Cereal leaf beetle has plagued wheat and other small grain farmers since the early 1960s.
As with most non-native insects, the beetle’s biology and ecology are an enigma, and it thrives in the absence of aggressive predators.
Although timed spraying controls the beetle if populations are low, it does little to control high beetle populations or prevent the losses sustained from the larvae’s incessant feeding.
To make cereal leaf beetle control less of a gamble, researchers at North Carolina State University and Virginia Polytechnic Institute and State University have discovered a way to help growers use scouting and thresholds more effectively.
Native to Europe and Asia, cereal leaf beetle was first discovered in southern Michigan in the early 1960s and has become one of the primary pests of small grains.
The most destructive life stage of the beetle is the larvae, which skeletonize the leaf, decreasing the plants ability to photosynthesize and giving the field a frosted appearance. Losses can reach 40 percent.
Federal agencies, researchers and growers have tried numerous efforts to control the pest.
After the beetle’s arrival, Michigan and Indiana were placed under quarantine, and small grains in other states had to be treated before transport. When those efforts failed, large-scale spray programs began, but these failed to stop the spread of the beetle as well. By 1970, all quarantine and eradication efforts had ended, and cereal leaf beetle was well established in the U.S.
Researchers turned to IPM tactics such as host plant resistance and biological control, but these tactics gained little traction with wheat growers.
Resistant wheat plants did not produce the yield of the more susceptible varieties, and imported parasitoids worked well in some areas but did not thrive in others. In addition, continued use of regular calendar sprays further reduced the predator population.
Growers had good reasons for a prophylactic spray program; many would mix insecticide with a nitrogen application or a fungicide that they were using anyway.
To many growers, the tank-mixes made economical sense, and many argued they didn’t suffer a yield loss from cereal leaf beetle because they had protected their field.
In addition, growers who wanted to scout had to do so between March and June, an extraordinarily long scouting season, and many of them scouted the fields on their tractor, flattening the wheat tillers and reducing yield.
A lot of disagreement
Even though growers insisted that the timed sprays saved them money, researchers say the prophylactic sprays are risky depending on the beetle population in the field.
“Calendar spraying works well when you have a low number of insects,” says Dominic Reisig, Extension entomologist at the Vernon James Research & Extension Center at North Carolina State University.
“However, when you have a high number of cereal leaf beetles, spraying doesn’t manage them at all and you wind up with a huge yield loss anyway. It’s a much riskier strategy than following IPM practices.”
To convince growers to use thresholds that had been developed several years ago by North Carolina State researchers, Reisig and other entomologists from North Carolina State and Virginia Tech knew they had to find two things: a way to shorten the scouting season and make scouting more efficient, and proof that using the thresholds would be more profitable or economically sensible than their calendar spray program.
To shorten the scouting season, researchers turned to degree-days. Degree-days begin with a biofix of either the first trap catch or a specified date and then count the number of degree days based on the number of degrees the ambient temperature is above the insect’s optimum temperature.
Counting degree-days helps the grower know when to begin and stop scouting. Because scouting often begins before threshold, if insect populations reach threshold, the grower is less likely to miss the chance to spray before populations increase to a number that will make the spray ineffective.
With funding from the Southern Region IPM Center, researchers discovered that the optimal degree-day model used 182 degree days, with a development threshold between 8 and 25 degrees Celsius.
Using that model for eastern Virginia and eastern North Carolina, researchers found that cereal leaf beetle egg-laying peaked between April 6-12, with the average date of April 8, and larvae, whose population peaked about 17.5 days after egg peak, usually peaked between April 19-29.
In 2011, egg peaks started earlier but larval peaks began later in the season, but they were not too far from the dates of the previous year.
Using the degree-day model, growers would begin scouting their fields one week prior to the predicted time of peak egg lay. When larval counts reached the threshold of 25 larvae per tiller, growers would spray their fields.
If population counts never reached threshold before the end of the scouting season, growers could save their insecticide for the next season, saving money on insecticide and conserving predators like lady beetles.
Once the degree-day model was established, scientists used it to create a prediction map of the state to forecast periods for egg peaks for counties in North Carolina and Virginia.
Reisig plans to use the map next year to create an alert system for growers to signal the beginning and ending of the scouting season, depending on where they live. Reisig said that as they tested the map this past spring, the dates they had estimated in each region accurately predicted egg peak and larva peak.
Reisig and other researchers are still trying to understand the beetle’s biology, since understanding why beetle populations are high in some areas and not in others will further alert growers to whether or not they may need to apply an insecticide.
But Reisig says that even without understanding why the beetle prefers certain areas over others, using the thresholds to time insecticide sprays is much less risky than spraying on a calendar schedule.
“If you use our degree-day model to help you know when to scout, and you use thresholds, you’re going to be able to manage the beetle before it reduces yield,” Reisig says.
“If you spray prophylactically, you’re taking a chance that there will be a lot of beetles and you won’t be able to control them, or the beetle isn’t around at all and you’re just reducing your predators.”