Black walnut tree bug

Asked June 26, 2018, 1:05 PM EDT

Hello, I have several black walnut trees on my property (Northville Twp). For the past couple of years we lose leaves in June. We just noticed a tiny grub in the base of the leaf stem. All of the trees are losing leaf stems. Is this the invasive species I’ve been reading about?? The trees still look fine and I’m not seeing grub evidence on the bark. But then, I’m not sure what to look for! Kate Pyle I tried to get a picture of the stem with a live grub but it appears they’re done. Fewer are falling now. But I am seeing another growth (pic 1)....

Wayne County Michigan

1 Response


You had butternut curculio (Conotrachelus juglandis) in the stems (petioles) of the leaves. Here are details from MSU Extension -. “At first glance, the leaves look perfect, but on close inspection, the end of the petiole that attaches to the tree is dark brown and broken instead of cupped and green. A small hole may be seen close to the end of the petiole... The adult is a brown weevil approximately 0.25 inches in length. Both the larva, which is the juvenile, and the adult do damage to black, English, Carpathian or Persian walnuts, butternut and Japanese heartnut. There is no treatment for butternut curculio. By the time the tree owner sees the leaves floating down, the damage is finished. Smart gardeners will water the trees if there has been little rain. If the lawn is fertilized, it should provide plenty of nutrients for the tree. This will help the tree to grow new leaves”. The complete article can be read here-

The reddish growths look like walnut petiole galls, a growth caused by the tree’s response to a small mite.

The ‘invasive’ species you may have read about could be the walnut twig beetle, which transmits the Thousand Cankers Disease(TCD). The beetle is actually native to the American southwest. Here is info, so you can monitor your trees-,5664,7-324--457991--,00.html

And please report it if your trees develop symptoms as described in the above links-,4570,7-350-79136_79237_81077-367462--,00.html

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