White Pine Weevil

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Cucujiformia
Superfamily	Curculionoidea
Family	Curculionidae
Subfamily	Molytinae
Tribe	Pissodini
Genus	Pissodes

Scientific Name

Pissodes strobi

Common Name

white pine weevil

Compiled by Whitney Cranshaw, Colorado State University:

WHITE PINE WEEVIL

Pissodes strobi (Peck)

Coleoptera: Curculionidae

Hosts:

Colorado blue spruce, Engelmann spruce. White pine is the common host in eastern states.

Damage and Diagnosis:

White pine weevil damage.

White pine weevil is sometimes the most serious insect pest of Colorado blue spruce in landscape plantings, particularly at the higher elevations. In other parts of the U.S. this insect is also an important pest of white pine. Feeding by the developing insects causes the top terminal (leader) to suddenly wilt and die in early summer. Upper branches are affected less frequently. This damage can subsequently result in a bushy, deformed tree that may be considered aesthetically undesirable.

Immature stages (larvae) of the white pine weevil feed underneath the bark of the spruce leader, girdling the plant. When sufficient damage has been done, the top growth will wilt and curl, becoming completely dead in a few weeks. Only the top leader and upper branches are affected by the insect and damaged leaders have an overall characteristic appearance involving a "shepherd's crook" appearance with the needles turning a gray-blue color. Often, at the base of the damaged growth, there are small (1/8-in) round exit holes in the bark made by the emerging insects.

Once the top leader is killed, some side branches will change their growth habit and begin to grow upwards to take the place of the killed leader. If successful, these new leaders will form main trunks and multiple main trunks will occur above the damaged area. This changes the form of the tree from its normal tapering growth to one that is more densely bushy. This can be considered to detract from the appearance of the tree.

The adult stage of the white pine weevil is a small (1/4-in) snout beetle flecked with brown and white patches. A feature it shares with other weevils is the long, curved snout with elbowed antennae arising from it. Larvae are almost identical to those of bark beetles, looking like a grain of cooked white rice. The larval head is brown. The larvae are found within the terminal growth of spruce.

Life History and Habits:

White pine weevil larvae.

The insect overwinters in the adult stage, under leaf litter and in other protected areas. After snow melts and temperatures begin to warm, (mid-March to early May) the weevils become active and females seek out spruce trees. They feed on the cambium of main branches near the leader and insert eggs into the feeding cavities that are formed. Small points of oozing pitch on the main leader are indicators of this feeding and egg laying activity.

Eggs hatch in one to two weeks and the young grubs (larvae) tunnel downward underneath the bark. Damage increases as the insects grow and wilting starts to become noticeable in June and July.

When full-grown, the white pine weevil larvae tunnel deeper into the stem and form a cocoon made of wood chips in which they pupate. In about two weeks the adult beetles emerge through small holes they chew through the bark. The chip cocoons remain behind and are a useful means of diagnosing old white pine weevil injury.

Adult weevils feed on the needles, buds and twigs of spruce for several weeks before going into a dormant condition for overwintering. Some minor chewing injury to buds may result if infestations are severe.

Related and Associated Species:

A closely related weevil species, P. terminalis (Hopping), occurs in the tops of lodgepole pine, producing similar injury.

Magdalis lecontei Horn is a weevil associated with terminal growth and small branches of ponderosa and, occasionally, lodgepole pine. Larvae make tunnels in a similar manner to the white pine weevil, but leave fine-packed borings in the tunnels rather than coarse shredded material. Adults of M. lecontei are usually blue-black, somewhat metallic and have a prominent snout. Attacks are almost always confined to recently dead or dying branches.

Management:

Insecticides applied in spring when adult weevils feed on trees and lay eggs can provide control. Standard bark beetle or wood borer insecticides should provide control if used at rates labelled for the above insects. These include the insecticides carbaryl (Sevin), chlorpyrifos (Dursban), and permethrin (Astro). Timing of these treatments will vary by location and year but usually should be made in late March or early April. Only the upper areas of the tree need to be treated.

Mechanical removal of infested terminals while the insects are still present can provide some future control if other sources of weevils are not in the area. For there to be any control value, this should only be done in June or July before adult emergence. Terminals should only be cut as far down as necessary to remove the weevil larvae. Pruned terminals should be destroyed, as the insects will be able to complete development in the prunings.

If top growth has been killed, proper training of a single side branch as a replacement leader can help to salvage the future appearance of the tree. Often several side branches will begin to grow upward and the healthiest of these should be favored. Temporarily binding competing shoots or pruning the tips of competing shoots will allow for a single leader to again be established. This new leader should be annually protected with insecticides until the tree is no longer highly susceptible to attack. Such a decision can be based on the incidence of attacks to hosts in the area.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.