What's Infesting My Live Oaks?

Have been getting a lot of questions about several insects in oaks and what to do about them.  The first is a caterpillar that is feeding on leaves, often high up in the foliage where they go unnoticed until either frass (poop), or mature caterpillars start “raining” down out of trees.  These are most likely yellownecked caterpillars, Datana ministra (See photo).  Larvae are black with four yellow stripes running along the length of their bodies and one around their “neck” just behind the head capsule.  They are thinly covered in long white hairs and reach up to 2 inches long just before pupation.  The adults are small, rarely noticed reddish brown moths.  Females deposit large clusters of eggs (up to 100 or more) on the foliage of several woody plants, including oaks.  As caterpillars emerge, they congregate and begin feeding, skeletonizing leaves which may severely defoliate sections of trees,especially as caterpillars reach maturity.  Then, they use thin strands of silk to drop to the ground where they pupate within the soil.  Treatment is not typically recommended on large, well-established trees as it is costly and often difficult for homeowners to safely get insecticides up to where larvae are feeding.  In many cases, by the time the caterpillars are noticed they are nearly mature and will not be causing damage much longer.  We do not anticipate another generation this year and the defoliation will not cause long term damage to otherwise healthy trees.

Yellownecked Caterpillar
Photo by Drees

Caterpillar frass (poop) under oak tree where yellownecked caterpillars are feeding
Photo courtesy of  Cristina Villalobos 

Another concern is regarding the white to yellowish fuzzy balls and/or pea-sized hard, brown balls attached to the underside of oak leaves (see photo).  These are both types of galls caused by two different species of tiny cynipid wasps (the wool-bearing gall wasp, Andricus quercuslanigera, and probably Belonocnema sp., no common name).  Females deposit eggs singly on oak leaves and the insect’s activity causes a reaction in the tree so that it produces additional growth, or a gall, around the egg or larva.  This serves to protect the developing wasp. 

Oak Leaf Galls

In general, galls may be induced by many different species of tiny wasps, mites, fungi, and bacteria.  They often have complicated life cycles and are very specific to a single plant or tree species.  In many cases these gall-laden leaves will drop to the ground early, but there is no cause for alarm.  Although they might be unsightly, most galls will not negatively impact tree health.  For whatever reason, populations/pressure seem to fluctuate from year to year and they often do not re-infest the same tree the following year (please note this is not always the case).