Young spruce stands and open-grown spruce stands of any age should be regularly monitored for YHSS. Once crown closure is achieved, susceptibility drops significantly and close surveillance is not necessary. In our experience, defoliation typically occurs in similar locations within stands year after year. Regular scouting of such areas should increase the likelihood of identifying YHSS before damage occurs. Both risk rating and hazard rating are important components of pest surveys. Risk rating is used to identify where defoliation is likely to occur. Hazard rating is used to identify where damage is likely to occur after defoliation. In addition, pheromone traps can be used to monitor population fluctuations.

Stand Risk and Hazard Rating....

Identifying stands susceptible to YHSS outbreaks (high risk) or vulnerable to damage if outbreaks occur (high hazard) allows managers to prioritize scouting and control efforts. Risk and hazard rating could also help managers avoid establishing plantations on sites likely to be defoliated and damaged.

A risk-rating model was developed and validated for white spruce plantations in Minnesota (Morse and Kulman 1986). All the variables used in this risk rating system were topographical and are readily available. The model, which incorporated slope and aspect, correctly classified stands into light or heavy defoliation categories on 70 out of 100 plots used for validation. Stands on steep, south-facing slopes were most likely to sustain high defoliation. The final risk rating model was:

where P was the probability of defoliation. Steepness was measured in seven 5 percent categories with an eighth category for slopes greater than 40 percent. Slope aspect (x) was a circular variable, measured in degrees from north. It was transformed to sine(x) (north vs. south) or cosine(x) (east vs. west).

Cook (1976) attempted to relate physiographic traits to stand vulnerability using such variables as percentage clay at 30 cm below soil line, percentage sand at 60 cm, thickness of B2+B3 horizons, and soil nitrogen levels. His results indicated that stands growing on heavy, poorly drained soils were most likely to sustain heaviest defoliation. Direct measurement of the variables used in Cook's model, however, is likely to be impractical in most cases.

Detection Survey....

The purpose of a detection survey is to learn whether YHSS or its damage is present at any particular time or place, as well as to map its range. A detection survey for YHSS can be done on the ground or from a low-flying aircraft. Ground surveys can locate either feeding larvae or the defoliation that results from YHSS feeding. Larval presence is seasonal. Defoliation, on the other hand, is present over a long period of time and is very characteristic. YHSS defoliation is concentrated in the top third to half of the crowns of scattered trees or in clumps of trees, and both new foliage and old foliage are often consumed. In contrast, eastern spruce budworm, Choristoneura fumiferana L. (Lepidoptera: Tortricidae), feeding is usually found on new foliage only, and occurs over most trees in a stand. In addition, spruce budworm larvae produce silk webbing that remains on twigs and foliage until the winter following defoliation, whereas YHSS larvae do not produce any silk webbing.

Aerial surveys to detect defoliation may be less effective for YHSS than for other defoliators. YHSS defoliation and tree mortality within plantations is often spotty and may go unnoticed. Morse and Kulman (1984) used small-format, 35-mm color-infrared and color film at a scale of 1:9600 to assess mortality in 11- to 12-year-old white spruce plantations in Minnesota. Results indicated that photos were accurate when used to estimate numbers of live spruce. However, the aerial photos were less accurate when used to identify dead spruce, because of poor contrast with ground cover. Photography was taken in northern Minnesota in mid-May when ground vegetation was brown in contrast with the green of living spruce trees. The authors did not think the color infrared film increased accuracy of mortality estimates over color film.

Ground surveys would be required with all aerial surveys to confirm that YHSS was the responsible agent because other defoliators or tree-killing agents can cause similar damage.

Population Evaluation Survey....

Evaluation surveys are used to determine the current insect population level or the actual or potential injury from a given insect population. These surveys often require specific sampling procedures and correct timing. Population evaluation surveys can be used to monitor population trends or to make decisions about possible management options.

Pheromone trapping may be useful in some areas to monitor YHSS population trends, an important component when determining the need for control. Pheromone trap catches were examined by Morse and Kulman (1985), who monitored YHSS populations in northern Minnesota. A population trend index I was used to signify whether populations were building (I > 1) or declining (I < 1). Unfortunately, YHSS trap catches alone were not directly related to population size, trend, or defoliation. However, combining trap catch data for both YHSS and a major YHSS hymenopteran parasite, Syndipnus rubiginosus, whose sex pheromone had also been identified (Eller 1982, Eller et al. 1984), resulted in significant correlations with subsequent defoliation levels. Fewer than one S. rubiginosus per trap was caught during the pre-outbreak phase compared to an average of 28 parasites per trap in the post-outbreak phase.

Morse and Kulman (1985) developed a model using pheromone trap catch of YHSS sawflies (S), parasites (P), and their interaction term (S*P), as well as tree height (Ht), to significantly predict the defoliation trend (I) in subsequent years. The model was:

They concluded that if damage had been observed in previous years, and the trend index (I) was > 1, direct control should be considered. However, a trend index < 1 indicated that a natural population reduction was likely to occur and no direct control was required. Morse and Kulman (1986) suggested pheromone trapping should be concentrated on high-hazard areas or sunny locations where ovipositional activity was expected. They also noted that the model was applicable only to areas where S. rubiginosus was an important parasite of YHSS.

Degree-day models are often used to improve timing of insect survey or control efforts, since phenological development varies among years. Morse and others (1984) developed a degree-day model to forecast emergence of adult YHSS. However, problems with determining a consistent starting point for accumulation of degree-days make this model difficult to implement. More research to link adult emergence with conventional degree-day accumulations (e.g., base 50° F) could provide a useful tool for prediction of adult activity and oviposition.

Surveys of YHSS egg or early larval populations can allow managers to implement control treatments if severe defoliation is expected or predicted. However, no sampling scheme has been developed for YHSS eggs or early larvae that could be implemented in field operations. One model was used by Houseweart and others (1974) to predict the numbers of eggs and first-instar larvae per tree using tree height and counts of these life stages on a sample of shoots. This model involved two regression equations: one that estimated number of eggs or larvae per branch, and the other that estimated number of branches per tree. Both regressions were used together to obtain an estimate of the number of insects per tree. This model was used to develop YHSS life-tables and is probably not practical for field surveys.