Forest Pests: Insects, Diseases & Other Damage Agents

Csóka, György and Kovács, Tibor (1999): Xilofág rovarok - Xylophagous insects. Hungarian Forest Research Institute. Erdészeti Turományos Intézet, Agroinform Kiadó, Budapest, 189 pp.

Xylophagous insects play a multifunctional and very important role in forest ecosystems. In this book we can discuss their roles only briefly, highliting only a few of the more important facts and points of view.

Some xylophagous insects develop either in dead trees and shrubs or in dead parts of still living woody plants. These are called saproxylic. A couple of decades ago they were not considered to play a significant functional role in the functioning of forest ecosystems. Now it is a well known fact that this earlier opinion was fundamentally wrong, and that undervaluation of their importance came simply from lack of knowledge of their importance. The saproxylic species play an important and irreplaceable role in the decomposition of dead woody plants. The first steps of this decomposition process are taken by the saproxylic species settling on the freshly dead trees (some species of longhorn beetles for example). Without their contribution the nutrient cycling of the forests would become slower and nutrients accumulated in the woody plants would get back to the soils as reusable substrate much later. Only these species are able to start cutting up the sometimes very hard and resistant wood tissues. Other organisms would not be able to do this and to open tunnels into the heartwood through which other decomposing organisms, such as fungi, can get inside.

Not estimates are available for how much wood tissue is needed for the full development of a xylophagous larva, such as a large longhorn larva. Considering the fact that the N (nitrogen) content of dead (and dry) wood is very low (usually under 1 %) and the N content of the larvae is very high, it is easy to conclude that the larva must consume a quantity of woody tissue many times its own body weight. In the case of larvae feeding in living tissues which are richer in N, this ratio is expected to be somewhat lower.

After these insects have taken the first steps in decomposition of wood, the chewed-up wood and their frass becomes available for other macro-, and microscopic organisms. The saproxylic insects take part not only in the first part of the decomposition process, but also in all successive stages. There are some species (flower beetles belonging to the dung beetle family for example) which feed in very decomposed wood. These species help to mix the wood compost into the soil. Further decomposition from this point becomes the duty of small arthropods such as springtails and mites, and even smaller microscopic organisms.

The xylophagous species developing in living woody plant usually consume a minor part of the food plant and do not cause any significant harm. It is unnecessary to prove that a tiny shoot boring creature, for example, attacking a low percentage of the shoots of a large oak tree, will cause only a negligible physiological effect. Of course the xylophagous insects - just like any other parasitic organisms- when massively abundant can have serious impact on the health of the foodplant, occasionally even causing mortality. In general, massive outbreaks of xylophagous insects rarely occur in healthy forests under natural conditions. Massive outbreaks of xylophagous insects (mainly outbreaks of bark beetles) most frequently occur in plantation-like, poor and even-aged stands planted on poor, suboptimal sites. It is also evident that these outbreaks - in addition to the unusual sylvicultural conditions- are also helped by the long periods of drought, and in some cases by environmental pollution.

Many xylophagous insects are known to be vectors of serious pathogens of woody plants. In many cases these pathogens have far more serious impacts on the foodplant than the insect's feeding itself. Probably the most well known example for this is the fungus Ceratocystis ulmi (causing Dutch elm disease) transported by the elm bark beetles (Scolytus scolytus for example). It is already known that this relationship is a mutualistic one, which is advantageous for both the fungus and the bark beetle. The bark beetle larvae need dying trees for their development (a healthy tree's self defence probably could prevent their full development) and this is provided by the fungus killing the tree. The fungus gets a " free ride" in return from the dispersing bark beetle, essential for it to infect new foodplants. Functionally similar relationships are known between woodwasps and fungi, and between some longhorn beetles (Monochamus spp.) and nematodes living in trees. These relationships are undoubtedly fascinating even if they sometimes lead to significant damage to human efforts.

The xylophagous insects, particularly their larvae, serve as a food source for many other species of animals. Some of these predators are highly specialised and can exclusively feed on xylophagous insects.

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