Issue 10, June 25, 2012
Insect Galls and Insecticides
Insecticide applications rarely provide adequate control of gall insects and mites. Basic to this is an understanding of how galls form and their internal structure.
Insect and mite galls form in meristematic plant tissue. Meristems are areas where the cells are undifferentiated in the growing areas of the plant. They include areas of leaf, stem, and flower buds, root tips, and stem cambiums. Based on chemical messengers, hormones, produced by the plant, these cells will develop into those of leaves, stems, flowers, fruit, roots, xylem, phloem, or bark. They are referred to as embryonic or undifferentiated cells as well as meristematic cells, which indicate their plasticity to form various mature tissues.
Gall-making insects and mites produce chemicals during their feeding or other activities which cause these meristematic cells to form galls rather than the plant tissues they were originally likely to produce. It is thought that physical damage by the gall-maker may cause the gall to form in some cases. Although caused by the insect or mite, the gall is plant tissue similar to a tumor in its creation and make up.
From the plant's perspective, it is walling off and compartmentalizing the attacking insect or mite from the rest of the plant. Plants use various methods to isolate themselves from injury or attacking organisms. From the gall-making insect's or mite's perspective, the plant gall provides an adequate supply of food protected from predator attack. Galls are excellent examples of the co-evolution between plants and insects.
In the plant's effort to compartmentalize the gall-maker, the nutrient and water conducting vessels, phloem and xylem, do not run through the gall tissue. With systemic insecticides being transmitted within the plant primarily via the xylem, the flow of them into gall tissue is greatly reduced. Thus, systemic insecticides typically provide little gall control.
The tissue within the gall is composed of cells that are not typical of those found in other areas of the plant. This results in locally systemic or other insecticides that move or disperse through cells to move even less in gall tissue. The outside covering of the gall is not structured like other plant tissue as well, resulting in low chemical absorption rates.
Adult gall-making insects and mites are the only life stages that occur outside of the gall, so they are the only ones exposed to insecticide or miticide residues on the outside of the plant. The majority of gall-makers are parasitic wasps, and adult wasps have hard, thick exoskeletons that do not absorb insecticides and other external chemicals very rapidly.
In summary, neither systemic nor contact insecticides typically provide much control of gall-makers. Some gall numbers can be locally reduced by pruning off and destroying the galls, but we typically have to rely on nature in the form of parasitic wasps and other natural enemies to control galls. (Phil Nixon)