Weevil biology

 

Hylobius abietis (the large pine weevil) and species of Hylastes (black pine beetles) breed principally in stumps and roots of felled trees of a wide range of conifer species. Whilst adult large pine weevils feed on the living bark of most woody or herbaceous plants, black pine beetles seem to be restricted to the roots of conifers. The common names of both insects are therefore misleading.

Felling a coniferous crop produces a large increase in breeding material for both insect species, whilst plant material suitable for adult feeding is reduced. Young trees used for restocking are liable to be heavily attacked by adult pine weevils feeding on the stem from the root collar upwards and by adult black pine beetles tunnelling in at the root collar and feeding on the main part of the root system. Heavy damage can completely girdle stems and cause plant death.

Hylobius generally causes more tree losses than Hylastes, although Hylastes can at times cause very high plant mortality. The two insects often attack the same plant, but being below ground, damage due to Hylastes is easily overlooked and its importance underestimated. If measures are not taken to protect transplants on restocking sites during the first 2 years after planting, an average of 50% of plants may be expected to be lost, although mortality levels vary unpredictably from site to site and often from year to year within a forest. Such losses increase the establishment costs substantially through the cost of insecticide application, replacing plants and additional weeding.

Adult Hylobius may live for up to 4 years and may attack at any time of year when it is warm enough for insect activity. Most Hylobius feeding occurs between early March and November, but there is a tendency for two peaks of damage to occur, one in spring before egg laying and the other in late summer before the adults hibernate underground. The timing of these feeding periods depends on both the physiological condition of the insects and the ambient temperature. The relative magnitude of these peaks will vary from forest to forest and from year to year.

Although adult Hylastes feed throughout the year, the main breeding season is between March and September, the eight week larval feeding period allowing two generations each year.

Many adult Hylobius can emerge from a single stump and associated root system, resulting in populations as high as 150,000 adults per ha. Most will feed on adjacent mature trees or on weed species on site, but heavy feeding damage on trees planted on the site is also very likely. The period during which plants remain vulnerable to damage varies considerably, depending on plant size and species as well as insect populations. Typically, after two growing seasons, 11/2 x 11/2 Sitka spruce (Picea sitchensis (Bong.)) will no longer be susceptible to damage. Douglas fir (Pseudotsuga menziesii (Mirb.) (Franco)) on the other hand remains susceptible for several growing seasons. No successful means of controlling the population in the stumps is currently available and it is therefore necessary to protect the plants directly.