White pine weevil, or commonly also known as spruce weevil (Curculionidae: Pissodes strobe) is considered as one of the most destructive forest pests of conifers in North America along the coastal of Washington, Oregon and British Columbia (Alfaro 1989, Hoover 2011), causing severe problems to reforestation (Alfaro et al. 1995). It was first described by W.D. Peck, Professor of Natural History and Botany at Harvard University in 1817 (Belyea and Sullivan 1956). But it was not until the twentieth century when the weevil was considered as a serious forest pest to a number of trees from growing successfully (Belyea and Sullivan 1956), including Spruces species such as Norway and Serbian, Colorado blue, Austrian pines, and in some cases, Douglas-fir (Hoover 2011). Repeated damage caused by the spruce weevil can make the infested stand unmarketable.
Life Cycle
Larval stage
Females lay eggs underneath the bark of trees. Therefore, during the larval stage, this species is found inside the bark, making their way by consuming the parts of the infested tree (Belyea and Sullivan 1956). The larvae have brown head with white body that is divided into thirteen segments (Peirson 1922). When the larvae are mature, they are actually a little larger than their adult form, being approximately 7 mm long (Peirson 1922).
Pupa
After the larval stage is the pupal stage. The color of pupa is white, but has a mandible with brown tips (Peirson 1922) as shown in figure 2. However, as pupa matures, the color darkens. The snout and legs become brown as their form changes more representative of an adult form. The size is about similar to the one of an adult.
Adults
Adult spruce weevils are not big, being only about 4 – 6mm long (as shown in the Figure 3). The color of the body is usually reddish brown with lighter colored patches on the elytra (McIntosh 1997). Spruce weevils have the long snout like beak, a distinct characteristic of species of weevil, in which small antennae arise (Belyea and Sullivan 1956, McIntosh 1997).
In a year, there is only one generation of spruce weevil (Peirson 1922). Although, it depends on the season and location, adult spruce weevils emerge from hibernation in mid-March to April, when the temperature has reached above 70F (Belyea and Sullivan 1956, Silver 1968). Females then mate from April to May on the upper part of the terminal shoot (Belyea and Sullivan 1956). The mated females start to lay eggs in niches in the tip of tree leader, or apical shoots, from the previous year (Silver 1968, Alfaro 1989). Usually a female deposits multiple eggs in single leader, leaving thirty to forty punctures on the leader (Peirson 1922). A single female sprue weevil may oviposit approximately 200 eggs within six weeks (Belyea and Sullivan 1956). In about a week, larvae hatch from eggs and start to feed on the bark of the infested trees until they become mature enough to pupate in a chamber known as “chip cocoon” (Figure 4) which is filled with shredded wood. In 10 to 15 days, new adults emerge from the pupae (Peirson 1922) and come out from the holes at the base of the dead terminal of the host tree late summer from July to August. All the adults emerge by mid-November (Silver 1968). However, these newly emerged adults do not mate in fall because they are mostly focusing on obtaining nutrients by feeding off of the old stand and matured stand of the same year (Peirson 1922). As the temperature decreases through fall and winter, the adults find suitable hibernation sites under the leaf litter where they stay until the next March and April (Belyea and Sullivan 1956), but usually they find a site near the tree they last fed on.
Distribution
For a species to survive, it requires suitable environment where adequate food resources are available (Peirson 1922). For spruce weevil, its distribution depends on the distribution of its host trees such as white pine. In 1922, spruce weevil was found in east US including Virginia, North Carolina, South Carolina and northeast of Georgia (Peirson 1922).
Damage
Usually the species in the genus Pissodes are polyphagous (LÅngstrÖm and Day 2007). For example, one of the species in the genus Pissodes, P. piceae, thought to only feed on silver fir, Abies alba, but it is found to feed off of different fir species including A. grandis, A. nordmanniana, and A. nobilis (LÅngstrÖm and Day 2007). Spruce weevil has been known to favor Eastern white pine, but like other species in the genus Pissodes, it has a wide range of host trees. When spruce weevil was newly described, some entomologists thought spruce weevil inhabit mostly in the mature dying trees to bore into the sap wood and rarely choose to attack young trees (Peirson 1922).
Being able to flight, the adult spruce weevil can attack more than one tree by feeding and depositing eggs. Despite the flight ability, the spruce weevils usually fly to nearby trees that are young, less than 5 years, and three feet tall (Belyea and Sullivan 1956). The damage is done by adult weevils upon feeding and punctures created from oviposition, but larvae add a huge amount of additional damage. But most importantly, the damage by spruce weevil results in loss of two to three years of terminal growth (Connola 1961).
From May through July, larvae are found mostly under the bark of infested terminal leader and the number of living weevils per leader was the highest (Silver 1968). They make their way downward as they chew and burrow around the stem, destroying the phloem (MacAloney 1930). The consumption of phloem by the larvae causes the tree to wilt, droop, and die eventually. Although larvae consume a large amount of food for being voracious feeders, the food is digested within an hour (Peirson 1922).
Lumbermen in New England have seen forked or crooked stem in cabbage pine terminal shoot dies repeatedly (Peirson 1922). This causes the laterals under the injury to strive for supremacy, leading to the forked top (Peirson 1922). In the standpoint of a lumbermen, this lost the value. Similar circumstance is shown in white pine trees.
