Control of the boxelder twig borer in the Prairie Provinces

Introduction

The boxelder twig borer Proteoteras willingana (Kft.) infests only Manitoba maple in the Prairie Provinces. It may attack trees of any age, from first-year seedlings in nursery plots to mature trees in urban and rural plantings. Usually the pest is not numerous enough to cause extensive damage because natural control keeps it in check. When natural control fails, however, as occasionally happens, severe injury can occur to the trees. Heavy outbreaks stunt established trees by killing much of the current tip growth on twigs and branches. The pest may also spread to nursery plantings; since it can survive on seedlings in winter storage, it may be disseminated through infested stocks.

DAMAGE

Two important kinds of injury are caused by the twig borer. The dormant buds on the trees are destroyed from mid-August to early fall and during late April and early May of the following spring. Later, in May and June, the new terminal growth on the branches and twigs is hollowed out causing it to develop into spindle-shaped galls. Some of the attacked tips may survive but most of them dry out, split open and die. Both types of injury prevent normal growth and if repeated in successive years will seriously stunt infested trees. In addition, some stripping of leaves occurs during mid-summer.

GROWTH STAGES AND DESCRIPTION

The borer has four stages of development - adult, egg, larva, pupa.

The adults are small, mottled-gray moths with a wing spread of approximately one inch. The eggs are roundish flattened, about 1/50 inch across and pearlywhite to greenish. They have a flange-like edge which holds them securely to the leaf surface. Newly hatched larvae are tiny caterpillars, about 1/16 inch long, greenishwhite, with black heads. Mature larvae are 1/4 to 1/2 inch long, stoutish, yellowish-white, with black or brown heads. The pupae are approximately 1/3 inch long, reddish-brown, with transverse rows of backward pointing spines on the abdominal segments.

LIFE CYCLE AND HABITS

The boxelder twig borer has a one-year life cycle. The moths are present from late June to late July. They fly with a darting motion and are most active in the evening and on warm cloudy days. The moths frequently rest on the trunks of trees and on the ground but seldom on the leaves. The females commence egg-laying soon after they are adults and each may produce 100 eggs or more. Most egg-laying occurs during the evening.

The eggs are stuck firmly to the leaves. Usually they are placed singly on the under-surface close to the midrib of the leaf or the larger veins and often in the angles between the midrib and the veins. They incubate in 10 to 14 days and hatching occurs during July and early August.

The larvae or caterpillars pass through seven stages during growth and require about eleven months for completion of their development. They remain on the foliage for about three weeks after hatching where the first two stages and part of the third are spent. They feed on the leaf tissues, stripping small areas - usually close to the midrib or veins. Either leaf surface may be attacked but the underside appears to be preferred. The caterpillars cover themselves and their feeding areas with webbing and particles of frass for protection. The remaining stages of larval growth occur in the leaf buds and tip growth on the branches and twigs. This period extends from mid-August to the following early July. Upon leaving the leaves, the caterpillars crawl down the leaf stems and bore through their bases into the new buds beneath where they overwinter. Very little food is eaten during this time. The openings leading into the new buds are closed with webbing and frass so that the caterpillars are still protected after the old leaves are cast off. Beginning in early April, most of the caterpillars emerge from their winter quarters and burrow into other buds where they feed actively. Each caterpillar may destroy two or three buds during this activity. In May or June most of the caterpillars abandon the buds and bore into the new stem growth at the tips of the twigs and branches. Here burrowing and feeding becomes quite extensive, resulting in tunnels which reach one inch in length or more. This activity stimulates the infested tips to swell abnormally, forming spindle-shaped galls. The caterpillars remain within these galls until they attain full growth during June and early July and then emerge from them to pupate. The caterpillars, which continue to live in the, buds where they overwintered, leave them for short periods to feed on other buds and vacate them only when they are mature and ready to pupate.

Pupation occurs in small cells made by the mature caterpillars from leaf-duff and webbing in the debris or humus layer above the mineral soil. Pupal development requires from 10 to 18 days. The new moths emerge from late June to late July and a new cycle of borer activity is begun.

CONTROL

Natural control factors such as parasites, disease and weather usually keep the boxelder twig borer from doing extensive damage. Of these, parasites appear to be the most valuable. When natural control fails, however, larger borer populations occur, tree damage increases and may continue at a high level for two years or more before natural control again becomes effective. Instances are common where more than 20 % of the tips on the branches and twigs have been killed each season during the outbreak period.

Applied control practices will do much to prevent stunting of Manitoba maple trees by reducing borer abundance. Two methods - hand pruning and chemical application are recommended. They may be used separately but where they can be combined, there will be less damage and better control.

Hand Pruning:

Remove and burn in the late fall or early spring the secondary sucker growth which may have developed around the bases and on the lower trunks of the trees. Such growth is succulent and is frequently heavily infested with borer larvae. Its destruction will prevent the borers present from becoming adults and laying eggs on the tree foliage.
Cut off and burn the galled twigs on the trees, during May and June, to reduce the subsequent population of adults and the number of eggs laid. This operation will not affect the amount of current-year damage to the tips but it will decrease it for the following year, especially if the chemical treatment recommended below is applied.

Chemical Control:

The amount of current year damage to the buds and tip growth on Manitoba maple, evident by mid-June, will indicate the degree of urgency for chemical control of the twig borer. If control is required, treating the foliage with a suitable insecticide during the period mid-July to early August will be effective. This treatment is directed against the newly-hatched caterpillars which are more easily killed than the later stages.

Newer and frequently better chemicals for insect control are being produced. Improved techniques for their application are also being investigated. To ensure that the best materials and methods acceptable for twig borer control are being employed, contact PFRA Shelterbelt Centre at (306) 695-2284 before undertaking chemical control measures. If in doubt about the correct identification of the problem forward a sample of damaged buds and tips for examination with the inquiry.

SOURCES OF INFORMATION:

Extension Entomologist,
Extension Services Branch,
Manitoba Department of Agriculture, 711 Norquay Building,
WINNIPEG 1, Manitoba.

Pest Control Specialist,
Plant Industry Division,
Saskatchewan Department of Agriculture,
Administrative Building,
REGINA, Saskatchewan.

Entomology & Pesticides Section,
Crop Protection and Pest Control Branch,
Alberta Department of Agriculture,
Agricultural Building,
EDMONTON, Alberta.


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