greenbug spots” coalesce into larger areas.
Establishing level of risk
When greenbugs are present, careful scouting and monitoring can determine if a field is a candidate for chemical control. Various sampling methods can be used to assess the status of a wheat field in regard to greenbug infestations. An evaluation of potential greenbug control by natural enemies is encouraged before making any deci sions to apply insecticides. The most important determination to make is whether or not greenbug infestation is increasing over time.
The Cereal Aphid Expert System and Glance-N-Go sampling system are recommended for greenbug. Glance-N-Go is a greenbug scouting system used to rapidly tell if a wheat field is at risk for economic damage from greenbug. Walking in a zigzag pattern across a wheat field, wheat tillers are sampled to see if greenbugs are present, and the findings are recorded on standardized data sheets. If greenbug density is very high, the decision to treat might be made by taking as few as five, three-tiller samples spaced 30 feet apart. If greenbug density is low or spotty, a sample of as many as 90 tillers may be needed to make a decision. Results for every 15-tiller sample are com pared on the data sheet, and a decision is made whether: 1) more samples are needed, 2) sampling can be stopped and the field does not require treatment, or 3) sampling stops and a decision to treat is made. Information on the Glance-n-Go system can be found at http://my.iwheat.org/pest_sampler (must be signed in to use); the orginal version can be found at http://entoplp.okstate.edu/gbweb/. The most recent version of the Glance-n Go system also incorporates the presence of parasitoids into the decision process, but now you can use it on your mobile device!
Economic thresholds for greenbugs vary depending on the time of the year, plant vigor, climatic conditions, and the presence of natural enemies. As a general guide, it is recommended that chemical control may be considered if greenbugs are found at a level of 100 to 200 per linear foot of row for plants three to six inches high, 200 to 400 aphids per linear foot of row for plants four to eight inches high, and 300 to 800 aphids per linear foot of row for plants ranging from 6 to 16 inches in height.
On the other hand, the Glance-N-Go system uses a sliding scale that depends on the season (fall or spring) and the number of aphids recorded per 15-tiller sample. The decision-making aspect of Glance-N-Go is based on multiple years of field research which determined the ratio of tiller infestation levels to actual field infestation levels. It also incorporates a preselected threshold that users can determine using the Cereal Aphid Expert System.
Because greenbug is more exposed on the leaves, it is generally more easily controlled than Russian wheat aphid. However, some greenbug populations have shown resistance to organophosphate insecticides, and treatments also may be ineffective when weather is cold. Thus, multiple applications of the same insecticide class during the same season should be avoided, and applications should be made when weather is expected to be favorable (highs above 50°F or 10°C and no rain) for a few days following the application.
As with Russian wheat aphid, a healthy and well-watered crop can endure a heavier greenbug infestation than one suffering from water or nutrient stress. Although control of volunteer wheat probably has some impact on greenbug over-summering populations, greenbugs readily infest sorghum, and some common grasses (such as Johnson grass), throughout the summer.
Greenbug biological control by native predators and parasitoids is identical to that found for Russian wheat aphid. Conservation of naturally occurring predators and parasitoids can often preclude the need for chemical control. This aspect of the aphid’s ecology is incorporated into the Glance-n-Go management system.
Host Plant Resistance
Wheat varieties and cultivars resistant to greenbug have been available for many years, but the number of resistant varieties is often very limited. Currently available varieties include TAM-110 and TAM-112. Be sure to check current variety descriptions as new resistant lines are constantly under development.