The keys to reducing losses to defoliators are to make stands more resilient to defoliator feeding, predict and detect outbreaks at an early stage, and treat outbreaks prior to significant damage occurring. Management of defoliators may include direct suppression of high, damaging populations or indirect, preventative methods.
Short-term strategies (direct control) require:
The two possible objectives of direct control are population reduction and foliage protection. Population reduction is often the objective in areas where the intent is to bring the insect population down to very low densities and little or no defoliation can be tolerated (Table 7). This tactic is often applied to defoliators in the early stages of an outbreak, to prevent population buildup. Nuclear polyhedrosis virus (NPV) is used to achieve this goal with Douglas-fir tussock moth. The success of population reduction tactics is evaluated by assessing the mortality of insects due to applied treatment and a drastic reduction in damage in subsequent years.
When defoliator outbreaks are very extensive, foliage protection is usually the primary objective, given that some threshold of defoliation can be tolerated (Table 7). Population reduction is not usually a viable option in this scenario. When defoliator outbreaks cover extensive areas, direct control should generally target large, continuous blocks for optimal results rather than small discrete blocks. Treatment of larger areas is more effective because it minimizes the risk of moth re-invasion and repeated egg deposition. The success of foliage protection tactics is measured by comparing the retention of foliage in treated and untreated areas.
Long-term strategies to minimize damage caused by defoliators are accomplished through silviculture and treatment prescriptions. Long-term management tactics which can be implemented at the silviculture or treatment prescription stage are listed below. Treatments followed by an asterisk are experimental in nature.
Stand management prescriptions
Initial prescriptions for stand management should consider selection of tree species that are not host to major defoliators, especially in areas that are subject to outbreaks. Where alternate species are not an option due to ecological constraints, difficulties in establishment, or economic factors, consider monitoring and modifications to stand tending and harvesting practices. Harvesting practices should encourage the establishment of even-aged, single- storied stands, or mixed species stands. When considering a strategy for the management of defoliators, weigh the relative advantages and constraints of each strategy, and its associated treatments (Table 8).
Partial harvesting, thinning, or other similar treatments, should be carefully reviewed before any prescriptions are submitted or approved. These treatments may be appropriate for managing specific defoliators, but may compromise the integrity of the stand if root disease or other damaging agents are present.
Long-term strategies should be considered for stands located in high and moderate hazard ecosystems. Factors to consider include:
The Douglas-fir tussock moth is a cyclical defoliator of Douglas-fir in the semi-arid portions of British Columbia. In Douglas-fir stands, patch infestations up to 250 hectares are characteristic. Trees may die after one or two years of severe defoliation, and larvae are capable of killing trees in a single year if all foliage is consumed before trees can form the next year's buds. When defoliation is moderate, new buds are formed before most of the foliage is consumed and the trees may survive. The next year, however, these trees are wholly dependent on the flush from those buds, and a relatively small larval population can consume that foliage and kill the tree. Recovery of the foliage complement on defoliated trees takes several years following collapse of the tussock moth population. Larger, severely defoliated trees may become susceptible to attack by Douglas-fir beetle, Dendroctonus pseudotsugae.
Douglas-fir tussock moth has potential impacts on human health as well. Irritating hairs on larvae and egg masses may cause tussockosis, a severe allergic reaction in some people. Individuals may find it impossible to continue working or living in or near a tussock moth outbreak. This may warrant direct control of the tussock moth in populated rural and urban settings.
Tussock moth outbreaks begin as localized epicentres, spreading and coalescing into larger areas of defoliation. Areas of tussock moth defoliation are very elevationally delimited, occurring in low elevation stands only. The delimited pattern of defoliation is largely due to:
By combining the historical occurrence of outbreaks, both in terms of area affected and periodicity of outbreaks, with stand parameters influencing hazard (Table 9), the relative risk in a particular area can be estimated.
The building phase of a tussock moth outbreak takes 1 to 2 years. Detection of increasing insect populations during the building phase is critical, and unless detected at this stage, significant damage could occur. High population levels persist for 1 to 4 years, then collapse due to natural control agents which include parasites, predators (mainly birds and ants), pathogens, and starvation due to the forced consumption of older, less nutritious foliage. Another factor in the collapse of the population is caused by a species-specific NPV (nucleopolyhedrosis virus), which is always present in the population at low levels. The virus is spread through insect-to-insect contact, causing populations to decline rapidly. Six to eight years elapse before populations again reach damaging levels.
Population density, year in the outbreak cycle, and the current incidence of disease in the population will affect next year's damage levels. Egg sampling can be used to predict the level of defoliation for the coming year, but this level will be reduced if the outbreak is in its third or fourth year. If dead larvae are commonly found, or if egg masses are small, distorted and incompletely covered with hairs, the population is infected with virus and no significant additional defoliation will occur.
The Douglas-fir tussock moth is the only defoliator that currently has an operationally calibrated pheromone-based population monitoring system.
Egg mass density can be measured and used to predict the level of defoliation for the coming year.
Larval sampling techniques are not recommended for predicting outbreaks or estimating population fluctuations. Adult monitoring and egg mass sampling are the preferred and more accurate methods. However, larval sampling should be done when any type of direct control program is implemented. Larval sampling to determine spray efficacy is outlined in the section on western spruce budworm management. The time interval of post-spray sampling should be every two weeks until >50% of the insects assessed are pupae.
