Thistle caterpillars cause the most injury during V3-V4 soybean stages in Iowa. The larvae web together with the leaves of soybean and will consume about 37.5 in of tissue per larva. 97% of this tissue consumption occurs during the last two larval instars. Caterpillars will feed for 2-6 weeks until pupation.
Last year, we experienced 3 to 4 generations in the areas that had significant pressure. Treatment threshold is 30% vegetative defoliation.
Caterpillars have started in Pottawattamie and Cass counties in late May and early June. Field Sales Agronomist Scott Small has already been spraying for caterpillars.
"Right now, the caterpillars are pretty scattered but we're starting to see leaves folded with webs and worms are about 1/2 in long. We'll start to see those painted moths out here in the next week," Small tells us about his experience with the caterpillars.
High-pressure areas help identify where problems with these caterpillars may occur.
Western Carroll and Sac counties
“I don’t have them…yet, said Larry Christensen veteran Field Sales Agronomist in Carroll and Sac counties. “Last year, I think we sprayed every field a year ago.” If you’re making a plan to spray in the next few weeks, make sure you’ve scouted for butterflies. With the wind we’ve had recently, it’s hard to see them fly. I’m not seeing any webbing as of yet in our area.”
Field Sales Agronomist Ryan Albers scouted these caterpillars recently and encourages growers to scout field edges asap to avoid defoliation.