When terminal leader is damaged and bent, lateral branches on the side try to grow upward immediately below the damaged terminal leader, competing to become the new terminal leader (Belyea and Sullivan 1956). However, this process causes the tree to be permanently to be crooked or forked in some serious cases where multiple lateral branches compete for the dominance as shown in Figure 5. However, in most cases, only one branch becomes the new terminal leader, forming a crooked posture of the host tree.
Control Methods
When it comes to controlling a pest population, one can use variety of methods from mechanical to chemical treatments. Although spruce weevil seems very devastating, there are a few different control methods that have been successful of keeping the population at a level.
Natural Enemies
As previously mentioned, females lay eggs in a single terminal, ranging from twenty to forty eggs. However, not all of them make it to the adult stage. Instead, out of the eggs laid in the terminal, rarely more than five of them emerge as adults successfully (Peirson 1922). This is due to the high percentage of parasitism that keeps the mortality rate up for the weevils.
Animal world is very competitive and it is also shown in the life of larvae of spruce weevil. The first ones to hatch make their way downward on the stem, feeding on the phloem as they leave a wet trail of frass for others to follow (Peirson 1922). However, later-born larvae will die shortly from either starvation or disease. The highest percentage of parasitism occurs during the pupation stage. Out of all the larvae that went through the pupation stage, nearly fifty percent of them die from parasites. The effective species of parasites are Eurytoma pissodis, Lonchaea cortices, and Microbracon pini (Peirson 1922, and Taylor 1930). These species were able to parasitize 6, 5.5, and 2.5% of the larvae respectively (Taylor 1930). There are other parasitic species that are not as effective as the above three, and they are Eupelmus pini, Rhopalicus pulchripennis, Coeloides pissodis, Calliephialtes nubilipennis, and Spathius spieces (Taylor 1930).
Plant Defense System of Plants
From primary metabolism, plants are able to produce many different chemical compounds, known as secondary metabolites, that contribute to plants’ color, taste, and odors (Bennett and Wallsgrove 1994). At first, the role of secondary metabolites was unknown, but now it is known as the defense mechanism carried out by plants themselves. When plants are attacked by either pathogens or herbivores, they are able to produce secondary metabolites to defend themselves from them.
When spruce leaders are damaged from feeding and egg punctures, the secondary metabolite production is activated, producing “pitch” to kill the eggs and larvae. According to the study done by Silver (1968), pitch produced by the spruce trees was able to kill 13% of eggs and 5.6% of larvae. It is hypothesized that pitch drowned the eggs in the leaders and broods that were not developed (Silver 1968, Dixon and Houseweart 1982). Not only trees can benefit from pathogens while being attacked by weevils, they can also protect themselves by producing pitch to kill undeveloped eggs and larvae.
Chemical Control
A few decades ago, the effects of a few insecticides using portable mistblower was tested to control the white pine weevil (Connola 1961). From the study, it was proved that the portable mistblower can be used to control the weevil population. The two most effective insectides from this study were lindane and DDT and these two were the lowest cost (Connola 1961). However, because of the environmental issues of DDT, it is not recommended for use anymore.
An experiment where trees were treated with either oxydementon-methyl or acephate to test their effectiveness of protection was conducted (Fraser and Heppner 1993). Both of the insecticides used in the experiment gave at least a year of protection from spruce weevil and harm on non-target organisms was reduced because trees were treated with insecticides individually (Fraser and Heppner 1993). More research is needed to see if the trees treated with the insecticides are protected for more than a year though.
Another form of chemical control will be utilizing pheromones such as aggregation pheromone – a mechanism known as chemical ecology. Beetle species are known to use aggregation pheromone to infest trees and anti-aggregation pheromone to prevent further infestation by the individuals of the same species. The possibility of pheromones used by spruce weevils was investigated in the 1980s. Two volatile compounds from two species of Pissodes were identified and hypothesized that these two compounds, grandisol and grandisal, could be aggregation pheromones (Booth et al. 1983). The attraction of the two compounds were tested in leaders against spruce weevils. They concluded that having grandisol and grandisal along with leaders was less attractive than males on leaders (Booth et al. 1983). Though, the combination was more attractive than just leaders alone (Booth et al. 1983). From an isolation experiment throughout the year, they found out that these compounds were produced by males when females were mature enough to mate (Booth et al. 1983). If the pheromones used by the weevils can be identified, then it will be useful in mass killing of weevils.
Silviculture
From previous observations and studies, it was shown that poorly stocked trees are more susceptible to the attack of weevils for providing abundant breeding sites (Belyea and Sullivan 1956). Not only that, when the crowns of open stands were free, the trees were more likely to be attacked as well. In addition, trees with height of more than three feet and exposed to more sunlight are susceptible to infestation by the weevil (Hoover 2011). It was also recognized that trees planted in a large density were less prone to the damage by the weevils because of the shade created by the trees growing in mixture (Belyea and Sullivan 1956). The reason for this is that adult weevils require certain temperature and humidity for their development which are limited in the shaded stands (Alfaro et al. 1995). Low temperatures and humidity can slow down the development of larvae as well, exposing them to a greater chance of attack from predators (Alfaro et al. 1995). To use silviculture to control the population of spruce weevil, it is recommended to plant pine in a mixture with other species to provide shading necessary to keep development of weevil slow.
Spruce weevil is a devastating pest species that affects the health of white spruce in the coastal to coastal areas of North America and Britisth Columbia, making them valueless when damaged. In order to effectively control the spruce weevil population, their biology and behavior need to be researched. Currently, there is no single effective method to manage their population, but variety of methods can be utilized to maintain their population relatively low in highly infested areas.