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  • Writer's pictureRachel

Thistle caterpillars, Painted Ladies, and Japanese Beetles, oh my!

You may have noticed in the last few weeks a fair amount of defoliation from Thistle caterpillars or Painted Lady caterpillars. What are these pests? Do they pose a threat to soybean yields? How do I know when to control them? We’ve also begun to see an increase in Japanese Beetles already. We will cover what you need to know as we get farther into the growing season and the peak levels of Japanese beetles occur. This blog will cover the lifecycle and impact of both thistle caterpillars as well as Japanese beetles and what you need to be prepared for during this growing season. Read on!

Thistle Caterpillar

The thistle caterpillar is known as Vanessa cardui and is in the order Lepidoptera which consists of all butterflies and moths. You may know the adult as the Painted Lady butterfly you see flying around. Most of the thistle caterpillars we saw earlier this year are currently Painted Ladies flying around pollinating various plants. While thistle caterpillars can eat over 300 different species of plants, they are particularly fond of soybeans, partially due to the leaf shape of the plant.

As thistle caterpillars feed, they use a silk like web to tie the leaf up into an enclosure. Inside this protected enclosure, the caterpillar starts to eat. It eats the leaf until it is largely defoliated between the veins and then moves on to a new leaf. Caterpillars will feed for about 2-6 weeks before pupation. The most damage typically occurs during V3-V4. During this time they will consume 37.5 square inches of tissue per caterpillar! That is a substantial amount of material for their size, but this energy will be essential once the caterpillar undergoes pupation.

Each caterpillar is about 1 inch long. They are typically a gray brown color with two yellow stripes down their back. As they mature, the caterpillar grows increasingly longer, branching spines across its body. Caterpillars will pupate in a gray/white sack attached to the bottom of host plant leaves. Pupation is typically 7-17 days long. After pupation the adult emerges. The adult is referred to as a Painted Lady butterfly. They are orange, white, and black with 5 black spots on their hindwings and are generally 2-2 7/8 inches in size.

The Painted Lady butterfly doesn’t overwinter here. Rather, they migrate north from southern Texas and Mexico. Once here, they have two generations a year. So, while we already saw one generation of caterpillars, be on the lookout for the next generation later this summer. Adults will lay single eggs on a surface of a leaf. The egg will develop into a caterpillar in about 7 days. Typically, the caterpillars stay grouped together towards the field edges. When scouting a field, be sure to walk in a ways—generally the defoliation will be less the farther into the field you get. The defoliation also is generally confined to the upper canopy, so be sure to look farther down into the canopy when assessing defoliation.

Photo courtesy of UNL Cropwatch:

So, we’ve seen one generation; when do we look for the next? If the peak of caterpillar feeding was the last week in June, we can probably expect peak of the next generation in early to mid August, right about the time we will see high levels of other soybean pests like Japanese Beetles and Bean Leaf Beetles. We may start seeing the early phases of the 2nd generation as soon as the third week of July. While the second generation is typically not as damaging, simply due to the fact the plant is much larger and is able to withstand the same amount of feeding as earlier in the season. What we have to worry about is the fact that we also have a larger quantity of other insects feeding at the same time. This can push us towards that 20-30 percent defoliation threshold. (30% is the threshold for treatment during vegetative stages, and 20% during reproductive stages)

So, what was with the increased pressure of Painted Lady caterpillars this year? Like we mentioned before, Painted Ladies migrate north from southern Texas and Mexico. Mexico had abundant rainfall leading to an increase in vegetation. These optimal conditions resulted in optimum conditions for egg laying and caterpillar survival during the generations produced in Mexico, greatly increasing the viable population of Painted Ladies. This was noticeable across the country as high populations were reported in San Diego and other locations along their migration path. In fact, the levels were so exceptionally high this year that is was reported in national new sources such as NPR, NBC, and the New York Times. So, while the larval stage isn’t as enjoyable, take time to appreciate the record number of Painted Ladies in our neighborhood this year.

Japanese Beetles

Another pest that we can expect to be seeing more of soon is Japanese Beetles. We’ve started seeing the first emerge from the ground during the last week. They typically do emerge the last week of June, however, this year we are about 200 GDU behind the average accumulated GDU’s. Because of this, emergence is slightly behind.

Japanese beetles typically have one generation per year. They overwinter as grubs in the soil. In late June they emerge from the soil as adults. Throughout the summer they feed on a variety of plants including soybeans and corn. They however prefer roses, grapes, smartweed, and flowers. Overall, Japanese beetles can feed on over 350 different species of plants. The beetles will skeletonize soybean leaves and feed on the surface of corn leaves unless corn silks are available. If corn is silking, the beetles will clip silks. In August and September, the adults will return to the grass where they will lay eggs. These eggs will hatch and burrow into the soil as grubs. While the adults typically cause the most of the damage, grubs will feed on roots and cause damage. The grubs overwinter in the soil until the next June when the cycle repeats.

Photo Courtesy: Pioneer,

The beetles typically cluster together at the edge of fields, largely due to a sex pheromone emitted by the females. This often makes damage look worse than it actually is, particularly as you drive by from the road. Be sure to walk out into the field to check for a more accurate distribution of damage.

The threshold for control in soybeans is the same for all insects—20% during reproductive stages and 30% in vegetative stages. Control in corn is sometimes harder to assess. The following guidelines must be met: there must be 3 or more Japanese beetles per ear, the silks must be clipped to less than ½ inch and pollination must be less than 50% complete. Again, remember to check farther in than the turn rows as the Japanese Beetles will group up together and make it difficult to get an accurate count of the whole field.

Have any questions about either pest? Get in contact with your agronomist to discuss more about the lifecycle and methods for control.


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