Leafhopper populations are increasing in the northern RRV. Fields which held low numbers on Friday have significantly increased populations this week. These are all winged adults and so are likely the populations from the southern part of the state that are migrating north. We don't have any data on what impact on yield these higher populations of leafhoppers may have on small grains but sap-feeding leafhoppers generally don't impact yield. Having said that, leafhopper populations in typical years are much lower; in dry conditions, sap feeders have been known to exacerbate drought stress. Generally, leafhoppers are more important as vectors of the disease, Aster Yellows (AY). Caused by a phytoplasm, AY can infect wheat, and under the right conditions cause yield loss. Symptoms show up a couple of weeks after infection by the leafhopper and include yellowing of leaves, often accompanied with reddish or purple coloration (similar to BYDV).
There have been several reports from the northern RRV of yellowing in small grains fields (see figure at bottom). While the primary symptom of Aster Yellows is yellowing leaves, it's felt that leafhopper populations have not been established in the northern part of the valley long enough for AY to be the cause of this discoloration. On the other hand, there's some indication that yellowing now being seen in the northern RRV is likely related to nitrogen and potassium deficiencies. Even in fields which typically have high potassium or good nitrogen levels, the dry soil conditions may be making these nutrients unavailable to the plants. So, yellowing may not necessarily be AY, but something else.
There have been a lot of questions about adding insecticide along with the next herbicide application in efforts to kill off leafhopper populations. Technically, not a difficult practice; almost any insecticide labeled in small grains will kill leafhoppers, and the small, thin crop canopy means there will be less of a deleterious effect of applying insecticides with the lower pressure and larger drop size you get from herbicide nozzles. But, as we currently have no data on the effect on grain yields of these high leafhopper populations, nor on how much AY is being transmitted in the field, we cannot provide a recommendation for or against this practice.
If you considering treating fields, there are a few points to keep in mind about your expected outcomes (you may need to modify your expectations):
1) AY phytoplasm is transmitted very quickly by the leafhoppers (just like non-persistent virus by aphids). If you have heavy populations of leafhoppers in your fields, plants may already be infected with AY. We know from experience that insecticides, both foliar and seed treatments, are not effective in managing quickly transmitted plant diseases (e.g. PVY in potatoes). In the time the insecticide takes to do its job, the disease can be transmitted. Consequently, don't be too surprised if there are AY infected plants in fields later this season after you had successful leafhopper control.
2) AY symptoms can be similar to those caused by a number of nutrient and other disease factors, including early-season tan spot, BYDV, nitrogen or potassium deficiencies. Removing the leafhoppers will not be effective in solving the underlying cause of yellowing.
3) The thin crop canopies make for a reduced expectation of insecticide residual. They allow UV light, wind, and any moisture available onto all the leaves. These are the environmental factors that break down the active ingredients in insecticides. The plants are also still relatively small, meaning more vegetative material is going to develop, none of which will be protected by insecticide.
4) Shorter residual means potential re-infestation by immigrating populations of leafhoppers. Keep scouting fields.
5) Early spraying may remove predator insects that can limit later populations of aphids. If we get aphids later in the season, sprayed fields may require re-treatment. If aphids infest local grain fields, keep a close eye on their numbers (85% stems with more than one aphid present).
6) Leafhopper populations in the south are decreasing so we may well see similar dynamics here in the next week or two.
Bottom line - we can't recommend spraying, we can't recommend not spraying - we just have no data. But even if a field is treated, you may have to modify your expectations.
- Ian MacRae (UMN), Jochum Wiersma (UMN), Janet Knodel (NDSU), and Bruce Potter (UMN)