Description of the Insect....

Taxonomy - Pikonema alaskensis (Rohwer) is in the order Hymenoptera, suborder Symphyta, superfamily Tenthredinoidea, family Tenthredinidae. The approved common name in North America (Entomological Society of America) is yellowheaded spruce sawfly. The YHSS was originally described as Pachynematus alaskensis by Rohwer (1912). In 1937, Ross erected the genus Pikonema, based on adult genital characters, including ovipositor sheath length and shape of the male subgenital plate (Ross 1937, 1938). Pachynematus ocreatus and Pachynematus picea are other synonyms of Pikonema alaskensis; P. ocreatus has been called the brown-headed sawfly in the literature (Mitchener 1931).

Egg - Eggs are pearly-white and ovate with a finely stipuled shell. Eggs average 1.3 mm long and 0.7 mm wide (Nash 1939). As the embryo develops, the egg swells and black eyespots become evident.

Larva - Full-grown larvae are about 18 mm long (fig. 1). They have a mottled yellow to reddish-brown head; the body is olive green, with six gray-green longitudinal stripes. A small spot is present near the prolegs on all except the last segment of the body. Larval color variation is frequent in this species, with color ranging from green to rose (Morton 1948). Forbes (1949) found three distinct larval color phases in the Maritime Provinces of Canada: yellow, brown, and green. All larval stages have a characteristic shiny, oily appearance. Several keys and diagrams exist to distinguish YHSS larvae from other sawflies (Schaffner 1943, Craighead 1950, Wilson 1977), including other sawflies found on spruce in Ontario (Lindquist and Miller 1971).

Figure 1. Full-grown larvae of the yellowheaded spruce sawfly.

Prepupa - The prepupa is the cocoon-spinning, last larval stage. Prepupae are similar to the last-instar larva, though prepupae contract to about 8.5 mm in length after the cocoon is spun (Nash 1939).

Figure 2. Cocoons of the yellowheaded spruce sawfly.

Cocoon - The cocoon is thin, dark brown, and fibrous, often with attached soil granules (fig. 2). Both ends are blunt, and one end is wider than the other. Female cocoons (4.6 x 11.3 mm) are larger than male cocoons (3.8 x 9.4 mm) (Houseweart and Kulman 1976a). The pupa, which forms inside the cocoon, is a light cream color. When it emerges, the adult sawfly cuts a circular emergence hole at the larger end of the cocoon.

Adult - Males and females appear similar, though male YHSS (7.5 mm long) are slightly smaller than female YHSS (9 mm long) (Rohwer 1912). Antennae are setaceous, nine-segmented, and about one-half the length of the body (Morton 1948). The basic color of the adults is straw yellow. In some individuals, the head is black only on the frons around the ocelli and mouthparts; on others, the head is almost entirely black (Morton 1948). The thorax may also have black markings. Ross (1938) reported entirely black adults. Morton (1948) did not observe black adults, but stated that males tended to be darker than females. Reproductive systems of both males and females have been described (Morton 1948) and diagrammed (VanDerwerker et al. 1978).

Distribution....

The geographical distribution of YHSS coincides generally with the range of spruce in North America. The range of YHSS extends from Alaska to Newfoundland, south to Wyoming and Idaho, across Michigan, Minnesota, and Wisconsin and into the Northeastern United States (Nash 1939, Ross 1938, Shenefelt and Benjamin 1955).

Hosts and Host Preference....

YHSS has been reported to feed on all spruce species native to North America (table 1) (Houseweart and Kulman 1976b). It has also been reported to feed on Norway spruce, a widely planted exotic in North America (Nash 1939).

Table 1. Hosts of the yellowheaded spruce sawfly in North America

Common Name	Native or Exotic	Scientific Name
Black spruce	Native	Picea mariana (Mill.) B.S.P.
Black spruce	Native	P. pungens Engelm.
Engelmann spruce	Native	P. engelmannii Parry ex Engelm.
Red spruce	Native	P. rubens Sarg.
Sitka spruce	Native	P. sitchensis (Bong.) Carr.
White spruce	Native	P. glauca (Moench) Voss
Norway spruce	Exotic	P. abies (L.) Karst.

