Q – Why are there two peaks of feeding damage?
A – It is the adult weevils that damage plants in the forest. Although the insects will feed at any time during the year if temperatures are high enough, there is a tendency for two major peaks of damage to occur. The first is in spring (April–June) before egg laying and the other in late summer (August–November) 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 unpredictably from forest to forest and from year to year.
Q – Are all restocking sites at equal risk of damage?
A – Although young trees of all species, both conifers and broadleaves, are susceptible to damage by the large pine weevil, the insect only develops in the stumps of felled conifers. The species and size of the previous crop may affect the population; for instance plantations of small prematurely felled Lodgepole pine quickly generate high populations of Hylobius compared to spruce sites. On the other hand, stumps in older Douglas fir, Scots pine and larch crops remain suitable for insect development for several years although, overall, the Hylobius population may be smaller. Lodgepole pine sites must be regarded as very high risk whilst all other species will result in medium to high risk.
Q – How quickly will weevils develop in an isolated block with no previous felling?
A – Hylobius adults can fly considerable distances and may walk considerable distances. For this reason, all areas of felled conifers are likely to be invaded by weevils migrating to the site during the first summer after felling. The nearer the site is to older felled areas, the larger the initial population is likely to be and therefore the greater the risk of damage. The proximity and population density of previous generations of adults determines the rate at which the insects invade the site and exploit the stumps. Large areas of contiguous felling over a number of years will tend to generate greater problems of Hylobius damage, but no area of conifer felling can be regarded as free from risk.
Q – Can we control weevils by burning or removing the brash?
A – Stumps under the brash mat generally have fewer Hylobius adults there than in more open areas. Because it deteriorates fairly quickly, the brash mat itself will not contribute significantly to the insect population. In fact adult feeding tends to be reduced to plants within an undisturbed brash mat. When felling debris is cleared from the site by use of controlled burning, heat from the fire does not penetrate deep into the ground. Hylobius larvae under the bark of the stumps are unaffected by the fire and the blackened surface will absorb heat from the sun to increase the rate of insect development. When the insects later emerge as adults there is usually no vegetation on the site on which they may feed other than the young trees.