Outbreaks....

It appears that outbreak populations of YHSS can be found somewhere within the range of spruce every year. Figure 5 illustrates the outbreak history of YHSS in Minnesota, Wisconsin, Michigan, and Ontario during the past few decades as reported in their respective annual pest reports (updated from Haack and Mattson 1993). Although no consistent reporting procedures were used among the three States and Ontario to estimate defoliation severity, these observations illustrate that outbreaks occur frequently and cause noticeable impacts. Most reports of YHSS defoliation have dealt with white spruce rather than black spruce, perhaps reflecting the greater market value for white spruce and its wider use in roadside plantings and as a Christmas tree. Morse and Kulman (1985), who worked in Minnesota, noted that local outbreaks generally lasted about 3 years.

Figure 5. Outbreak history of yellowheaded spruce sawfly in Michigan, Minnesota, Wisconsin, and Ontario as reported in their respective annual pest reports. Data were available for the years 1968 to 1993 for Minnesota (25 years; missing 1973), 1953 to 1993 for Wisconsin (40 years; missing 1956), 1950 to 1993 for Michigan (42 years, missing 1951 and 1973), and 1960 to 1993 for Ontario (34 years). Defoliation severity values were assigned as follows: severe defoliation = 4, heavy = 3, medium = 2, low = 1, no YHSS defoliation reported = 0.5, or annual report not located for a particular year = 0. The most severe YHSS outbreak condition reported in each annual report was the value used to rank defoliation severity for that particular year and region. See details in Haack and Mattson (1993).

YHSS defoliation was reported to be heavy or severe in at least one location in Ontario in 97 percent of the 34 annual forest pest reports observed (fig. 5). Similarly, YHSS defoliation was reported as high or severe in 60 percent of the 25 Minnesota reports, 23 percent of the 40 Wisconsin reports, and 19 percent of the 42 Michigan reports. This trend may reflect the acreage of forest land in each State or Province that is covered by spruce (table 2). For example, black and white spruce cover 41.8 percent of Ontario's 93 million acres of forested land (Ontario Ministry of Natural Resources 1987), 9.6 percent of Minnesota's 14.8 million acres (Miles and Chen 1992), 2.3 percent of Wisconsin's 14.8 million acres (Raile 1985), and 3.3 percent of Michigan's 18.6 million acres (Leatherberry 1994a, 1994b; Schmidt 1993, 1994).

Table 2. Estimated black spruce and white spruce area, number of live trees, and net volume at the time when various forest inventories were conducted in Minnesota, Wisconsin, and Michigan

State	Spruce species	Year of inventory	Area	Thousands of live trees	Net volume	Source*
Minnesota	Black	1962 1977 1990	1,152,000 1,042,000 1,322,000	--- 844,736 1,039,448	--- 552,013 745,825	1 1,4 4
	White	1962 1977 1990	57,300 79,200 93,800	--- 63,944 78,662	--- 183,887 295,108	1 1,4 4
Wisconsin	Black	1968 1983	235,900 273,000	203,992 262,568	92,206 123,955	5,9 5
	White	1968 1983	75,200 61,400	53,573 75,451	81,855 199,628	5
Michigan	Black	1966 1980 1993	421,000 520,600 460,800	--- 362,257 328,395	257,000 343,500 363,058	6 6 2,3,7,8
	White	1966 1980 1993	146,600 100,100 145,200	--- 130,353 144,760	272,500 396,700 492,458	6 6 2,3,7,8
*Source: 1 = Jakes (1980), 2 = Leatherberry (1994a), 3 = Leatherberry (1994b), 4 = Miles and Chen (1992), 5 = Raile (1985), 6 = Raile and Smith (1983), 7 = Schmidt (1993), 8 = Schmidt (1994), and 9 = Spencer and Thorne (1972).

Parasites, Predators, and Pathogens....

Figure 6. A survivorship curve for yellow-headed spruce sawfly taken from Houseweart and Kulman 1976a.

A 3-year life-table study of YHSS populations in Minnesota indicated that egg survival was high, 97 percent, as was survival of early instars (1 to 3), 73 percent. Survival decreased to 16 percent for later instars (4 to 6) (Houseweart and Kulman 1976a). Survival of cocoons was 44 percent. A survivorship curve for these YHSS populations resembled a Type I curve as described by Slobodkin (1962), where mortality of eggs and early instars (1 to 3) is minimal, but mortality of late instars increases rapidly (Houseweart and Kulman 1976a) (fig. 6). Similar survivorship curves have been reported for Neodiprion spp. sawflies (Lyons 1977). Type I survivorship curves are fairly unique since mortality rates for most insects are relatively constant (e.g., Type III curve).

Infertility, desiccation, and predation were probably responsible for YHSS egg mortality in the Minnesota life table study. Egg parasites were not observed, and their absence presumably contributed to high YHSS egg survival (Houseweart and Kulman 1976a). Pointing (1957) also reported finding no egg parasites after a survey in Ontario. The only recorded egg parasites of YHSS were found in Maine and were Trichogramma minutum Riley (Nash 1939, Duda 1953), and Tetrastichus n. sp. (Duda 1953). Duda reported that in one case, about a third of the YHSS eggs were parasitized by Tetrastichus. A survey in 1976 failed to relocate either of these egg parasites in Maine or Nova Scotia (Thompson and Kulman 1980). Wilson (1971) mentions T. minutum as a common egg parasite, though he did not provide location data for these observations.

