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Boring insects - fact sheet
Stem- and bud- boring insects can be major pests of trees and shrubs. Damage is caused by the feeding activities of larvae from certain groups of beetles, moths and wasps. The most destructive borers in economic terms are: stem-boring longicorn (cerambycid) beetles; stem-boring cossid and hepialid moths; bud- and stem-boring (xylorictid and helicoverpid) moths; and stem-boring Siricid wasps. Leaf miners, since they bore tunnels through leaf tissue, could be reasonably considered to be boring insects, but they generally are treated as a separate category as regards control.
Many boring insects begin life as eggs laid under bark by free-living, winged adult females. Larvae commence feeding on the wood of stems upon hatching, and emerge from the stem as adults to repeat the cycle. The amount and type(s) of damage inflicted on the host plant depend on the species of borer and any associations that it may have with other organisms such as fungi or bacteria.
Beetles: Eggs are generally laid under bark via cuts made by the adult females, or in lesions from other sources of damage. On hatching, the larvae tunnel into the heartwood of the tree, feeding on the wood as they go. The mouthparts of boring beetles are extremely strong, enabling them to withstand the continual demands that cutting through tough woods places on the jaws during their juvenile life. The grating and scraping sounds of longicorn larvae, in particular, can be heard quite distinctly if you place your ear against the trunk of an affected tree, or sometimes even if you stand near such a tree. Some species can eventually produce quite complex gallery systems rivalling those of termites.
Some species of beetles, the most well-known of which is the European Elm Bark Beetle (Scolytus multistriatus), inoculate the wood of their hosts with a fungus that breaks down the timber and allows the beetles to digest it. The fungus which causes Dutch Elm disease, Ceratocystis ulmi, is extremely pathogenic to Dutch Elm and other elms (Ulmus spp.), and can kill trees within weeks of infecting them. Dutch Elm disease is not known in Australia yet, so any sudden dieback or signs of beetle activity noticed on elm trees should be reported to your local Council or Department of Agriculture office immediately.
Moths: Eggs are laid in much the same ways as beetle eggs - on, in or under the bark. Whereas stem-boring beetles tend to be gregarious, stem-boring moth larvae are more solitary, with usually only one grub per tunnel. A prominent example of a moth borer is the Waratah Bud Borer Xylorycta luteotactella. The larvae of this insect feed inside both the stems and developing flower buds of the waratah, and the insect is a significant pest in commercial waratah flower crops.
The infamous heliothis moths or Cotton Bollworms, Helicoverpa zea (plus closely related Australian native species H. punctigera and H. armigera, and cotton pests) bore through stems and developing cotton buds in a similar way, and can ruin cotton crops. Unfortunately, these caterpillars are extremely difficult to control by chemical means, and have become intractable pests in cotton crops worldwide (see below).
Wasps: Although their feeding activities under the bark of pine trees result in serious damage to the stems of their hosts, Siricid wasps (e.g. Sirex noctilio or Sirex Wasp) do not actually kill their hosts by boring, but like the Dutch Elm Beetle, the cause of primary damage is the activity of an associated fungus (Sphaeropsis sapinea). Host trees are inoculated with fungal spores by the female Sirex wasps when laying their eggs under the bark. It is the fungus which kills the tree en route to providing a rich food source for developing wasp larvae. In the case of Pinus radiata, the most significantly affected of Australian plantation pine timbers, the wood is made useless by softening, a prominent blue-grey staining and larval tunnels.
Borers in plants are notoriously difficult to control once they have established colonies within their hosts. Non-systemic pesticides have no effect since the chemical does not reach its target; and in many instances, systemic pesticides are not much more successful. This may be partly due to the poison being too diluted by the time it reaches its target, or not actually reaching the affected region at all. Systemic pesticides also are very toxic, and for this reason their use is discouraged. Another possible side effect of highly toxic pesticides is the creation of new pests, which can breed up in large numbers if their natural enemies are killed off by these chemicals (see below).
If you are lucky, the borers may only be present in a lateral branch or stem, in which case the problem can be solved by pruning and removing the affected limb(s). In the case of a large tree, a qualified arborist/tree surgeon should be employed to do the removal safely. Usually, however, the borers are well established in the trunk or main stem of the plant by the time they are discovered, so control by pruning is not feasible. The plant then usually has to be removed before it becomes dangerously unstable (if it hasn't become so already), and before it infects other susceptible plants growing nearby. If you need further information, the National Arborists Association of Australia (ph. 02-9970 7899) can recommend either a course of action or a member arborist who can evaluate your problem at first hand on-site.
The best way to control borers, whether beetle, moth or wasp, is to prevent plants from becoming stressed or weakened by other factors, such as poor nutrition, attack by other insects or mites, pH imbalance, mineral toxicities, lack of water or any combination of these. A healthy tree is more likely to fend off borer attack. Some eucalypt species respond to borer attack by producing protective resins in large quantities that flood the borers’ galleries. This can often be seen as runnels or streams of reddish-brown resin pouring down the trunk: if you see this phenomenon, it is highly likely that the affected tree is being devoured from within by one (or more) species of borer!
Beetle larvae can be killed reasonably easily if their tunnels open out on the surface of the stem or trunk - although by the time the hole has appeared, they have probably caused considerable damage. Some control is better than none, however, and may prevent further damage and subsequent loss of plants. The larval insect can be stabbed by inserting a straightened coat hanger or a similar length of light gauge (e.g. 14 or 16 swg) fencing wire down the hole.
Other bud-boring moths have even greater impact on agricultural crops such as cotton. Cotton Bollworms (Helicoverpa spp.), the world's most significant agricultural insect pests, have become resistant to most pesticide types through overuse and misuse, a feedback system now known as the ‘pesticide treadmill’. One of the most spectacular examples of this occurred in South America in the 1950s and 1960s - an entire regional cotton industry collapsed when pest insects became resistant to even the most toxic and persistent pesticidal chemicals available at the time. Over-application in both amount and spray frequency led to increasing resistance in the moth populations, which in turn led to the use of increasingly toxic pesticides at higher rates and at increasing frequencies, which again led to further increases in moth resistance, and so on. Hitherto harmless insects also became severe pests in both cotton and food crops, as the counterbalancing natural enemies of these new pests disappeared in the face of the enormous pool of persistent chemicals entering the food web. The ramifications affected the human population rapidly, not only as the result of total crop failures, but also in the form of many deaths from disease borne by pesticide-resistant parasites such as fleas, lice and ticks.
Controversy still surrounds the use of pesticides in the cotton industry: in Australia the potential buildup and harmful effects of highly toxic pesticides and their residues in inland waterways such as the Murray-Darling river system are currently causing much concern. Many attempts have been made at developing biological and integrated pest management systems in the cotton industry worldwide to achieve long term sustainability, but marked success and wide acceptance have so far been elusive.
Occasionally, as in the case of the Sirex Wasp in Pinus radiata plantations, an effective biological control agent is discovered and successfully used to control a pest. The organism which now has Sirex under control in South Eastern Australia's pine plantations is a nematode or 'eelworm', a tiny threadlike organism (quite unrelated to segmented worms such as earthworms) which infects young wasp larvae and renders them sterile. This nematode has been spectacularly successful in controlling Sirex, and is specific to this wasp, so there are no harmful side effects.