The course of action selected in an area should be consistent with the long-term stand management objectives. Different treatments may be selected to meet different objectives. Therefore, each stand has to be considered individually. Long-term management strategies are stand manipulation tactics including:
Conversion of species on high hazard sites to Ponderosa pine or the promotion of mixed Douglas-fir and Ponderosa pine stands is one long-term silvicultural strategy. Harvesting high hazard stands is another long-term option if regeneration is already present on the site or can be achieved post-harvest. Changing to some other land use, such as grazing could also be considered. Many susceptible stands are located on hot, dry sites where shelterwood harvesting is necessary to maintain protective shade for seedlings. By removing large overstorey trees, and thinning intermediate age trees, stands may be made slightly less susceptible to tussock moth damage.
Successful management of the Douglas-fir tussock moth depends on careful monitoring of populations within high hazard stands during the non-outbreak and building phases. Once the outbreak begins, the number of viable treatment options decreases significantly (Table 10). Where management objectives warrant, treatments should be applied before there is noticeable defoliation, in order to reduce the tussock moth population, rather than after defoliation has appeared with its associated growth loss, dieback, or mortality.
Total removal of an infested stand, including all logging debris, could control localized outbreaks of the tussock moth. However, unless regeneration was possible or conversion to other land uses was the objective, this is not a recommended treatment. Shelterwood cutting within an infestation would not produce any protection from the insect for the remaining trees and, if tops and branches of logged trees containing egg masses are left on site, dispersing, newly hatched larvae may concentrate on the remaining trees. Thus, harvesting (clearcut or shelterwood) of an infested stand may not be feasible unless the regeneration is already established and can be protected by a virus or insecticide treatment.
Resource issues, stage of the outbreak, and condition of infested trees should be considered when deciding which insecticide to use (Table 10). There is a five- to eight-week incubation period after virus application before the larvae cease feeding, and unless the trees can withstand additional defoliation, a chemical insecticide treatment may be necessary. Therefore, virus is best used when populations are very low, as seen in the early building stage of an outbreak. The advantages of virus treatment are species specificity and the need for only one application. This virus is highly contagious to tussock moth larvae, so only small amounts are needed to bring an outbreak under control. NPV is ideal for reducing the population a year before significant defoliation is expected. One year into an outbreak, chemical insecticide treatment is often a better choice as it will prevent further damage. The main advantage of insecticide treatment is the speed with which further damage is prevented, but other environmental constraints may exclude its use.
Harvesting susceptible trees before there is damage may be a viable management strategy. If harvesting is the preferred treatment, then all logging, processing, and slash burning should be completed before egg hatch to prevent the spread of the insect to nearby stands. Replanting with a non-susceptible species such as Ponderosa pine or changing to some other land use such as grazing may also be considered. The outbreak may also be allowed to run its course, but the salvage value obtained from dead trees may be considerably lower than from green trees, and seedling survival may be decreased if the shading effect of the overstorey is lost.
If virus or insecticide treatment is preferred, then it should be applied as soon as the larvae have hatched from egg masses and have moved to the foliage. NPV is specific to native tussock moths found in B.C. Ground or air application of virus is feasible as the virus will spread (±50 metres) from the point of application within the first year via insect-to-insect contact. Aerial treatment is necessary when stands are large or not easily accessed. This is ideal for early treatment of incipient outbreaks a year before significant defoliation is expected. Treatment at the building phase of an outbreak will cause the population to collapse in that particular stand. Therefore, no subsequent treatments will be necessary and no significant defoliation will occur.
Once visible defoliation is detected (year 1–3 of an outbreak), virus can still be applied causing a population collapse, but defoliation will be significant. In later stages of the outbreak cycle, chemical insecticides may be a better choice as they will prevent further damage to the weakened trees, reduce possible allergic reactions, and lessen the chance of Douglas-fir bark beetle attacks. Application of biological or chemical insecticides should occur as soon as 80% of the larvae hatch from egg masses and have moved to the foliage. Table 11 outlines the schedule of events for management of the Douglas-fir tussock moth.
Virus mixing and application:
Premix these ingredients the day before spraying, at staging sites near the treatment areas. Add the virus the morning of the treatment. Mix it thoroughly to remove lumps.
Use fixed or rotary wing aircraft, or truck-mounted spray systems to apply the virus mixture. It is applied at 10 litres per ha, with a desired spray droplet diameter of 100–250 microns. To ensure good droplet deposition, spray during periods of low temperature, high humidity, and low wind velocity. The two virus products registered for use in Canada and their application rates are listed in Table 12. NPV is only registered for use by the provincial or federal government.
Mating disruption is a technique which employs the use of pheromones to disrupt insect mating because the "confused" male is unable to find the female. The pheromone is sprayed from helicopters or from ground spraying devices to saturate the atmosphere around the adult insects with the synthetic pheromone. This technique is still in the experimental stage of development.
The pheromone, impregnated into spherical polyvinylchloride beads, is applied at 72 g/ha in water (with small quantities of adjuvants, surfactants, and stickers). Application of the pheromone should be done in late July, prior to moth flight. This technique is suitable for application to small, building populations or for urban situations (Table 10). The mating disruption technique allows a "second chance" at controlling Douglas-fir tussock moth in the 1st year of an outbreak. For example, if epicentres were located and treated with virus in May, but other small patches of defoliation were discovered by mid-summer, the opportunity exists to treat those areas the same year with pheromone (provided applicable pesticide use permits are in place).