Forest Practices Code - Terminal Weevils Guidebook

[Terminal Weevils Guidebook Table of Contents]


Introduction

Terminal weevils are found throughout most regions in British Columbia on pine and spruce. Feeding and larval mining by the weevil kills the terminal growth, and although this damage causes no direct tree mortality, unacceptable growth loss and stem deformation may occur in young stands. Tree volume and wood quality may be reduced by terminal weevils.

The two terminal weevil species of economic importance in British Columbia are the spruce or white pine weevil, Pissodes strobi (Peck), and the lodgepole terminal weevil, Pissodes terminalis (Hopping) (Table 1). The species are similar in appearance but differ in their life histories, hosts and damage. Spruce weevil is found throughout the range of spruce except the Queen Charlotte Islands and parts of the outer coast. The range of spruce weevil is limited by the number of degree-days available for completion of development. Lodgepole terminal weevil is found throughout the range of lodgepole pine in British Columbia, with a lower incidence of attack at the higher elevational range of lodgepole pine and at higher latitudes.

Table 1. Common and scientific names and hosts of the two most important species of terminal weevils in British Columbia.

Forest-level impacts have become apparent in parts of coastal British Columbia that have suffered from severe spruce weevil infestations. Over the last decade, species selection guidelines for the Vancouver and Prince Rupert forest regions have virtually eliminated the planting of pure stands of Sitka spruce in highly productive valley bottom sites disposed to weevil damage. In most of these sites, Sitka spruce was the "preferred" species because of its superior growth and high lumber value. However, because of the risk of weevil damage, Sitka spruce is now classified only as an "acceptable" species. Due to height suppression caused by the weevil, many former spruce stands planted in the early 1960s and 70s have naturally converted to stands dominated by other acceptable conifers, usually western hemlock, amabilis fir and western redcedar. In the interior, severe weevil damage (> than 20% currently under attack) has been noticed in the Interior Cedar Hemlock zone and moister portions of the Sub-Boreal Spruce wet cool subzone. Spruce plantations in these and similar areas now pose management problems.

This guidebook describes the biology, damage symptoms and impacts of each species, and recommends management options where appropriate.

Life history and biology

Life history and damage

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 (Table 2). These differences should be noted, as they 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 weevil develops from egg to adult in one summer. In most areas in British Columbia, 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 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, 1 mm long

Larva stout, legless yellowish-white grubs, up to 1.2 cm

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 0.4 to 1.0 cm.

Life histories and damage due to weevils are described further in Tables 2 and 3. Figures 1 and 2 depict various life stages of the insects, as well as the damage caused by weevil attack.

Table 2. Life history and symptoms following attack by the spruce weevil and lodgepole terminal weevil

Figure 1. Diagrams showing, clockwise from upper left, spruce weevil chip cocoons with emergence holes, adult weevil recently emerged from spruce leader, and shepherd's crook and multiple leaders of attacked spruce.

Figure 2. Diagrams showing, clockwise from upper left, lodgepole terminal weevil feeding puncture with egg and fecal pellet in lodgepole pine terminal, larva in pith, pupa, adult on needles, and old attack causing a fork.

Table 3. Symptoms of terminal weevil attack

Spruce weevil population dynamics

Attack rates by spruce weevils in spruce stands typically increase rapidly within a few years of initial attack. Figure 3 illustrates weevil attack rates over the life of a stand in British Columbia. Attacks commonly begin five years after planting, when leaders become large enough to attract weevils (e.g., 1.5 cm in diameter and 40 cm in height for Sitka spruce, smaller for white and interior spruce). Many damaged trees develop multiple tops, which may be attacked annually. Weevil populations and attack rates stabilize when average plantation heights are between 2 and 10 m, and then begin to decline when tree height is >10 m. Severe infestations have an average maximum of >30% attacked leaders per year. The spruce weevil is not a problem in older stands.

Figure 3. Typical course of a spruce weevil outbreak on open-grown Sitka spruce in B.C. The population passes through a stage of rapid increase, a phase of insect-host equilibrium, and a decline phase (source: Alfaro et al. 1994).