In a life-table study in Minnesota, the impact of parasites, predators, or other agents was much greater among instars 4 to 6 than among instars 1 to 3 (Houseweart and Kulman 1976b). No other data are available to confirm this trend, although most of the reported parasites of YHSS have been reared from late-instars and cocoons. In North America, 32 Hymenoptera and 9 Diptera species have been reported to parasitize YHSS (table 3). Larval parasitism rates averaged 12 percent in Maine, 47 percent in Nova Scotia (Thompson and Kulman 1980), and 1.5 to 21 percent in Minnesota (Houseweart and Kulman 1976b, Valovage and Kulman 1986). Of these 41 parasite species, apparently only a few have a significant impact on YHSS populations. In Minnesota, the tachinid fly Bessa harveyi is the dominant larval parasite. Bessa harveyi is a polyphagous parasite that concentrates on the predominant host in its range (Valovage 1979; Valovage and Kulman 1983, 1986). In Minnesota, YHSS was a second preference behind the larch sawfly, Pristophora erichsonii (Htg.) (Turnock and Melvin 1963). Another larval parasite, the ichneumonid wasp Syndipnus rubiginosus was also common in Minnesota (Rau 1976). The icneumonids Rhorus bartelti and S. rubiginosus were the most common parasites of YHSS in Maine (Thompson and Kulman 1980, Luhman 1981). In Nova Scotia, the ichneumonid Aderaeon bedardi and B. harveyi were prevalent (Thompson and Kulman 1980). Other ichneumonids in the genus Rhorus were reported as common YHSS larval parasites in southern Ontario (Raizenne 1957) and in Saskatchewan (Bradley 1951).

Cocoon parasitism may also be important. The ichneumonid wasp Endasys pubescens was responsible for 15 to 35 percent of the parasitism occurring at three sites in Minnesota from 1972 to 1974 (Rau 1976). Rau noted that this species was also common in Manitoba and Ontario.

Table 3. Reported parasites of the yellowheaded spruce sawfly¹

Diptera Phoridae Rhyncophoromyia conica (Malloch) Megaselia pulicaria (Fallen) Tachinidae Bessa harveyi (Townsend) Euphorocera sp. Diplostichus lophyri (Townsend) Palexorista bohemica Mesnil Zygobothria gilva Hart Spathimeigenia aurifrons Curran Spathimeigenia spinigera Townsend Hymenoptera Bethylidae two undetermined species Braconidae Ichneutes pikonematis Mason Icheutidae proteroptoides Viereck Chalcidoidea Brachymeria compsilurae (Crawford) Eulophidae Tetrastichus sp. Ichneumonidae Adelognathus sp. Aderaeon bedardi (Provancher) Ctenochira pikonematis Townes & Townes Ctenochira quebecensis (Provancher) Cubocephalus sp. Endasys pubescens (Provancher) Endasys subclavatus (Say) Hyperbatus marmoratus Luhman Lamachus angularius (Davis) Excavarus velox (Walley) Hypamblys sp. Lamachus lophyri (Ashmead) Lamachus ruficoxalis (Cushman) Lethades sp. Mastrus laplantei Mason Mesoleius sp. Pleolophus indistinctus (Provancher) Rhorus bartelti Luhman Rhorus gaspesianulus Luhman Rhorus nervierxii (Provancher) Smicroplectrus incompletus Walley2 Syndipnus gaspesianus (Provancher) Syndipnus rubiginosus Walley Pteromalidae Tritneptis diprionis Gahan Tritneptis klygii (Ratzeburg) Trichogrammatidae Trichogramma minutum (Riley)

¹Parasite information taken from Houseweart et al. (1984).
²Recorded from Pikonema sp.

Although birds, insects, and small mammals are known to feed on larvae, no quantitative estimates have been made of their overall impact on YHSS populations. Houseweart and Kulman (1976b) suggested that predation of cocoons was more important than predation of larvae. In a 1973-1975 study, 67 percent of overwintering cocoons were killed by predators: 28 percent by insects and 39 percent by small mammals. Primary insect predators were elatarid and carabid beetle larvae (Schoenfelder et al. 1978). The most common small mammal predators found in areas with YHSS were the masked shrew, Sorex cinereus Kerr, and the short-tailed shrew, Blarina brevicauda (Say) (Houseweart and Kulman 1976b).

Effects of pathogens on YHSS are largely unknown. An unidentified "wilt" disease may have killed a large number of YHSS larvae in Maine in 1947 (Duda 1953), but the causal organism was not identified. YHSS larvae were tested against one strain of Bacillus cereus. In that test, 100 larvae were fed the Mu-3055 strain. Of these, 16 larvae died, whereas the control treatment had no mortality. It was concluded that the B. cereus strain tested had relatively low pathogenicity against YHSS (Heimpel 1961).

