The onset of warmer weather is bringing out the first wave of perennially pesky garden insects. Two we watch for this time of year are rose sawfly (order Hymenoptera, family Tenthredinidae) and eastern tent caterpillar (Malacosoma americanum).

First, we diligently scout our target species, looking for the first signs of their presence. Then, we determine if they are approaching unacceptable population counts or damage levels. Our tolerance level for both pests is very low due to past experience. As we have learned, both insects can cause a great amount of damage very quickly.

Rose sawfly larvae begin appearing toward the beginning of May, roughly when red buds and crabapple blooms are fading. There are three species: 1) The bristly roseslug, Cladius difformis (Panzer), 2) The roseslug, Endelomyia aethiops Fabricius, and 3) The curled roseslug, Allantus cinctus Linnaeus. Each species generates differing numbers of generations, some producing one, two or many multiple generations, all the way to first frost. Horticulture staff examine the underside of rose leaves, looking for little green larvae. Treatment begins shortly after they first are detected. Staff spray spinosad, a substance generated by a soil bacterium, every two weeks until populations drop off.

Eastern tent caterpillar is a native insect known to most midwestern gardeners for causing serious defoliation on a range of different species. We frequently find them on crabapple and cherry trees in the gardens. There are a number of controls recommended in the literature; however, our consulting arborist made a novel suggestion. Instead of spraying or cutting and burning the branch or anything else, we simply tear open the “tent”. Caterpillars venture out from their abode to feed during the day, returning at night for protection. Removing their home gives them no place to go, and it exposes them to birds who enjoy a little twitching treat. We have not employed any sprays since adopting this tactic.

Josh Steffen, Horticulture Manager