Age of initial attack, rate of population increase, maximum intensity, and duration of weevil attack will vary, depending on host availability and environmental factors, such as climate, that influence both the host and the weevil. Long-term monitoring of stands in various hazard zones and subzones is currently the only means of providing an accurate estimate of maximum attack rates. Generally, observations have shown that attack levels in low hazard areas usually remain low (<5%). However, within high hazard ecosystems, low attack rates in young (10- to 15-year-old) stands do not adequately reflect the peak level of infestation. Stands must be >15 years old if peak levels of annual attack are to be obtained (for both the interior and coast).

Lodgepole terminal weevil population dynamics

Attack rates by lodgepole terminal weevil fluctuate greatly from year to year depending on weather, host quality and density. Annual attack rates of >25% have been documented but usually average about 5%. Trees are susceptible soon after establishment (1 m height or more), but the highest rate of attack is seen in stands between 2 m to 5 m in height. Stands spaced to low densities at an early age are at increased risk from weevil attack, because more vigorously growing trees produce longer, thicker leaders, which in turn provide better nutrition for the weevil. High levels of weevil attack are also observed in high density stands, but the defect caused by attacks is generally less severe due to the less branchy nature of tightly grown trees.

Impacts of terminal weevils

Terminal weevils reduce the quality and quantity of merchantable timber. When the terminal is killed, lateral branches turn upwards and compete for dominance. Usually within one or two years one lateral assumes dominance, and stem form is not greatly affected. In other situations the impact is greater, as defects may develop in the form of creases, crooks or multiple tops (Figure 4), which may persist to rotation age and which will affect wood quality.

A simulation model called the Spruce Weevil Attack Trial (SWAT) model (developed by the Canadian Forest Service) provides an estimate of the volume production at rotation following a defined period and intensity of weevil attack (Figure 5). For example, a 30% attack level simulated over a 30-year period will reduce volume production by nearly 40% at the time of rotation. The introduction of decay fungi as another form of defect may also be attributed to weevil damage. This relationship has not been formally investigated for either pine or spruce.

Figure 4. Defects caused by terminal weevil attack.

More immediate effects from spruce weevil attacks are visible when height growth is suppressed following several years of repeated attack. Attacked trees do not reach minimum target heights and are not considered free growing. Suppressed and deformed trees can be culled during spacing operations. Heavily damaged stands that are also marginally stocked may be rehabilitated, but this usually is done only after all other silvicultural options have been exhausted (see the "Stand tending" section under "Management strategies"). Stand rehabilitation has not yet been necessary in white or interior spruce plantations. The impact of spruce weevil on stands that are rehabilitated would equal the volume loss brought about by a regeneration delay of 10 to 20 years.

There are 35-year-old permanent sample plots at Klanawa, on Vancouver Island, containing Sitka spruce with a history of severe weevil attack. Recent examinations of them have shown that the degree of defect is substantially lower than first predicted. Severity of crooking has diminished since the stand was first examined five years ago, and the sawlog potential has increased. However, the density of knots and compression wood formed around the defects may significantly reduce lumber strength and quality.

Figure 5. Simulated typical losses in merchantable volumes at three weevil attack rates in open-grown Sitka spruce plantations on the south coast of British Columbia (source: SWAT Canadian Forest Service).

The lodgepole terminal weevil also causes noticeable defects in trees attacked. In high-hazard stands within the IDF and MS biogeoclimatic zones, 95% of stems may be attacked one or more times during a 10- to 20-year period, with some trees attacked more than three times in this time period. Up to 60% of lodgepole terminal weevil attacks in highly susceptible stands form major defects (crooks, forks or stagheads). The most common defect is a crook. The largest trees in stands are most likely to be attacked. Height loss due to weevil attack averages 30% of the annual potential height increment in the year of attack, and 17% in the year following attack. Low-density stands are more likely to be impacted by the lodgepole terminal weevil.


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