Terminal weevils biology and history
Adults of both species excavate feeding niches and lay eggs in the phloem of the current (lodgepole terminal weevil) or the previous year's growth (spruce weevil). The eggs hatch and the larvae mine the phloem tissue, eventually girdling and killing the terminal shoot.
Mature fourth instar larvae form depressions in the sapwood (spruce weevil) or the pith (lodgepole terminal weevil), where pupation occurs. Adult weevils emerge by chewing holes through the bark and then disperse to overwintering sites.
There are important differences between the spruce weevil and lodgepole terminal weevil life histories (see table). These differences are important in timing treatments. The spruce weevil lays its eggs in the phloem tissue of one-year-old growth, whereas the lodgepole terminal weevil lays its eggs in the newly expanding terminal shoot. Spruce weevils develop from egg to adult in one summer.
In most areas in British Columbia, the lodgepole terminal weevil develops from egg to larva the first summer and from larva to adult the second summer, overwintering in the first year as a larva in the terminal shoot. Lodgepole terminal weevil suffers high egg and larval mortality, often with no weevils emerging from an attacked terminal.
Although egg laying by the lodgepole terminal weevil may not result in successful adult emergence, the terminal shoot is usually killed by the larvae mining. Both spruce weevil and lodgepole terminal weevil may attack more than one terminal each summer. General descriptions of lifestages are:
- Egg – pearly white, 1mm long
- Larva – stout, legless yellowish-white grubs, up to 1.2cm
- Pupa – white, resembles adult; spruce weevil pupae are found in chip cocoons dug into the sapwood, and lodgepole terminal weevil pupae are found in chamber in pith
- Adult – reddish brown to black with cream markings, long curved snout, body length 0.4cm to 1.0cm
A table shows the life history and symptoms following attack by the spruce weevil and lodgepole terminal weevil: