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Update on Aster Leafhoppers in Wheat

by Ian MacRae, Jan Knodel, Bruce Potter, Jochum Wiersma

High populations of Aster Leafhopper (also called 6-spotted Leafhopper) have been reported in small grains over the past couple of weeks. Starting in the south but now spreading to northern MN and ND. Aster Leafhoppers are greyish leafhoppers; the adults have clear wings and 6 spots between the compound eyes (Figure 1). Other than their coloration, the adults and nymphs both very much resemble potato leafhopper. The leafhopper uses it's piercing sucking mouthparts to feed on the plant's sap. The damage caused by Aster Leafhopper feeding is more localized than that produced by potato leafhopper. Feeding may produce localized necrosis or stippling (Figure 2), however, damage is much less than that caused by the Potato Leafhopper.

While they may overwinter as eggs in parts of MN, the sudden arrival of large populations of adults, together with the lack of nymphs present, indicates they arrived here from somewhere else. Like many other snowbirds in this area, part of the northern plain's populations overwinter somewhere warmer and return to the north when the weather once again becomes bearable! Once active in the region, Aster Leafhoppers feed on a wide variety of grass and broadleaf plants, crop and non-crop alike. Adults may move between host plants and follow what's green and available.

Disease Vectors - These insects can be economically important in wheat when they vector of Aster Yellows (AY). Feeding injury of aster leafhoppers is less important than disease vectoring. AY has a very wide host range and causes economic losses in several vegetables and ornamentals. If you have seen purple coneflowers with green distorted flowers, you have seen the aster yellows plant disease. Aster yellows is caused be a phytoplasm; an organism similar to a bacterium but without cell walls. When AY infects wheat, it produces symptoms very similar to Barley Yellow Dwarf Virus (BYDV).

Research has shown that heavy infections of AY can cause yield loss in susceptible wheat varieties. There are anecdotal observations from wheat varietal plots in northern Minnesota, which indicate that AY symptoms may be more severe, or that the disease is aggravated under wet conditions.
Aster leafhoppers acquire the phytoplasm by feeding on an AY infected plant for a minimum of 30 minutes. Acquisition of the phytoplasm increases with longer feeding times. The AY phytoplasm requires another two weeks, to incubate within the aster leafhopper before the leafhopper can transmit the disease to new plants. Consequently, immigrating aster leafhoppers, arriving already infectious for AY, are more likely to vector the disease into fields than the smaller overwintering populations which have to acquire and incubate the phytoplasm before they can infect plants. However, once it acquires the phytoplasm, a leafhopper remains infectious for an extended period of time. Although the acquisition phase may be long, it takes a very short feeding period by the leafhopper to transmit the disease to uninfected plants. Generally speaking, the more disease vectors that are present, the greater the potential for that disease to spread.

Feeding Damage - Other than vectoring AY, there is little data on the impact of very high levels of aster leafhoppers. In most years the populations of aster leafhoppers are lower and their feeding injury has little or no impact on wheat yield. We have no data indicating if this is the case with very high populations of aster leafhoppers such as we are seeing this year.

Management -Unfortunately, there is no clear cut answer as to whether treatment of an individual field is warranted; we have no action thresholds for this insect as it is rarely a problem (the only mention we can find of treatable levels is from a 1935 paper that refers to clouds of leafhoppers at one's feet). There are a number of factors to be considered before making individual decisions.

  • High numbers of vectors increase the chances of disease spread
  • There is little data indicating that direct feeding damage causes wheat yield losses. There is no treatment threshold (clouds not being a very useful term).
  • The rapid transmission of AY may mean that fields with heavy populations of aster leafhoppers may contain plants already infected with the disease, so killing aster leafhoppers to avoid AY may not be effective.
  • There's not yet a canopy, plants are small, rapidly adding new leaves and have maximum exposure to wind, sun and moisture, meaning insecticide residual is going to be shorter than later in the season. So there's no guarantee that treated fields may not become re-infested with aster leafhoppers.
  • Treating with a broad spectrum insecticide will kill beneficial organisms and may lead to higher aphid populations and BYDV. Bird-cherry oat aphids are already present in southern MN and BYDV is a more serious threat to wheat yield than AY. Populations of aphids in the field would favor treating the field and influence the insecticide used.
  • Both Aster Yellows and Barley Yellow Dwarf must be transmitted by insects (aster leafhoppers and aphids respectively). If you see discoloration in the absence of these insects, it isn't AY or BYDV, look for some other cause!
The bottom line is this will have to be an individual's decision and is a field by field situation, please use the facts that we've mentioned above and make the best decision for your production system.

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