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Prevention and Control Tactics
Tactics to prevent and control YHSS have been proposed or have been used to reduce YHSS damage. Prevention, especially through the use of silvicultural practices, should be stressed for long-term YHSS control. Biological Control.... No large-scale biological control programs have been attempted for YHSS management. Houseweart and Kulman (1976a) recommended targeting YHSS eggs and early larval stages for introduction of biocontrol agents, because of the high survival of these stages in a life-table study. Late YHSS larval stages and cocoons were well controlled by existing parasites and predators. Unfortunately, few parasites have been identified that would impact either the egg stage or early larval stage. As mentioned earlier, the two identified egg parasites are Trichogramma minutum and Tetrastichus n. sp. No collections of Tetrastichus n. sp. from YHSS eggs have been made since that reported by Duda (1953), despite searches in the 1970's (Thompson and Kulman 1980). Trichogramma minutum is a generalized egg parasite of many species, but none of the extensive Minnesota studies have recovered it as an egg parasite of YHSS. Therefore, although Wilson (1971) considered T. minutum as a common egg parasite of YHSS, its generalist nature and infrequent collection record from YHSS eggs suggest that it has little potential as a control agent for YHSS. Mechanical Control.... Handpicking YHSS larvae from foliage may be a viable control alternative for ornamental trees or similar situations. Kusch and Cerezke (1991) suggest that young larvae may be washed off the foliage with a strong jet of water. They also suggest that ornamental spruce that have been severely attacked for 1 or more years be pruned to encourage new growth and to reshape the crown. Silvicultural Control.... Several studies indicate that YHSS impacts can be minimized by not establishing new stands or plantations on sites susceptible to YHSS outbreaks. South-facing slopes (Morse and Kulman 1984a) and sites subject to poor drainage or with heavy, clayey soils (Cook 1976) should be avoided. Avoiding such sites will also reduce white spruce plantation failure due to drought, frost, flooding, and exposure (Stiell 1958). Rauscher (1984) reported that these sites were low in productivity. Further, Harding (1982) found that white spruce plantations in Minnesota that grew on slopes greater than 10 percent tended to be low in productivity. Site preparation should be monitored closely to minimize loss of soil organic matter and nutrients from the site. Removal of organic matter through windrowing may contribute to tree mortality in the presence of YHSS defoliation (Morse and Kulman 1984a). Site disturbance also may reduce populations of small mammals that play a major role in YHSS population dynamics through cocoon predation. Planting density should be carefully considered. Because of shading, dense stands may be less susceptible to YHSS outbreaks and may provide greater stocking in the event of defoliation-induced mortality. Overstocking, however, may result in stagnant growth or increased competition, potentially reducing the ability of trees to recover from stress. Foresters can take advantage of the YHSS affinity for light when managing young spruce stands. Morse and Kulman (1984b) suggested maintaining a light overstory over young white spruce stands to reduce YHSS defoliation. Thinning or release of white spruce stands should be delayed until trees are about 10 feet tall (Anonymous 1992). This may be most appropriate on sites with south-facing slopes, which are likely to support high YHSS populations. Rauscher (1984) noted that a nurse-canopy of aspen or white birch that reduced full sunlight by 25 to 30 percent would allow maximum height growth of white spruce seedlings and would protect them from spring frost damage as well. Mixing fast-growing shrubs with roadside spruce plantings may also provide sufficient shade to reduce YHSS susceptibility for several years. Insecticides.... Application of insecticides to control YHSS on ornamentals, Christmas trees, shelterbelts (Rose and Lindquist 1985) and forest plantations (Anonymous 1981) has been necessary in some cases. Many insecticides will control YHSS if applied at the proper time. Rose and Lindquist (1985) recommended that on white spruce, insecticides be applied about 10 days after bud caps have been shed. The Minnesota Department of Natural Resources recommended spraying as soon as larvae are found (Anonymous 1992). DeGroot (1995) summarized experimental field trials with chemical insecticides for control of YHSS, conducted in Canada from 1972 to 1979. DeGroot noted that in comparison with other forest insect pests, control of YHSS is not difficult. Table 4 identifies products that have been historically registered or used for control of YHSS. Table 4. Historical insecticide recommendations for control of YHSS
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