Environmental factors affect YHSS host preference and the ability of trees to tolerate defoliation. Studies with other sawfly species have suggested a link between poor growing conditions or environmental stress and sawfly success (McLeod 1970, Knerer and Atwood 1973, Smirnoff and Bernier 1973, Averill et al. 1982, Wagner and Evans 1985). Morse and Kulman (1986) observed that tree mortality attributed to YHSS defoliation exhibited a clumped distribution in Minnesota white spruce plantations. Clumps or pockets of mortality were most common in open areas on steep, south-facing slopes, where trees were smaller. Ovipositional preference of YHSS for sunny locations with resultant higher rates of defoliation and, presumably, lower tree vigor due to water stress on the steep slopes, likely contributed to the observed mortality patterns.

Another Minnesota study conducted during an exceptionally wet summer indicated that excessive moisture could also reduce tree resistance to YHSS defoliation (Cook 1976, Cook and Hastings 1976). More defoliation occurred on poorly drained soils with high silt and clay content than on sites with sandier soils. The authors noted that soil moisture in August was 23 to 35 percent higher in areas of heavy defoliation than in areas of light defoliation. However, soil moisture levels between heavily and lightly defoliated areas differed by only 5 to 7 percent in two plantations and by 20 percent in the third plantation during the time of larval feeding in June and July. Thus, the large difference in August soil moisture measurements may have been a result of reduced transpiration and water uptake by heavily defoliated trees.

Host Availability....

YHSS has been a notorious problem in plantations, but rarely causes damage in naturally regenerated spruce stands. The reasons for this are not well understood, though forest monocultures have long been recognized as likely areas of pest concern (Graham 1925). Bartelt and others (1982a) and Pointing (1957) speculated that host odors played a role in female oviposition behavior. Abundance of non-host trees in a naturally regenerated stand may impact the ability of females to locate suitable hosts. In the Lake States, white spruce rarely occurs naturally in pure stands, normally making up approximately 30 percent of a total stand (Rauscher 1984). Haack and Mattson (1993) used circumstantial evidence to speculate that the large reliance on pine in reforestation efforts in the Lake States led to an increase in outbreaks of pine-feeding diprionid sawflies. It seems probable that a similar relationship could exist between increased planting of white spruce in the Great Lakes region and increased YHSS outbreak frequency (fig. 5). The many spruce plantations established throughout the Great Lake States in the 1930's and 1960's may have contributed to more frequent YHSS outbreaks in this region.

Host Phenology....

Phenology and effects of previous defoliation can also affect YHSS population dynamics. Since synchrony of YHSS adult emergence and new shoot expansion affects oviposition success, any change in the timing of bud flush or bud size could have implications for subsequent YHSS generations. Cook (1976) noted that oviposition was more common on smaller, tighter buds produced by trees that had sustained moderate to heavy defoliation the previous year. However, these trees also tended to flush later than trees not previously defoliated. Pointing (1957) speculated that delayed bud break caused by prior-year defoliation may enable trees to recover from attack. These trees would be less likely to be infested the following year if bud burst was significantly delayed relative to YHSS adult emergence.

Host Nutrient Availability....

Several projects addressed relationships among soil nutrient availability, YHSS success, and defoliation in Minnesota white spruce plantations. One repeated observation was that YHSS defoliation tended to be higher when availability of soil nutrients, particularly nitrogen (N), was low. Cook (1976) reported that low soil N was associated with severe defoliation in Minnesota. An unrelated study found that heavily fertilized white spruce sustained lighter defoliation than unfertilized trees (Popp et al. 1986). Windrowing, commonly used for site preparation of spruce plantations, can concentrate organic matter in windrows, affecting availability of nutrients on the site (Morris et al. 1983). Morse and Kulman (1984b) found that trees growing near windrows were larger and sustained less defoliation than trees growing further from windrows. If low N sites produce trees with low N foliage, then the observations that severe defoliation is common on low N sites could reflect increased feeding by YHSS larvae to compensate for low foliar N levels (Mattson and Scriber 1987, Scriber and Slansky 1981).

However, YHSS success may be reduced if foliar N levels are too high. When white spruce trees were fertilized with NH4NO3 at 0, 224, and 448 kg/ha, foliar analysis indicated that a transition from N deficiency to sufficiency occurred at the intermediate fertilization level (Popp 1982, Popp et al. 1986). Ovipositing sawflies did not discriminate among the three fertilization regimes. Larval survival, however, was greatest on trees treated with the intermediate fertilization regime. Survival was lower when sawflies fed on trees with very high or very low foliar N levels. The authors speculated that the transition from deficiency to sufficiency provided the optimal N levels for feeding larvae. Higher N levels may have disrupted ratios among N and other nutrients (House 1969, Ingestadt 1960), altered amino acid levels and availability of N to sawflies (Cockfield 1988, Durzan and Steward 1967), or changed allelochemic levels (McCullough and Kulman 1991, Nienstaedt and Teich 1972).