Although YHSS feeds on a variety of trees, host preference appears to vary geographically. In Minnesota, white spruce is the only commonly utilized host (Houseweart and Kulman 1976b) even though black spruce is abundant and often grows in close association with white spruce. Naturally occurring differences in YHSS defoliation were assessed in an 11-year-old plantation in Minnesota containing 1 black spruce and 23 white spruce seed sources (Pauley and Mohn 1971). No consistent variation in YHSS defoliation among the white spruce seed sources was discerned in 3 years of observation, but black spruce trees were never attacked. In a second Minnesota plantation, YHSS defoliation was observed on 65 percent of the white spruce trees and less than 2 percent of black spruce trees. Defoliation of black x white spruce hybrids was intermediate (24 percent). In Ontario, however, YHSS may preferentially select either black or white spruce in areas where both species occur (Pointing 1957). A provenance plantation of black spruce in southwestern Michigan, consisting of several North American seed sources, was severely defoliated in 1993 and 1994 (McCullough 1994).

Synchrony between bud burst and emergence of YHSS females may play a key role in determining host suitability (Pointing 1957). Bud burst in white and black spruce is typically distinct; bud burst of white spruce precedes that of black spruce by about 10 days (Blais 1957).

Life History and Habits....

YHSS has a single generation per year. It is capable of prolonged diapause, which is discussed below.

Egg stage - In field investigations, average fecundity ranged from 36 eggs in Ontario (Pointing 1957) to 64 eggs per female in Minnesota (Houseweart and Kulman 1976a). Eggs in sunny locations were observed to hatch in as little as 4 days, while eggs in shaded locations took 8 to 12 days to hatch (Pointing 1957).

Figure 3. Full-grown yellowheaded spruce sawfly larvae feeding on white spruce.

Larval stage - The duration of the larval feeding period varies regionally. Larvae fed from 10 to 21 days in Minnesota (Valovage and Kulman 1986), 26 to 30 days in Manitoba (Mitchener 1931), 30 days in Ontario (Pointing 1957), and 30 to 40 days in Maine (Nash 1939).

First-instar larvae consume only small parts of the new needles. As the larvae develop, they skeletonize new needles at first and then consume them entirely (fig. 3). Late-instar larvae can feed on foliage from previous years (Wilson 1971). If complete defoliation occurs, larvae may feed on the tender cortical tissue of new growth, at times severing the central axis of current year shoots (Pointing 1957).

VanDerwerker and Kulman (1974) determined that male YHSS larvae have five stadia and that about 66 percent of females have an additional sixth stadium. Earlier studies reported seven to eight stadia (Forbes 1949) and six to nine stadia (Duda 1953). VanDerwerker and Kulman (1974) speculated that inadequate diet likely altered the number of molts in these laboratory studies.

When they stop feeding in mid-summer to late summer, larvae drop from infested trees and search for cocooning sites in the soil. Cocooning has been reported to begin in mid-July in Manitoba (Mitchener 1931), in late July to August in Wisconsin (Shenefelt and Benjamin 1955), and from early July (Nash 1939) to early August (Duda 1953) in Maine. Yearly weather fluctuations undoubtedly play a large role in determining development times.

Ninety-five percent of cocoons are located within 2 cm of the soil surface, and most occur within 0.1 cm of the surface (Schoenfelder et al. 1978). Cocoon density is much greater under crowns of defoliated trees. However, many cocoons are found outside of the area directly beneath tree crowns, especially in young, open stands (Rau et al. 1979).

Prepupal and pupal stages - Larvae do not pupate immediately after cocooning but remain as prepupae for several months. The prepupal stage is characterized by suppressed development or metabolic rates and is considered a period of diapause-mediated dormancy (Tauber et al. 1986). Although YHSS diapause does not require chilling temperatures for termination, pupation will not occur until late winter (Bartelt et al. 1981).

Some individual prepupae, in both field (Duda 1953) and laboratory reared (Bartelt et al. 1981, Eller et al. 1989) populations, underwent prolonged prepupal diapause for 2or more years. This phenomenon has been reported for other sawfly species (Drooz 1960, Sullivan and Wallace 1967) and may be a population survival mechanism, especially for insects found in unpredictable environments.

Adult stage - Warm temperatures in the spring initiate development of the pupae, and emergence of adults is highly synchronous. In Minnesota, 63 percent of YHSS adults emerged within a span of 3 days and 94 percent emerged within 8 days (Bartelt et al. 1982a). Reported dates of emergence have ranged from 30 May through 8 June in Manitoba (Mitchener 1931), from 10 June through 17 June in Ontario (Morton 1948), and from 19 May to 29 June in Minnesota (Morse et al. 1984).

