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Understanding Pest Contour Maps

Monday May 01, 2023

Contour maps showing results from the WSU Extension insect monitoring network are available on the Potato Decision Aid System (DAS). These maps use color to help you visualize where and how many insect pests were found each week.

Low, moderate, high, and very high numbers of insects are represented by blue, green, yellow, and red zones respectively. The ranges are based on years of monitoring these insects in the Columbia Basin.

MAKING THE MAPS: Contour maps are constructed by interpolating insect monitoring network data, i.e., by using data gathered at individual fields to predict pest numbers at other points on the map. A method called "inverse distance weighting" is used. The data from all monitoring sites factor into the prediction, but the data from sites that are closest to the point on the map being estimated give the greatest weight. The maps (below) show insect monitoring data at individual sites (left) and the contour map after interpolation (right).

INTERPRETING THE MAPS: The contour maps DO NOT represent action thresholds for pest management. The insect numbers are NOT correlated to insect feeding injury or incidence of insect transmitted diseases. The colors are simply a way to show the range in pest numbers from insect monitoring. The WSU Potato Alerts e-newsletter is issued weekly during the growing season to put these numbers into context, and to support management decisions. 

There is danger in always associating BLUE zones with low risk, and RED with high risk. Here are a couple of examples...

BLUE ZONES CAN BE IMPORTANT: Potato psyllids slowly make their way into potato fields in late May or June, and it's typical to intercept just one or two of them on the four yellow sticky cards that are deployed around potato fields. This is a low number represented by blue zones on the contour maps. But risk-averse potato growers should be prepared to manage potato psyllids as soon as they show up in potatoes. So, the blue zones are important for assessing risk. Click HERE to review psyllid management strategies.

RED ZONES ARE NOT ALWAYS IMPORTANT: Beet leafhopper (BLH) numbers in August and September can be very high, but late onset of purple top (the phytoplasma-caused disease spread by BLHs) does not result in significant losses. The same can be said for lygus bugs. The plants accumulate more lygus feeding injuries when infestations occur early in the season vs. late. Applying insecticides to control these pests late in the growing season is not necessary. So, in this case, red zones are not important for assessing risk.

The RATE of insect population growth can be important. This is why the contour maps have a scrolling feature that allows you to look back in time. Scrolling helps visualize the rate of population buildup. 

DON'T WAIT FOR THE PEAK:  Areas on the contour maps where insect pest numbers are increasing each week and are approaching a high peak population are usually more at risk of insect feeding injury and insect-transmitted diseases, than areas where the buildup is slow and is only likely to reach a moderate peak. The series of contour maps (below) illustrate the buildup of insect pest numbers over several weeks. In many cases, the pests should be managed several weeks BEFORE the numbers reach their peak. The areas that are blue or green in the first few weeks, are generally the first to turn red in the following weeks.

Knowing how the insects are being monitored is important.

The traps used to monitor beet leafhoppers, potato psyllids, and potato tuberworm are place outside of the potato fields or on the edge of the field. So, the numbers shown on the maps represent populations moving into potato fields, and it is up to the grower to manage them once they enter. In other words, the numbers and colors on contour maps do not indicate whether growers are controlling these pests.

Lygus bugs and aphids are monitored inside potato fields, by shaking plants over a small bucket to dislode the insects. So, the numbers shown on contour maps DO indicate that growers are controlling them.