Various means are available to apply pesticides to trees for protecting them from insect and mite pests. These techniques include foliar and soil applications and microinjection or trunk injection.
Microinjection involves placing or injecting small amounts of a pesticide into the sapwood and cambium tissues of a tree. Holes are drilled, or the bark is punctured, about 4 to 6 inches apart. These openings are placed near the base of the tree or root flare with the Mauget system or within the lower few feet of the trunk with the Wedgle system. Material is then injected into the sapwood via low pressure. The tree then takes up the material through the vascular (water-conducting) system, whereby it is distributed throughout the tree. The small holes or punctures eventually compartmentalize or heal during the growing season.
Microinjection works because pressure in the xylem is lower than atmospheric pressure outside the tree (negative pressure)—primarily due to transpiration, or water loss from the leaves. Liquids injected into the xylem tissue under negative pressure are taken up and distributed within the tree’s sap stream.
This technique is best done in the fall or spring, when movement of the liquid within the tree’s water-conducting tissues is highest. Microinjection is used to place insecticides, fungicides, and fertilizers into trees. In general, uptake takes about 2 to 4 hours; however, this period depends on weather, tree age, tree health, and soil temperatures. The uptake of the material occurs best when the soil temperature is above 45°F. However, the material may take longer to move in stressed trees or in locations with inadequate soil moisture. This application technique is best performed by professionals trained in its use.
Although this technique may be used on many types of trees, some do not accept it. For example, American elms, which don’t have a porous xylem, are unable to take up large volumes of liquid this way.
The advantages of microinjection are essentially elimination of spray drift, reduced risk of spills or leaks, minimal exposure to the public, prevention of wash off (because the material is placed inside the tree), prevention of direct contact of pesticide to applicator (because it’s a closed system), prevention of loss due to volatilization, and minimal impact on beneficial insects and mites.
Some concerns with microinjection include that (1) wounds need to be small and as shallow as possible, (2) the proper number of capsules are used and placed at the proper depth, (3) injection holes should never align vertically, (4) tree health can be compromised due to wounding, and (5) there is a possibility of uneven distribution within the tree crown.
The primary insecticide injected into trees is imida-cloprid (Merit, Imicide, or Pointer). This systemic insecticide affects sucking insects (aphids, soft scales, and mealybugs), leafminers, and woodboring insects (bronze birch borer). Timing is important, as the material must be applied before insects start feeding.
This technique is currently being used to deal with the Asian longhorned beetle in Chicago. Imidacloprid is injected into trees to protect them using compressed canisters (Mauget system). Imidacloprid is readily transported within the xylem into stems, twigs, and leaves, where newly emerged adult beetles feed before females lay their eggs beneath the bark. In addition, research has demonstrated that the eggs and early instar larvae are killed.
Due to environmental concerns with foliar or spray applications, microinjection is gaining acceptance as a method that poses minimal impact on the environment while still providing effective control of plant-feeding insects.