Question
I have a client who has 2 trees already infested with the Locust Borer.  The first one has already snapped off from the decay inside the tree and the wind.  I checked with another source and it stated that you could apply a drenching spray in late August, since the larve are already inside his other tree.  What type of spray could you suggest?  Thank you. (Zone 5)

Answer
Soil-applied Systemic Insecticides
Systemic insecticides applied to the soil are taken up by the roots and translocated throughout the tree. The most widely tested systemic insecticide for control of EAB is imidacloprid, which is available for use by professional applicators and homeowners. Professional use formulations of soil-applied imidacloprid include Merit?75WP, Merit?75WSP, and Merit?2F. The homeowner formulation of imidacloprid is Bayer AdvancedTM Tree & Shrub Insect Control. Additional formulations of imidacloprid with different brand names are also becoming available.

All imidacloprid formulations can be applied as a drench by mixing it with water and pouring it directly on the soil at the base of the trunk. The application rates for the homeowner and professional formulations of imidacloprid are very similar (1.3 and 1.5 grams of active ingredient per inch of trunk diameter, respectively). Soil drenches offer the advantage of requiring no special equipment to apply (other than a bucket or watering can). However, surface layers of organic matter, such as mulch or leaf litter, can bind the insecticide and reduce uptake. Before applying soil drenches, it is important to remove or pull back any mulch or dead leaves so the insecticide solution is poured directly on the mineral soil.

Merit formulations can also be applied as soil injections, which require special equipment, but offer the advantage of placing the insecticide directly into the root zone. Injections should be made only deep enough (2-3 inches) to place the insecticide under the turf or mulch layer. Soil injections can be made either at the base of the trunk or on a grid pattern extending to the edge of the canopy. Recent studies have found that soil injections made immediately adjacent to the trunk (within 6-18 inches) are more effective than those made on a grid pattern under the canopy. Density of fine roots is very high at the base of the trunk and declines quickly as you move away from the tree because large radial roots diverge like spokes on a wheel prior to branching into smaller roots that ultimately terminate in feeder roots. This pattern of root distribution can be clearly observed on trees that have been recently uprooted in a storm, or when taking soil cores under the canopy, many of which will be devoid of fine roots.

Optimal timing for imidacloprid soil injections and drenches is mid-April to mid-May , which allows the 4-6 weeks that are necessary for uptake and distribution of the insecticide before larvae begin to establish in mid- to late June. One study with small trees indicates that imidacloprid soil drenches can also be applied successfully in the fall.

EAB larvae damage the vascular system as they feed, which interferes with translocation of systemic insecticides. Studies are underway to determine how much injury a tree can sustain before systemic insecticide treatments are rendered ineffective. It is probably unlikely to save trees showing more than 50 percent dieback, and any damage can reduce the effectiveness of systemic treatments.

Trunk-applied Systemic Insecticides
Several systemic insecticides can be injected or implanted directly into the trunk of the tree. Some formulations are applied by professionals, while others are available to homeowners. Imidacloprid is available in several professional use formulations that are injected directly into the trunk using various application systems. These include IMA-jet? which is injected using various ArborjetTM injection systems; Mauget Imicide?micro-injection capsules; and PointerTM, which is injected using the Arborsystems WedgleTM Direct-InjectTM injector system. Another insecticide option is Mauget Inject-A-Cide B?micro-injection capsules, which contain Bidrin?(dicrotophos). Systemic trunk implants available for purchase and application by homeowners include ACECAP?97 Systemic Insecticide Tree Implants and Bonide?Systemic Insecticide Bullets, both of which contain acephate as the active ingredient. Both products are applied by inserting insecticide-containing capsules into holes drilled in the base of the tree trunk.

Trunk injections and implants have the advantage of being absorbed by the tree more quickly than soil applications, and can be applied where soil treatments may not be practical or effective, including trees growing on excessively wet, compacted, or restricted soil environments. However, trunk injections and implants do injure the trunk, which may cause long-term damage, especially if treatments are applied annually.

Optimal timing of trunk injections and implants is between mid-May and mid-June. Studies have shown that Inject-A-Cide B injections made as late as August can kill insects in the tree, although substantial feeding damage will have already been done. If the option exists, applications should be made earlier to prevent larval establishment.

Most efficient uptake of trunk-injected insecticides occurs when trees are actively transpiring. Best results will be obtained when injections are made on sunny days in the morning when good soil moisture conditions prevail. Uptake will be slow on cloudy days, during hot afternoons, and when the soil is dry.

Protective Cover Sprays
The objectives of protective bark cover sprays are to kill newly hatched larvae on the bark before they can enter the tree, and depending on the label, adults as they feed on foliage prior to laying eggs. Products that have been evaluated as cover sprays for control of EAB include OnyxTM (bifenthrin), Tempo?(cyfluthrin), Sevin?SL (carbaryl), Orthene?(acephate), and BotaniGard?(contains spores of the insect-infecting fungus Beauveria bassiana). Some of these have been much more effective than others in university trials .

Protective cover sprays are designed to prevent infestations and must be timed precisely to be effective. Because protective residues must be present on the bark before eggs hatch to prevent infestation, they must be timed to coincide with adult emergence and oviposition, which is difficult to monitor because there are no effective pheromone traps for EAB. However, first emergence of EAB adults corresponds closely with full bloom of black locust (Robinia psuedoacacia), which can serve as a useful phenological indicator for accurately timing applications. Best results with cover sprays have been obtained when two applications are made, with the first as black locust reaches full bloom  and the second four weeks later.