That sounds like a good title for some horror movie sequel, but that’s not where I’m going with it today. As I’m sure many of you noticed, we’re getting an abundance of butterflies… AGAIN! These are Painted Lady Butterflies… although I’ve heard them called many things… some I can’t repeat. These are the adults of the caterpillars that were devouring many soybean fields, and lots of other plants, a few weeks ago.
Fortunately, the adults do not injure crops as they only feed on the nectar of flowers. They are a bright, colorful, addition to our summer landscape and really don’t do any damage unless you consider smearing up your windshield as you drive down the road. However, the larvae of the painted lady butterfly, sometimes called the thistle caterpillar, can be another story.
To understand when and how these defoliating insects impact soybeans, one of their favorite food sources, you need to understand a little about their life cycle. Painted lady butterflies do not overwinter in Nebraska. They migrated and moved with wind currents from the southern U.S. and Mexico earlier this summer, arriving in Nebraska in June. Where they are concentrated depends a lot on weather patterns, so just because we had a lot of them this year does not mean we will have an abundance of them in 2020.
Unlike some insects that lay large masses of eggs, Painted Ladies will lay individual eggs on the leaves of thistles, soybeans, and over 100 other species of plants that serve as a food source to the larvae once they hatch. In five to seven days, the eggs hatch and the larva begin feeding. Because the eggs are laid over several days up to a week or more, it is common to have different sized caterpillars on a single plant.
The larvae feed an average of four weeks, but sometimes up to six weeks. However, the majority of their feeding damage occurs towards the end of the larval stage as their size and appetite increase. To protect themselves from predators, they often pull several leaves together and connect them with a fine webbing, forming a sheltered area to feed.
Mature caterpillars are usually 1½ to 1¾ inch long and can vary in color, but are covered with numerous branching spines. Once they mature, they form a pupa or chrysalis which hangs from the underside of a leaf and can be blue, brown, or green in color. In seven to 17 days, a new adult Painted Lady butterfly emerges and starts the life cycle over again.
So the butterflies we are seeing now are the adults of the thistle caterpillars that were feeding on soybean fields a couple weeks ago. Knowing the life cycle, we know we can expect to see more larvae feeding in about four to five weeks and can plan our scouting accordingly.
Knowing how to scout is just as important as knowing when to scout. I won’t go into the full explanation on how to scout your fields for defoliating insects, but I can tell you most people will overestimate the amount of defoliation because thistle caterpillar feed at the top of the plant where it is most visible. An easy way to estimate defoliation can be found at https://go.unl.edu/g2259. I really like the images that help you more accurately estimate the percentage of defoliation.
This method applies to all defoliating insects… grasshoppers, bean leaf beetles, thistle caterpillars, wooly bear caterpillars, and any other type of defoliating insect. At this stage of growth for the soybean plant, we don’t want to see defoliation exceed 20% of the total leaf area. Frequently defoliation on the upper leaves is greater, but when we consider the whole canopy, the defoliation has not reached this threshold.
So be prepared to check your fields because I can just about guarantee we will have more thistle caterpillar feeding… and possibly other defoliators. The important thing to know is when that damage just looks bad versus when it is actually reducing your yield… and your profitability. For more information on managing thistle caterpillars and other defoliating insects, contact your local Nebraska Extension office.