Morse and others (1984) used degree-days to forecast YHSS adult emergence. Using the last date of snow cover as a starting point, they determined that adult emergence required 343 degree-days, using a temperature threshold of 1.6°C (35°F). Degree-day accumulation was based on estimated soil temperatures at a 0.1 cm depth predicted from daily minimum and maximum air temperatures. Yearly differences between predicted and observed emergence dates ranged from 0 to 6 days. Unfortunately, determining the last date of snow cover is difficult in years when traces of snow linger.

Newly emerged females fly directly to host trees, often in an upwind direction. Bartelt and others (1982a) and Pointing (1957) speculated that a host-derived odor is involved in host-finding behavior though none has been isolated. Females land on a spruce tree and search for suitable current-year foliage. They saw slits into the bases of needles and deposit a single egg into each slit. Commonly, females lay most eggs on a single shoot, but occasionally they fly a short distance to oviposit on other nearby shoots. Ovipositing females rarely fly to other trees. Thus, the entire egg complement is usually laid on a single tree. Females normally live 3 to 14 days and apparently consume only water (Pointing 1957).

YHSS females preferentially oviposit on partially developed current-year shoots, during the period of active needle elongation. Pointing (1957) reported that in Ontario, YHSS eggs were found on shoots whose bud cap or scales covered 40 percent or less of the needles, or on shoots from which the scales had recently dropped. Eggs were not found on shoots with caps covering more than 60 percent of the new needles. In Minnesota, peak oviposition occurred when 80 percent of new shoots were just barely retaining the bud cap (Houseweart and Kulman 1976a). Pointing (1957) observed that YHSS females rarely succeeded in ovipositing on fully developed shoots. He speculated that needles that had diverged from the axis of developed shoots were too flexible to support YHSS females as they cut egg slits.

Mating is not required to initiate YHSS oviposition, although nonmated females produce only male offspring (Houseweart and Kulman 1976a). Males hover around host trees and scramble over foliage until they locate a female. Mating, as in other sawflies, is end-to-end with both individuals dorsal side up, and it lasts about 10 to 15 seconds (Bartelt et al. 1982a).

Ovipositing YHSS females show a marked preference for spruce growing in full sunlight. The positive response of YHSS to light was demonstrated when all light-competing vegetation within 1.5 m of host trees was removed in a 5-year-old white spruce stand overtopped by aspen. Defoliation by YHSS was six times greater on released trees than on overtopped controls (Morse and Kulman 1984b). Once a plantation or an open-grown natural stand reaches crown closure, YHSS susceptibility is greatly reduced. This preference for sunlight remains poorly understood.

The apparent preference of YHSS for light may arise from aspects of YHSS pheromone chemistry. Mating and ovipositing YHSS show a marked preference for spruce growing in full sunlight, and larval populations were consistently higher in the upper crown than in the lower crown (Houseweart and Kulman 1976b).

YHSS females produce a multicomponent sex pheromone to attract males (Bartelt et al. 1982a, Bartelt and Jones 1983). The major component of the female-produced sex pheromone is an aldehyde compound, (Z)-10-nonadecenal. This aldehyde is produced by oxidation of any of 10 (Z,Z)-9,19 dienes ranging from 28 to 37 carbons in length (Bartelt and Jones 1983). The oxidation reaction occurred readily only when virgin females were placed in full sunlight (Bartelt et al. 1982b, Bartelt and Jones 1983). The 10 dienes were located on wings, legs, heads, thoraces, and abdomens; they amounted to approximately 10 percent of the hydrocarbons in females but only 0.1 percent of the hydrocarbons in males (Bartelt et al. 1984).

A secondary alcohol-based pheromone, (Z)-5-tetradecen-1-ol, is used by males to locate virgin females at close range. The combination of the primary component and the secondary component was as attractive to males as were virgin females (Bartelt et al. 1983). Bartelt and others (1983) speculated that host odors and perhaps other secondary components were involved in the complete YHSS pheromone system. They found that mating occurred much more frequently when females were present on spruce foliage than on nonspruce foliage (Bartelt et al. 1982a).

The pheromone system of YHSS appears to be vastly different from that of diprionid sawflies, which do not use hydrocarbons. The YHSS pheromone system is more similar to that found among Diptera, where pheromonal activity is associated with unsaturated hydrocarbons (Bartelt et al. 1982b).

Effects of YHSS Defoliation....

Figure 4. White spruce tree following severe defoliation by the yellowheaded spruce sawfly.

