By Linda Williams, DNR Forest Health Specialist, Woodruff; Linda.Williams@wisconsin.gov or 920-360-0665 

Basswood leaves show damage after feeding of introduced basswood thrips. / Photo Credit: Wisconsin DNR.

For the second consecutive year, damage from introduced basswood thrips (Thrips calcaratus) is significant in some northeastern Wisconsin counties. Introduced basswood thrips are tiny, invasive insects that feed inside tree buds in early spring. Leaves are then deformed when they expand and can look like frost or wind has damaged them.

Basswood trees defoliated by introduced thrips will have tufted foliage and parts of leaves missing. If the damage is severe, trees will attempt to send out a second set of leaves; we see this in many areas. Consecutive years of severe damage can weaken or kill a tree. If the tree survives, recovery can take many years, and they may still die from secondary issues such as Armillaria root rot.

Introduced basswood thrips (circled in yellow) are tiny insects that feed on expanding buds and create significant damage to basswood leaves. / Photo Credit: Wisconsin DNR.

Moderate to severe damage from thrips was first observed on basswood in Wisconsin in 1980, but the damage was initially incorrectly attributed to our native thrips. Additional research identified the culprit as introduced basswood thrips.

Since identification, the invasive thrips population has gone from very high numbers, which causes severe damage and a decline in trees, to 20 years of low populations with very little damage. Then, in 2020, damage from introduced basswood thrips was noted in Forest, Marinette and Oconto counties. In 2021, severe damage was reported in Rusk and Sawyer counties in northwestern Wisconsin and Florence, Forest, Marinette and Oneida counties in northeastern Wisconsin. In 2022, damage was observed in Forest and Oconto counties. Significant defoliation has been observed this year in Oneida, Forest and Marinette counties.

There is only one thrips generation per year, so the damage is done by the time defoliation is noticeable. Therefore, pesticides would have no impact. Instead, landowners should minimize stress on damaged stands and keep trees healthy during outbreaks. This will help trees tolerate defoliation and retain the energy to send out a second set of leaves.