Yield robbers in the form of thistle caterpillars are the surprise guests of South Dakota soybean fields as late September and early October harvest approaches.
Combined with typical caterpillar species presence, the eastern side of South Dakota is dealing with pest management questions. Specifically, four caterpillar species are showing up in fields, although three of the species are not at significant levels, the thistle caterpillar may be an issue for some soybean growers.
Green cloverworms, cabbage/ soybean loopers and alfalfa caterpillars are common visitors to South Dakota soybean fields, but in 2017, add thistle caterpillar to the list. Dr. Adam Varenhorst, South Dakota State University Extension field crop entomologist, says the unexpected arrival of thistle caterpillars may be due to a successful first generation in July.
Thistle caterpillar was present in July in low numbers for most of the state, but populations jumped in the second generation. In a few areas, growers are spraying for the caterpillars due to high percentages of defoliation. In a standard year, green cloverwom ranks as the No. 1 caterpillar pest, but thistle caterpillars are present in higher abundance in 2017.
Currently, most soybeans in South Dakota are between R4 and R5. The presence of multiple caterpillar species requires a shift in threshold determinations. Varenhorst recommends a focus on defoliation. “For this time of year, we drop thresholds from 30% to 20% defoliation. Look at all the leaves on the trifoliates to get a best estimate for leaf matter removal,” he explains.
Varenhorst says top leaves can be misleading and stresses consideration of the entire plant: “Some of these caterpillars will only damage leaves on top of the plant. If you only look at those top leaves, you might incorrectly assume a plant has 40% to 50% defoliation.”
At 20% defoliation, Varenhorst estimates 3% to 7% yield loss. Yet, spraying at 20% defoliation isn’t a given, he notes. If caterpillars are 1” to 1.5”, feeding could almost be finished and defoliation injury completed. However, smaller caterpillars translate to continued feeding damage. “If caterpillars are half an inch or less in size, you’ve got to figure on them feeding a lot more and potentially taking away yield,” Varenhorst concludes. “That’s when you’ll need to spray.”