The larvae feed on the roots of dying conifers, and the beetles on the crowns of the old trees, as well as on brushwood, young plants and the blueberry. Any forest owner would be able to put up with all this, but sooner or later there comes a point when the game is over: at the latest when the large pine weevil appears on regeneration areas after storm or beetle damage.
Fig. 1: Large pine weevil: The insect can be up to 1.5 cm long, and it is brown, with patches of yellow or light brown hairs on its elytra; its head is elongated to form a proboscis. Photo: Manuela Forster (LWF)
The scents emanating from freshly cut stumps attract the beetles. In spring, after storm damage or felling, the beetles fly to the damaged area and start feeding on the existing regeneration - naturally regenerated plants or newly planted trees. The feeding causes characteristic scars, sometimes described as “pockmarking”: In the process, the beetles gnaw the bark off, piece by piece.
The weevil needs fresh stumps to reproduce. It lays its eggs on them from May until August/September. The hatched larvae feed on the roots of the stumps. This feeding does not lead to any damage. Depending on the weather in summer and the time of egg laying, the first young beetles appear in August/September of the same year or a year later. During their maturation feeding, the young beetles gnaw on the bark of young coniferous plants. This is the late summer feeding, which, like the spring feeding, can be recognised by the typical pockmarked feeding pattern.
The beetles can become relatively old, living up to four years. Regeneration areas are therefore threatened by the weevil for at least two years, although the stumps are only suitable for breeding in the first year. In the second year, it is mainly the freshly hatched young beetles that cause damage with their maturation feeding. If fresh stumps appear after further storm and bark beetle damage, the threat is prolonged.
Fig. 2: Pockmark-like patterns caused by feeding: Typical damage pattern on a young Douglas fir. Photo: Cornelia Triebenbacher (LWF)
Fig. 3: Young, vigorous plants can often recover well from damage caused by the weevil's feeding. Photo: Cornelia Triebenbacher (LWF)
The beetles are true gourmets
It has been observed that the beetles feed specifically on planted spruce trees, but have so far spared natural regeneration. Weevils rely on their sense of smell and sight to find their host plant. Planted seedlings from the nursery have a different scent than the seedlings in natural regeneration on an area.
The beetles are polyphagous, i.e. they do not specialise in one tree species. They also feed on the bark of hardwoods such as birch, beech and oak. They cause no serious damage to hardwoods, however. Young Douglas firs and Sitka spruce are another matter, being particularly popular with the beetles - preferable even to spruce and pine. Given the choice between Scots pine and spruce, the weevil prefers the pine.
Adult weevils react to different light and temperature conditions. During the day they remain at the base of their host trees, only climbing the trees after dark. From ambient temperatures of around 8 °C they become active, but are still quite sluggish. Above temperatures of 40 °C, they fall into a sort of heat rigidity, which not infrequently leads them dying of the heat.
Larval development is also strongly dependent on temperature. In laboratory experiments, the larvae went into developmental dormancy at development temperatures between 10 and 20 °C, which could last several months before pupation. At temperatures above 25 °C, the larvae pupated after only a short resting time. This also explains why young beetles can sometimes even appear in the summer of the first year. This is a cause for concern in view of climate change.
The adult beetles fly to regeneration areas in spring, but then increasingly lose their ability to fly, until they can only crawl in summer. They need all their energy for reproduction. The ability to fly develops again in the late summer. The beetle often overwinters in the adjacent stand. When flying, it can cover several kilometres; its dispersal distances when crawling have been shown in studies to be 5 to 6 m.
The feeding of the beetles is dependent on the temperature and the tree species. Laboratory tests showed that the warmer it was, the hungrier the beetles were. At 10 °C they ate around 50 mm² of pine bark (about 7 x 7 mm); at 20 °C it was as much as 250 mm² (about 16 x 16 mm). With spruce bark, the values at the same temperatures were “only” around 30 mm² to 140 mm². At 20 °C, the maturation feeding of the females lasted between just under 12 days and 15.5 days, depending on the tree species.
Fig. 4: Freshly cut old-growth logs and freshly planted Douglas firs - perfect conditions for the large pine weevil. Photo: Sarah Mitze
The beetle feels at home in humus soils
Whether a weevil feeds on a plant or not is strongly influenced by the soil type. Only once they are in the immediate vicinity of the potential host plant do the animals make their decision. If humus or soil mixed with humus surrounds the plant, the probability of weevils feeding on it is almost 50% higher than with pure mineral soil. This is mainly because the beetles find better hiding places in the humus. This effect is usually only observed in the first year, however, because of the emerging accompanying vegetation.
What the forest owner can do
- The best countermeasure is to avoid large clear-cuts and regeneration areas. Since damage events cannot be planned, advance regeneration is always a great advantage. A canopy of more than 80 trees/ha significantly reduces damage.
- In addition, the use of larger and stronger assortments also reduces the extent of damage. From a root neck diameter of 10 mm, healthy plants can also withstand more severe feeding. However, this also requires a careful planting measure, in which the roots can develop properly.
- Disturbing the soil (to reveal mineral soil) in a radius of about 40 cm around the plant helps to reduce the likelihood of an attack. The same result is also achieved through a slight elevation of the plants (mounding). A fallow period of at least two years also helps if no new stumps are foreseeable. However, the onset of competing vegetation and economic losses must then be accepted.
- Protective collars and protection of the trunks with agents applied all round them (e.g. ecowax) have not shown any sufficient effect in LWF trials. Stump removal or large-scale mulching are also not advisable. We generally advise against using vehicles over extensive areas - not only on sensitive soils.
- The laying out of trap wood and trap bark works to a certain extent, but is very time-consuming: The traps need to be renewed approximately every fortnight. They should be checked several times a week. Since the beetles are not caught by the trap wood, they should be collected in the course of monitoring - or they continue to spread.
- The situation is different with various trap systems from which the beetles can no longer escape. As an attractant, the active ingredient alpha-pinene in combination with 70% ethanol has proved successful. So-called Nordlander traps have proved to be the most practicable in LWF trials. However, there are estimates of weevil population densities of several hundred to 70,000 beetles per hectare of bare land in the literature. Due to the beetles' reluctance to move more than 5 to 6 m, 100 or more traps per hectare would therefore have to be deployed and checked weekly. A critical catch number cannot be given, as this depends on the tree species, size and vitality of the plants. Moreover, it is not known how many beetles are actually present per unit area and whether all beetles are attracted by the trap.
- Crops should be checked for feeding damage especially in May and June and again in August/September. In checking, forest owners should look at ten plants respectively at ten locations. If smaller plants (root neck diameter up to 1 cm) show severe damage on at least 10% - i.e. several feeding spots per plant - the use of a plant protection product should be considered.