Plant galls are abnormal growths of plant tissue that develop in response to irritation from an external organism. They appear as swellings, bumps, or intricate structures on leaves, stems, or roots. Gall formation is a plant’s specific, localized response to a chemical or mechanical stimulus from the gall-maker. This article outlines strategies for managing galls currently present and preventing future infestations.
Understanding Gall Formation and Severity
Galls are induced by a wide array of organisms, primarily insects, mites, fungi, and bacteria. Insect galls are often the most noticeable, created by tiny wasps, midges, aphids, or mites that inject growth-regulating substances or lay eggs into the plant’s developing tissue. The plant then forms a protective layer of tissue around the invader, providing the organism with both shelter and a food source.
The severity of a gall infestation depends heavily on its location and the causal agent. Most leaf galls, such as maple bladder galls caused by eriophyid mites, are generally cosmetic and do not pose a significant threat to the overall health of a mature plant. Control measures are often unnecessary unless the infestation is severe or the plant is very young.
Galls that form on the stems, twigs, or crown of a plant, however, can be more serious, especially if they are caused by pathogens. Stem galls, like the hard, woody growths resulting from some gall wasps, can potentially girdle a branch, restricting the flow of water and nutrients. Bacterial galls, such as Crown Gall caused by Agrobacterium tumefaciens, are particularly damaging as they often appear at the soil line and can stunt or kill susceptible plants. Identifying the type of gall is a necessary first step to determine if intervention is warranted.
Non-Chemical Management of Existing Galls
Once a gall has fully formed, the organism inside is protected by the plant tissue, making chemical treatments ineffective. Therefore, the primary strategy for managing existing galls relies on cultural and mechanical control methods. Pruning is the most direct approach, but it must be timed correctly to remove the pest before it completes its life cycle and emerges to re-infest the plant.
For galls on leaves or small, non-essential twigs, removal should be performed as soon as they are noticed. When dealing with stem galls, prune the affected branch several inches below the visible swelling to ensure the entire gall-making organism is removed. Proper disposal is necessary; plant material containing galls should never be added to a home compost pile, as the temperatures are usually not high enough to kill the insects or pathogens. Instead, the material should be bagged and placed in the trash for landfill disposal, or incinerated where permitted.
Sanitation practices extend beyond simple removal; they include disinfecting pruning tools with a 10% bleach solution or isopropyl alcohol between cuts and between plants. This step is especially important when dealing with bacterial or fungal galls, which can be easily spread through contaminated equipment.
Cultural practices also play a strong role in passive management by promoting plant resilience. Maintaining balanced soil fertility and ensuring appropriate irrigation helps a plant naturally defend against or recover from pest attacks. Plants stressed by over- or under-watering or improper nutrition are more susceptible to severe infestations. Over-fertilization, particularly with high nitrogen, can lead to flushes of tender growth that are highly attractive to many gall-forming insects.
Chemical and Biological Strategies for Prevention
Chemical control is only effective as a preventative measure, targeting the vulnerable life stage of the gall-maker before the gall tissue forms. The most critical factor is precise timing, which is determined by the pest’s life cycle and the plant’s development stage. A common preventative approach for insect and mite galls involves the use of horticultural or dormant oils.
Dormant oils are highly refined petroleum products that smother overwintering eggs and adult pests, such as eriophyid mites and some gall aphids, on the bark and buds. These sprays must be applied in late winter or early spring, typically when temperatures are above 40°F for 24 hours, but before bud break and the emergence of new leaves. Applying oil after bud break can damage tender new foliage, a condition known as phytotoxicity.
Systemic insecticides, which are absorbed by the plant and move through its vascular system, can be effective against specific gall wasps and midges. They are best applied at the time of egg-laying or early larval development. For example, studies on certain gall wasps have shown that a foliar application of a systemic product, such as spirotetramat, immediately following bloom can significantly reduce gall formation.
Biological control offers a less chemically intensive option, particularly for pathogen-induced galls. For Crown Gall, applying a solution containing the beneficial bacterium Agrobacterium radiobacter strain K1026 to plant wounds before planting can prevent the pathogenic bacterium from infecting the tissue. Encouraging natural predators, such as parasitoid wasps, is a long-term strategy, as these beneficial insects are known to lay their eggs inside galls, killing the developing gall-maker.