Washington State University (WSU) researchers found some alarming numbers of beet leafhoppers (BLH) on their sticky traps last week. Several “hot spots” have developed in the central-west and south-west areas of the region, with fewer BLH showing up in the east. The average BLH count for all traps this week (33 BLH per card) was the highest they have recorded in twelve years of monitoring this pest. It is also very early to see this many BLH.
The maps below show interpolated BLH population densities across the region using our trapping network results. The pest density maps were developed and validated by the Crowder Laboratory, WSU Entomology. If you would like to zoom into these maps and/or scroll through maps from the previous weeks, visit the Potato Decision Aid System website by clicking HERE.
WILL WE SEE MORE PURPLE TOP THIS YEAR?
These unusually large and early BLH numbers are concerning. Early-season transmission of the purple top phytoplasma (spread by BLH) is more likely to result in severe plant symptoms and reduction in potato yield and quality. Purple top is seen in potato fields in the Columbia Basin every year, but its incidence and severity has generally been low. There has not been a serious region-wide outbreak of purple top since 2002-2003. But, that was before the regional BLH monitoring networks were established, so we do not have a good reference for how large the BLH populations were in those years.
The graph below shows seasonal comparisons (2017 to 2021) of weekly average BLH counts from our network of sticky card traps. If the average trap count continues to increase in 2021, I will have to change the scale of this graph (something I have not done since 2013).
EMERGENCE OF THE SPRING BROOD:
Almost all of the BLH they collected appear to be recently emerged adults of the spring brood, i.e., the first generation of BLH hatched from eggs laid by overwintered females. Beet leafhoppers tend to be lighter in color when they are newly emerged, but darken with age.
The pictures below show a light form (left) and darker forms (middle, right) of BLH. Photos courtesy of Andy Jensen, NW Potato Consortium.
WILL THE NUMBERS CONTINUE TO INCREASE?
This is rather early to see a peak in the spring brood numbers. Our trapping results over the last decade indicate that peak populations usually occur in early- to mid-June. Does that mean we will see the numbers continue to rise in the next few weeks? Or are we seeing an earlier peak this season?
A study conducted by O.A. Hills in the 1930’s showed that there are three overlapping generations of BLH each year in the Columbia Basin (see graphic below).
WEED HOSTS & BEET LEAFHOPPERS:
Last week’s report mentioned the host plant preferences of BLH and how controlling weeds, or managing the BLH overwintering in weeds, may contribute to BLH control. Click HERE to see that report.
PURPLE TOP RISK ESTIMATION:
If traps near your fields rise from low counts each week (< 8) to moderate-high counts (20-40) and continue to build to higher numbers (> 50, or even > 80) over the next weeks, then the chance of seeing purple top is probably higher. But, if the numbers decrease after peaking at low to moderate numbers each week, the risk of seeing purple top, beyond a few border plants, should be much lower.
USING INSECTICIDES TO CONTROL BEET LEAFHOPPERS:
There are many insecticides with various modes of action that can be used to control BLH and minimize the spread of the purple top phytoplasma. Most applications are made to target the spring brood in May and June, when plants are young and most susceptible to purple top. Applications made after mid-July will control BLH, but infections that occur late in the season are not usually significant and do not often warrant control.
SYSTEMIC INSECTICIDES APPLIED AT PLANTING:
Systemic insecticides applied at planting (or shortly thereafter) can be used to target BLH, but their effectiveness in preventing purple top in potatoes may be limited and is debated. Beet leafhoppers can acquire and transmit BLTVA quickly and spread may occur faster than the insecticides can act, especially if the residual level of the insecticide in the plant is too diminished by the time the pests arrive. Insecticides applied to the foliage in-season are preferred.
FOLIAR INSECTICIDES APPLIED IN-SEASON:
Foliar insecticides with long residual activity (10 to 14 days) are recommended. Residual activity and follow-up applications are important because BLH can continuously invade potato fields from surrounding areas. Residual activity may vary depending on the product, rate used, method of application, and plant growth. If plants are growing rapidly, it may be necessary to reduce the application interval, from 10-14 days to 7 days, to protect new foliage.
