Oak galls

Now that the weather has started to warm up and stay warm many of us are spending more time outdoors.  Several people have asked about growths on the twigs and branches of their oak trees.

Oak galls are caused are primarily caused by tiny wasps that lay their eggs on twigs.  The larva will then hatch and burrow into the stem.  The larva themselves do not form the gall.  Instead it is a formed by the tree in reaction to chemicals released by the larvae or other stimuli.  These stimuli incite plant hormones to form the gall.  These galls are rich in protein and provide a source of concentrated food for the larvae.  In addition to providing food the galls also protect the larvae.  The galls can vary in size and shape, depending on the type of oak and the insect causing them.  They can range in size from a fraction of an inch to several inches in diameter.  In many cases galls may grow together and form large masses.

In central and Southern Illinois the two most frequently encountered oak galls are the horned oak gall and the gouty oak gall.  Both of these are caused by wasps.  Horned oak gall occurs on Pin, Black and Water Oak.  Gouty oak gall occurs on Pin, Scarlet, Red and Black Oak.  Horned oak galls are brown, woody masses that can reach up to 2 inches in diameter and, as the name implies, are covered in horns that are 1/8 to ¼ of an inch long.  Gouty oak gall looks similar, only it doesn’t have any horns.  In most cases galls will not harm the trees, they are just unsightly.  However if the tree is young or there are an excessive amount of galls on older trees, they can become weakened.  This weakening along with other issues such as stress and disease can possibly lead to the death of a tree.

Unfortunately there are limited management options for oak galls.  For young and small trees the affected twigs and branches can be pruned out.  Large trees may be difficult to trim without special equipment.  Because the larvae are protected by the galls they cannot be reached by topical insecticidal or oil sprays.  Adults emerge at different times of the year making it difficult to time sprays.  In addition to these factors most insecticides are not labeled for them.  Fertilizing trees that have galls will encourage new, healthy growth, with late fall being the best time to apply fertilizers.

Ken Johnson

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