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.

This family has the highest number of species (ca. 50,000) in the order Coleoptera, and 1400 of them are recorded from Hungary. This makes the curculionids the most species-rich family of Coleoptera in Hungary. The head is snout-like and elongated into a rostrum, but in some groups the rostrum is very short. All species are phytophagous with a very broad range of appearance and life history. Their larvae are legless.

The broad-nosed snout beetles (subfamily Cossoninae) are represented by quite many species in Hungary. Their larvae develop in dead decaying wood.

In addition to a few of species belonging to the subfamily Curculioninae we pay special attention to the subfamily of bark beetles (Scolytinae) which have the greatest importance as xylophagous insects. Earlier this group was considered as a separate family in its own right.

The bark beetles are small (1-9 mm), usually cylindrical beetles. The family is very diverse, with more than 6,000 known species world-wide. In Europe there are approximately 300 species and ca. 150 in Central Europe. Most of them feed on the tissues of various parts of plants. A very interesting exception is a group commonly called ambrosia beetles. Adults of these species introduce the spores of fungi to their tunnels, and developing larvae feed from a fungal garden that is far more digestible than wood.

The bark beetle species native to Hungary usually have one or two generations per year, although some species have a lifecycle lasting two years. Developmental rate, and so the number of generations per year, are strongly dependent on environmental conditions such as food quality and climate. After emerging from their pupae, bark beetles leave the host tree but are still not sexually mature. Maturation usually follows 1-2 months of feeding in the adult stage, and in some species an overwintering diapause. Mating takes part on the trunk in some species, while in others it occurs under the bark in a chamber excavated specially for this purpose.

Mated females bore egglaying (oviposition) tunnels into species-specific tissues of the tree (bark, sapwood, heartwood) and lay their eggs on the tunnel wall. These tunnels are called egg galleries. In the case of polygamous species, in which a single male beetle may mate with several females, several egg galleries may radiate from one mating chamber. The larval galleries - made by the feeding larvae- radiate from the egg galleries. Bark beetle larvae are small, plump, slightly curved, legless and usually white. Larvae of many species construct a widened pupal chamber at the end of the larval galleries. The size and shape of both egg and larval galleries are typical of a given species, and in many cases are diagnostic enough for safe identification of the beetle species. Bark beetles have a free pupa on which the shape of the beetle and the body parts can be easily recognised.

When healthy trees are attacked by low numbers of beetles, they are able to flood the larval tunnels with resin or sap, and so kill the beetles and stop the attack. Most bark beetle species can therefore only attack weakened or sick trees whose self-defence systems cannot operate effectively. Many bark beetle species overcome tree defences by simultaneous mass colonisation, mediated in many species by an aggregation pheromone. One sex is the colonising sex, and this recruits individuals of both sexes to a single host tree. Colonisation above a certain density is disadvantageous, because the larvae consume the whole food source before development can be completed, and many species secrete anti aggregation pheromones once some threshold density is reached. The aggregation pheromones of one bark species can also act as anti-aggregation pheromones to other bark beetle species. Aggregation pheromones can be used in the control of bark beetle pests - this possibility is discussed in more detail in a later chapter. Other bark beetles weaken their food plant by infecting it with aggressive pathogens.

The bark beetle species feeding on a given host tree species partition the available resources. In case of the four species feeding on spruce (Pityogenes calcographus, Polygraphus poligraphus, Ips typographus and Dendroctonus micans) the smallest species (Pityogenes) colonises the thinnest stems in the crown followed in thicker stems by the slightly larger Polygraphus and later Ips and Dendroctonus which feeds on the thicker barked parts of the trunk. Similar partitioning is known in case of the ash bark beetles (Hylesinus and Leperisinus species) where there is positive correlation between the size of the beetle and the thickness of the bark on the trunk part attacked.

The bark beetles feeding in cambium and sapwood have the greatest economic importance among the xylophagous insects. During the last decade many European countries (Austria, Poland, Slovakia, Romania, the Czech Republic and Lithuania) have suffered massive outbreaks causing the death of many millions of m3 of wood, representing several tens of thousands of hectares of forest. The most common victims have been conifers, particularly spruce. Although the relative importance of spruce is much lower in Hungary than in the countries mentioned above, over the same period we have also experienced serious damage resulting from bark beetles.

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