Trap trees

Low pressure injection (or topical application to axe or chainsaw cuts) of unwanted or hopelessly diseased but living elms with the herbicides cacodylic acid (sodium dimethylarsenate) or monosodium methylarsenate (MSMA) kills elms within 14-18 days {[125],[792]}. As a result of this treatment, the dead elms become highly attractive to European elm bark beetles and H. rufipes. Beetles attracted to the trees will start colonization. However, due to herbicide-induced drying of the inner bark, the brood will fail either entirely or substantially and the production of beetle brood from trap trees is small {[232],[483],[779],[807]}. For this reason, these elms do not need to be quickly removed, unlike non-treated diseased elms.

O'Callaghan et al. {[792]} reported the colonization of elms treated with cacodylic acid by such bark saprophytes as Phomopsis oblonga. The latter fungus is known to negatively affect bark beetle reproduction (see enemies of the elm bark beetle). Since trap trees absorb more in-flight DED vectors than they contribute to the next generation, reduction of the beetle population takes place {[125],[351]}. In addition, trap trees absorb DED vectors of the current generation that might otherwise transmit the disease to healthy elms. However, the attraction of these beetles to trap trees ceases after continuous drying of the bark and colonization by saprophytic fungi. Instead of forming a gallery, many beetles will leave immediately after landing on a trap tree {[789],[790]}. Lanier et al. {[351],[793]} suggested that mortality of the current beetle generation can be maximized by baiting trap trees with synthetic pheromones in combination with treatment of the lower bole of the elms with such insecticides as chlorpyrifos.

The herbicide properties of MSMA and cacodylic acid are quickly inactivated upon soil contact {[787]}. Burning of wood containing the pentavalent organic arsenicals cacodylic acid or MSMA will often produce less arsenic compared to burning coal (45 ppm on average {[788]}). The chemicals can be transported via root graft connections from a trap tree to a adjacent healthy elm, causing the death of the latter within approximately 7 days {[351]}. Double chainsaw girdles and vertical cuts will separate healthy elm wood from diseased material.

The timing of herbicide application is important for the effectiveness of the method. To enable a sufficient distribution of the chemicals throughout the diseased elm, the sapwood may not be frozen and the herbicide treatment must be completed before the disease has progressed too far {[125],[351]}. The time required to treat one tree is approximately 15 minutes. The dosage of chemicals used depends on the bark surface area {[351]}.

Although the trap-tree strategy appears to be effective in reducing beetle immigration from green spaces and such areas as railroad rights-of-way, the technique is of little use in towns where trees cannot be spared for traps {[125],[351],[641]}.