Potential of Shoot and Twig Insects as Exotics

René I. Alfaro, Tia Heeley, and Leland M. Humble

Shoot and twig insects are a threat to regeneration and reforestation efforts worldwide. These harmful insects interfere with normal tree growth and seed production. In North America, spruce (Picea) and pine (Pinus) species are heavily damaged by shoot and twig insects. The damage is caused by adult feeding and oviposition and larval feeding and mining, which causes destruction of shoots, resulting in severe height growth reduction and trunk deformations such as crooks, forks, and top kill (Fig. 1). Although mining of lateral shoots (Fig. 2) is not generally considered significant to growth and timber yield, it is emerging as an important factor in forest seed production, as it reduces the number and fertility of staminate and cone-bearing flowers. Typical insects causing destruction of lateral or leaders occur mainly in the orders Diptera (flies), Coleoptera (beetles) and Lepidoptera (butterflies and moths).

Figure 1. Spruce tree severely damaged by the white pine weevil Pissodes strobi.	Figure 2. Lateral shoot of lodgepole pine attacked by European pine shoot moth. Rhyacionia buoliana.

The possible impacts of shoot insects as exotic pests can be analyzed from the point of view of their biological potential for establishment in a new habitat and also with respect to the probability that this habitat could harbor them. In their native ecosystems, these insects are components in the forest’s natural cycles, where they generally cause little or no damage. However, they can be serious pests in new environments, especially when they have the opportunity to fill a niche that was previously vacant.

The following discussion, therefore, centers on the potential of shoot insects to cause economic damage as exotics. The white pine weevil, Pissodes strobi, a native of North America, and the European pine shoot moth (EPSM), Rhyacionia buoliana, a native of Europe, are used as case studies. In spite of its enormous economic importance, the white pine weevil has not been reported as an exotic outside North America. The pine shoot moth, on the other hand, causes little economic damage in its native European habitat but has been introduced and causes severe damage in several parts of the world. Our objective is to compare these two insects (Table 1) to determine patterns that may account for this difference.

Table 1. Factors affecting the potential transfer of white pine weevil and European pine shoot moth life stages to new habitats. Low, medium, and high, refer to the probability of transport of a particular life stage.

Life cycle
The probability of establishment in a new habitat depends on the likelihood of the insect’s life stages being present (and surviving) in commercial commodities at the time of shipment. All life stages of shoot insects can be transported. Eggs deposited under the bark are protected from the environment or from predation and are difficult to detect by inspectors. Larval mining is easily detected, except in the earliest stages, because attacked shoots show characteristic resin flows, drooping, and discoloration. Transport of pupae is possible, but not likely, because these insects often pupate inside highly noticeable dead shoots, or in the soil, which is usually prohibited. Adults may be easily transported. Moths, with their night flying habits, readily fly to illuminated doors or windows of transport vehicles or planes. Adult beetles hide in crevices or under bark on logs and are, therefore, well protected, can survive long trips, and are easily transported. The length of the life cycle and methods of reproduction are also important. Pissodes strobi adults, for instance, can live up to 4 years, and females can carry a sperm load from several males, for more than one season. Therefore, introduction of a single mated female may initiate an outbreak in a new environment.

Dispersal ability is of paramount importance. Adults of shoot-attacking Lepidoptera, such as the EPSM, are strong fliers and can disperse quickly, sometimes aided by wind currents. Long-distance transport of Lepidoptera has been attributed to stormy weather. In contrast, weevils have poor dispersal abilities. They are poor flyers and often disperse by walking.

The probability of an insect moving into a new habitat is often related to the size of infestations in the habitat of origin. For example, the number of introductions of European Gypsy moth, a defoliator, to the West Coast of North America, is directly proportional to the size of the infestations in eastern North America. This does not seem to apply to Pissodes strobi. While it is one of the most important shoot-feeding insects in North America and develops large populations in young spruce plantations, it does not readily disperse long distances into new ecosystems. It has never been reported in Europe, where susceptible plantations and natural regeneration of native and exotic spruce are available. Even within North America, Pissodes strobi has not dispersed to all suitable habitats. The active weevil infestations of northern British Columbia have not reached the Queen Charlotte Islands, which are situated only 155 km off the coast. These islands contain extensive plantations of the preferred host, Sitka spruce, and have a suitable climate for P. strobi. We attribute this to the poor dispersal ability of P. strobi and to westerly winds prevalent during adult dispersal that hamper flight to the islands, blowing any flying weevils back onto the mainland.

Figure 3. Adult white pine weevil, Pissodes strobi.	Figure 4. Adult European pine shoot moth, Rhyacionia buoliana on an infested lodgepole pine, Pinus contorta shoot.

R. buoliana, like most Lepidoptera, is a good flier and can disperse rapidly. Its small eggs are laid singly and, therefore, are difficult to detect. The EPSM was first detected in eastern North America in 1914, on imported ornamental pines from Europe. It had reached a region that had abundant pine regeneration and an appropriate climate for the shoot moth to complete its lifecycle successfully. By 1924, it reached the west coast, covering a distance of 8,000 km in 11 years. Thus, long-distance movement was probably the result of transportation on infested stock. Countries such as Chile, Uruguay, Argentina and Brazil, which have large pine plantations, have also been infested by EPSM and have invested substantial resources in research and management. Since its introduction into Chile, the range of EPSM has expanded at a rate of approximately 50 km/year. This relatively local movement is attributed to adult moth dispersal.

Habitat Suitability
Two other factors important in the ability of shoot insects to colonize new territory are the existence of abundant host trees close to the point of introductions and suitable climate. Neither factor appears limiting in these case studies. P. strobi has a broad host range, encompassing many species of spruce and pine. Plantation forestry in the Southern Hemisphere (where pines are not native) provides ideal conditions for European pine shoot moth establishment.

European pine shoot moth has apparently been more easily transported to novel habitats than has the white pine weevil as a consequence of its overwintering habits. The presence of overwintering larvae in cryptic hibernaculae that are difficult to detect on shipped stock has led to inadvertent introductions of this pest around the world. After arrival, its good dispersal ability has allowed establishment and rapid population expansion.

Suggested Readings and Links

http://www.pfc.forestry.ca/nursery/pests/europea1.htm

http://www.pfc.forestry.ca/landscape/weevil/

http://www.pfc.forestry.ca/landscape/weevil/resistance/