Basswood leaves defoliated and trees looking thin

Linda Williams, forest health specialist, Woodruff, Linda.Williams@wisconsin.gov, 920-360-0665 

Basswood trees in Forest, Marinette and Oconto counties are looking very poor this year. The leaves are damaged, misshapen or completely missing. Several things seem to be happening, but the worst offenders seem to be a late frost/freeze and a suspected infestation by introduced basswood thrips.

Evidence of basswood thrips on a single leaf; some defoliation and dieback.

Evidence of suspected basswood thrips infestation in early spring.

Damaged leaves look similar to those impacted by basswood thrips, although due to COVID-19 safety measures, our sampling occurred too late to detect this insect in any number. Many buds were killed by a late frost/freeze, and although the trees were able to send out some new leaves following this event, the new leaves were then damaged by several other things, including wind and basswood leafroller, in addition to the apparent thrips damage we observed.

Dieback on a basswood twig (left) and crown dieback on a mature tree (right).

Newly emerging leaves were killed by a late frost/freeze (left). Basswood crown impacted by frost/freeze and suspected thrip infestation (right).

Fortunately, we expect these impacted trees to survive the damage, though they are likely to have reduced growth rates for this year.

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