Includes aphids, adelgids, eriophyid mites, flies, gall wasps, midges, psyllids and sawflies
Pest description and damage Plant galls are formed in response to an interaction with insects, mites, (or nematodes, viruses or fungi) and the plant tissues. Egg-laying, salivary secretions or mechanical injury initiate production of localized plant hormones which cause accelerated growth (larger plant cells or increased number of cells) of the growing tissue. The plant tissue grows around the insect or mite protecting it from weather and natural enemies. Galls can form in the developing tissues of any plant structure such as leaf, buds, stems, flowers, shoots or roots. Generally, damage is not significant to the plant, but may be considered unsightly by homeowners, or may be excessive in some years. Occasionally, infestations flare up to significant numbers and the plant will form abscission cells which cause infested leaves to drop from the host along with the insects or mites inside. Insect galls rarely kill plants and they are heavily parasitized in most years. Low numbers are not harmful to trees.
Several orders of insects have adopted this life style: methods of control will vary with the general group of insects.
- Eriophyid mites such as maple bladder gall mite or linden nipple gall.
- Psyllids or jumping plant lice which cause tip leaves to curl over the psyllid on boxwood.
- Aphids that form the petiole galls on cottonwood or the reddish galls on kinnikinnik.
- Adelgids such as the Cooley spruce gall adelgid which forms cone like galls on spruce (but not on the alternate host of Douglas-fir).
- Flies and midges such as honeylocust pod gall midge, beaked willow gall and willow pine cone gall.
- Gall wasps, tiny members of the order Hymenoptera (not stinging wasps) that form oak apple galls, thimbleberry galls or other galls.
- Sawflies (Hymenoptera) commonly forming willow pea galls and willow bean galls.
Biology and Life History Biology is variable depending on the insect or mite initiating the gall by ovipositing, feeding, or other means. Generally, there are two kinds of galls: closed and open. Closed galls form around and enclose the eggs or developing chewing insects such as tiny flies of the willow pine cone gall, pea gall, or honeylocust pod gall midge or rose galls. The insects must chew their way out of the closed gall. Open galls are formed by the salivary juices of sucking insects or mites. These galls have small openings that allow the mites or insects to move in and out of the galls, or the galls crack open on drying and free the insects inside. Removing green galls before adults emerge may break the life cycle. A few of these galls (removed before the insects have emerged) can be placed in baggies and tacked to a shaded wall under approximate natural conditions. When the first adults (and/or parasitoids) emerge, it's time to initiate controls if it appears the tiny adults are abundant. In some cases, the emerging insects may be parasitoids, so it may be wise to get them identified.
Pest monitoring Some species with the habit of causing significant damage to hosts, such as the honeylocust pod gall midge, can only be controlled when control measures target the adult stage. Thus, monitoring adult insect flight activity with sticky traps and repeated inspection is recommended. Degree-day or phenology models to predict adult flight can be found for some gall insect species.
Most galls can be viewed as an interesting partnership between plant and gall maker. Rarely are they numerous enough to warrant control. Most homeowners find they can tolerate gall insects once they know the plant is not going to die. Prune out or remove galls on infested twigs, buds or leaves if they are found to be objectionable.
Many gall insects are heavily parasitized once they become numerous enough for parasitoids to locate them. Pick galls and place in a sealed plastic bag with a bit of tissue to absorb moisture and wait for emergence of gall maker and parasitoids to determine the level of parasitoid activity.
For more information
Shour, M., L. Jesse and D. Lewis. 2005. Galls on Trees and Shrubs. Iowa State University Extension. IC-417 (http://www.extension.iastate.edu/Publications/IC417.pdf)
Robert P. Wawrzynski, R.P., J.D. Hahn, and M.E. Ascerno. 2005. Insect and Mite Galls. University of Minnesota Extension. WW-01009 (http://www.extension.umn.edu/distribution/horticulture/dg1009.html)