Garden Views: Shoo away those sawflies

When the snow melts, the pests come out to play. Every year on May 1, I go out in my yard and plan the destruction of the little green worms that have invaded my pine trees.

I’m talking about the larvae of the European Pine Sawfly, Neodiprion sertifer. They enjoy all my pines, Mugo, Scotch, and Jack but they find my Austrian pines especially flavorful.

Sawfly eggs overwinter in the current season’s needles, emerge in April and begin feeding on the previous year’s growth. The black-headed larvae measure just under an inch in length when fully grown, have a light stripe down their back, two light green and one dark stripe run along their sides.

Since they feed in groups toward the ends of branches, they are fairly easy to spot. When you find a cluster of them and close in for a better look, they will raise up on their rear legs. After a few moments, if you don’t disturb them, they will calmly go back to feeding.

They will eat all of the previous year’s needles on one branch, then move on to another. A branch can look somewhat like a shaved poodle’s tail when the larvae are finished. It will be bare out to the tip where the new growth begins.

After feeding, the larvae pupate in the soil. The wasp-like adults emerge in September and October and lay their eggs in the current year’s growth to overwinter.

When the calendar reads mid-April, depending on the weather, start looking for larvae on the branch tips or branches already stripped of needles. If you have PJM rhododendron, start looking for the larvae when the PJM blooms.

Fortunately, the chances that your tree will die from a sawfly infestation are low. However, the stripped branches look unsightly and create stress for the tree.

You have a wide variety of controls for the European Sawfly. Treat minor infestations by simply picking the larvae off the branches by hand on smaller pine specimens. However, if your tree is large you can use a powerful water spray to knock the larvae off the branches. My preferred method is a 3000-psi pressure washer. Care must be taken with this method as a larvae weigh less than a tenth of an ounce and can be blasted a long way. Your neighbor may take exception if you launch your sawfly infestation on to his pine trees half a lawn away.

If you are looking for a chemical control, insecticidal soap works well on young larvae. Hold off on control methods if the larvae are full-grown. The damage is already done. Start planning for next year.

If you have tried all other control methods and nothing else seems to work, many pesticides can be effective at controlling sawfly larvae. Read the labels to ensure sawfly larvae are listed as a target species. It also is important that you follow the label directions to protect yourself against inhaling or getting the product on your skin. Remember, you are working with a poison.

Spring will come eventually and you will find me firing up the pressure washer and singing “Shoo, sawfly.”

Dennis Langley is an Anoka County Master Gardener.

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