Research Highlights

Sentinel Plots Encourage Scouting, Integrated Pest Management

By Laura Temple

When farmers know a given insect or disease is in the area, they are more likely to scout their own fields. And scouting results in sound decisions about treating — or not treating — soybean fields to manage specific pests. 

That’s the driving force behind Pennsylvania’s sentinel plot program. Since 2012, about 20 soybean fields each year have served as untreated sentinel plots across the state. County extension agents scout weekly, and results are compiled and shared by John Tooker, entomology professor and extension specialist with Pennsylvania State University.

“Each week, the team reports both insect and disease pressure severity with a scale between 0 and 10,” Tooker explains. “I report those results in our Field Crop News weekly newsletter, often with pictures and links to more information, so farmers know what to watch for in their own fields. Our reports appear to provide a good view of what is happening across the state.”

The sentinel plots have become part of the Pennsylvania Soybean On-Farm Network research, multi-faceted research funded by the Pennsylvania Soybean Board. Each season, county educators can choose to participate, working with a cooperating farmer to identify a field to scout each week. Educators seek out fields planted without either insecticide or fungicide seed treatments, so insects and pathogens can colonize the sentinel plots. For the rest of the growing season, insecticides and fungicides will only be used if scouting reveals that pest populations exceed economic thresholds.  

“The goal is to characterize typical pest populations in soybean fields that we hope will prompt farmers to be curious about what is active in their fields and scout them,” he says. “The sentinel plots serve as a scouting guide, cliff notes or an early warning about issues in different parts of the state.”

The most common diseases that appear include Septoria brown spot, Cercospera leaf spot and white mold. Septoria brown spot tends to spread within a field, but it has not yet caused yield loss in sentinel plots, according to Tooker. The same is true for Cercospera leaf spot. White mold grows on a variety of hosts, and crop rotation helps manage it.

Common insects include bean leaf beetles, Japanese beetles, grasshoppers and stink bugs. Tooker notes that insects usually remain below economic threshold levels.

“If one population of some insect pest gets rolling, it can cause damage,” he says. “But the sentinel plot reports are not meant to trigger field treatments, but they serve as an alert to growers to scout for potential problems in their own fields.”

Tooker and the team of participating county agents report that farmers appreciate this information about what is happening in area fields. The information prompts questions about what could be happening in their fields and provides a practical way to engage with farmers. 

“Farmers have asked for similar information for other crops, including corn and wheat, but right now the effort is only funded for soybeans, thanks to the checkoff,” he adds.

Value of Integrated Pest Management

Sentinel monitoring indicates that economically damaging pressure from diseases or insects is rare in Pennsylvania soybean fields. Scouting and responding with insecticides or fungicides only when needed supports long-term yield and profit. 

“These fields demonstrate that Pennsylvania soybean fields don’t need insecticides or fungicides to be productive,” Tooker says. “If pest levels don’t exceed thresholds, fields aren’t suffering economic losses and pesticides are not needed. IPM is the best way to manage these fields.”

He explains that soybeans provide great habitat for beneficial predators, like lacewing, lady beetle and damsel bug. Such natural predators feed on damaging insects, keeping populations well below thresholds. 

“A strong population of beneficial insects provides a huge benefit if something like soybean aphids show up,” he adds. “They protect fields throughout the season and can prevent buildup of problem pests. Beneficial pathogens can work the same way.”

Tooker believes IPM practices decrease potential for development of pest resistance. They also decrease input costs and offer season-long benefits.

“For example, about 50% of the soybean seed in Pennsylvania includes an insecticidal seed treatment that takes out all early season insects, including thrips,” he explains. “Thrips have a high economic threshold, so they are rarely a problem. But, thrips sustain early season populations of predators that will remain in the field and feed on other pests throughout the season.”

The results from the sentinel plots demonstrate that no tradeoffs exist when forgoing insecticides. His team isn’t seeing economic damage early in the season, and killing beneficial insects with insecticides can actually create other problems, like increasing slug populations, which can be very damaging to soybeans. 

Tooker’s research has shown that slugs tend to be more common in fields managed with insecticides, because slugs are molluscs rather than insects, so they are not sensitive to most insecticides. However, insecticides can kill the insect predators that eat slugs, leaving the slugs free to eat soybeans.  

“We have only seen a need for three fields to be treated with insecticides in Pennsylvania since we started this monitoring program in 2012,” Tooker says. “We have monitored more than 200 fields since then, without exceeding economic thresholds.”

Published: Jan 15, 2024