Trees - Defoliation by YHSS (fig. 4) can significantly affect growth of white spruce trees. YHSS larval preference for current-year foliage results in loss of the most photosynthetically active tissue. In addition, YHSS defoliation occurs during spring needle expansion, when new needles function as major nutrient sinks (Kramer and Kozlowski 1979). Loss of new tissue can affect length and mass of current-year shoots and needles, as well as reduce the amount of photosynthate available for shootgrowth the following year (Pointing 1957, Kulman 1971a). Trees sustaining heavy defoliation produced shorter shoots and short, closely spaced needles the year following defoliation (Kulman 1971a). Kulman (1971b) showed that even 1 year of moderate defoliation of white spruce substantially reduced shoot and terminal length, although between-tree variation was high. Terminal shoot length recovered after 1 year, but length of lateral shoots was still reduced 2 years after defoliation. Heavy defoliation in 2 or more consecutive years greatly reduces growth; terminal and lateral shoots were 20 to 60 percent shorter than shoots of undefol-iated trees after 2 years of defoliation. Significant reductions in terminal growth were also observed in Minnesota, where regression analysis indicated that percentage defoliation and tree height accounted for 75 percent of the variation in terminal growth (Cook and Hastings 1976). Ford (1979) related YHSS populations to defoliation levels in a Minnesota white spruce plantation. Defoliation averaged 40 percent and was associated with a mean count of 512 larvae per tree. This level of defoliation caused 4 percent reduction in tree height growth and 57 percent reduction in lateral shoot length. Heavily defoliated trees produced smaller buds (Ford 1979), and bud flush tended to occur later than in lightly defoliated trees (Cook 1976). Severe defoliation can also lead to forked branches due to terminal bud mortality as well as attacks along the trunk by the whitespotted sawyer, Monochamus scutellatus (Say), a cerambycid beetle (Pointing 1957).

Stands - Some tree mortality is typically observed after YHSS defoliation. Morse and Kulman (1984b) reported that 2.3 to 2.7 percent of white spruce died in Minnesota white spruce plantations defoliated by YHSS between 1974 and 1979. However, they also observed small areas where mortality was as high as 15 percent. Individual white spruce trees were observed to survive even 100 percent defoliation (Kulman 1971b), suggesting this species may be fairly tolerant of defoliation, at least under certain conditions.

Long-term effects of YHSS defoliation have not been examined. Defoliation of other conifers sometimes results in higher long-term stand productivity due to decreased competition among trees and possibly enhanced nutrient cycling (Mattson and Addy 1975; Wickman 1980, 1988; Haack and Byler 1993). However, defoliation of white spruce by YHSS generally occurs before canopy closure, when competition for light is presumably low. Defoliation by YHSS is unlikely, therefore, to increase long-term growth. Future researchers should consider establishing permanent plots to collect long-term data on response of both individual trees and stands to YHSS defoliation.

Host resistance - Clumped or spotty defoliation and heavily defoliated trees adjacent to undamaged trees (Cook 1976) suggested possible intraspecific differences in resistance to YHSS. Nienstaedt and Teich (1972) observed that YHSS defoliation rates among 28 white spruce seed sources in 1968 were significantly correlated with defoliation in 1970. They noted, however, that differences among seed sources were apparent only when infestations were light; no differences were observed in another plantation when the same seed sources were heavily defoliated. This observation suggested female YHSS may discriminate among provenances only at low populations or prior to heavy defoliation. As populations and defoliation increased, fewer suitable buds may be available, restricting any potential female discrimination. YHSS preference for specific provenances might relate to timing of bud burst (Cook et al. 1978). Bud burst among white spruce provenances may vary by up to 28 days (Nienstaedt and Teich 1972). However, Connor and others (1982) found no differences in either bud development or YHSS oviposition rates when adult YHSS were caged on trees from 25 white spruce seed sources. They noted, however, that caging sawflies did not allow adult females to demonstrate any potential oviposition preferences. Also, only three trees per seed source were tested, making it difficult to detect subtle seed source differences. They could not detect any differences among seed sources in survival of fourth and fifth instars; survival of early instars was not examined.

Nienstaedt and Teich (1972) suggested that components of YHSS resistance be identified for subsequent white spruce breeding and selection programs. Numerous genetically based traits exist that could potentially affect YHSS. Leaf cortical monoterpenes differed significantly among a rangewide provenance plantation of white spruce growing in Michigan (Wilkinson et al. 1971), but allelochemicals have not yet been related to YHSS success. Other morphological and growth-related traits, including needle color and stiffness, also differed among white spruce provenances (Nienstaedt and Teich 1972), but possible effects on YHSS are unknown.