The Conifer Swift Moth and Spruce-Fir Decline
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The Conifer Swift Moth and Spruce-fir Decline Donald R. Tobi, University of Vermont, Entomology Research Laboratory, 655 Spear St., South Burlington, VT 05401; William E. Wallner, USDA Forest Service, Northeastern Forest Experiment Station, 51 Mill Pond Rd., Hamden, CT 06514; Bruce L. Parker, University of Vermont, Entomology Research Laboratory, 655 Spear St., South Burlington, VT 05401 Northern high-elevation spruce stands in Vermont have shown feeding habits. Different methods for measuring populations of considerable dieback over the past two decades (Siccama et al. hepialid larvae and adults were evaluated. The best method to 1982; Vogelmann et al. 1985) yet the cause(s) of this spruce assess larval numbers was random placement of '/p m2 soil mortality is not known. While acid deposition is believed excavations immediately adjacent to red spruce or balsam fir trees. responsible, Johnson and Siccama (1983) conclude that the The soil was removed down to the bedrock or inorganic soil evidence does not show a clear cause and effect relationship horizon and total numbers of larvae recorded. After testing between acid deposition and forest decline. More likely dieback several different types of traps, trap colors, and trap heights for is the result of many factors interacting to increase tree stress and collection of adults, we found the most effective to be a ?4 m2 eventually cause tree mortality. sheet of clear plexiglass suspended 15 cm above the ground. Both sides of the trap were coated with an even layer (2 mm) of In 1981, a large population of soil-inhabiting lepidopteran larvae "Tangle-Trap' '. This effectively captured moths flying into the were observed at elevations between 750 and 1100 m on Camels trap enabling us to collect, count, and remove moths from the Hump Mountain in Huntington Vt. Densities were as high as traps on a daily basis. Malaise traps also worked very well for 10/m%nd were believed to be feeding upon spruce roots (Arthur the collection of adults but because of their expense and bulk, Johnson, personal communication). In 1983, at an elevation of were used on a limited basis. 915 meters on Camels Hump where spruce decline is most evident, we found a similar infestation of soil-inhabiting larvae which was In 1984 on Camels Hump Mountain, five plots were established identified as the Conifer Swift Moth, Hepialus gracilis Grote. along the Burrows trail at elevations of 900,950, 1006, 1050, and Our 1985 studies turned up as many larvae as 100/m2. 1100 m. In early June, five L/4 m2 soil excavations were dug in each plot and numbers of H. gracilis larvae and pupae recorded The Hepialidae, commonly referred to as the ghost or swift (Table 1). Additionally, eight L/4 m2 sticky board traps were set moths, are primitive lepidopterans that have attained their highest up in a 12 meter diameter circle around plot center at each evolutionary development in the Australian region (Tindale 1938). elevation. Traps were checked daily and all hepialid moths The larvae are usually phytophagous, feeding on pasture grasses removed and counted thus providing us with a estimate of adult (Tindale 1933; Common 1970), or as stem borers of living trees densities by elevation (Table 1). H. gracilis larvae, pupae, and (Tindale 1953). Although not well documented, some are adults were found at all five elevations but with fewer at the lowest suspected to feed on tree roots (Tindale 1932, 1935, 1964). and highest plots. This suggests that habitat and food preferences are important determinants of density; the plot at 900 m was in the transitional zone with a large hardwood component and the Although hepialids may be serious pests elsewhere, there is little plot at 1100 m was pure balsam fir. Highest densities of all H. information on the biology and behavior of the Hepialidae in gracilis life stages were found where the predominant cover type North America (Engelhardt 1920; Prentice 1965; Kile et al. 1979; was spruce-fir where forest decline is most evident and acidic Wagner 1987). Sthenopis argenteomaculatus Harris, however, deposition is purported to be highest. was reported breeding in red maple and alder swamps in New York (Leonard 1926) and has also been recorded as a borer in In 1985 and 1986 adult trapping was continued in the five Burrows chestnut (Castanea sp.), oak (Quercus sp.), maple (Acer sp.), Trail plots and expanded to include a transect running from 700 willow (Salix sp.), poplar (Populus sp.), and alder (Alnus sp.) m to 1150 m on the western slope of Camels Hump. Sixteen plots, (Felt 1906). Sthenopis quadriguttatus Grote bores in the roots placed every 30 m of 'elevation on the transect, each contained of aspen (Populus tremuloides), cottonwood (Populus deltoides), two L/4 m2clear sticky board traps. Four of these traps were also and willow, (Furniss and Carolin 1977, Gross and Syme 1981). placed in each of the five Burrows Trail plots. Traps were set Another hepialid, Hapialus mustelina Pack., has been cited as out in mid June and monitored daily through mid August. H. a borer in spruce (Picea sp.) (Felt 1906: Packard 1895). gracilis adults were counted and removed daily and any unusual Our research objectives were (I), to develop survey procedures specimens returned to the laboratory for identification. to monitor H. gracilis populations and determine their elevational 'The Tanglefoot Co., 314 Straight Ave., S.W., Grand Rapids, and geographical densities and (2), to determine its life cycle and Michigan 49504 Table 1.--Total number of H. gracilis pupae and larvae; Camels Hump Burrows Trail plots Elevation 1984 1984 1985 1986 Plot (meters) Pupae Adults Adults Adults 1 884 1 63 0 14 2 945 9 216 2 191 3 1006 15 181 2 207 4 1067 19 275 0 242 5 1128 4 66 0 179 Total H. gracilis adult catches in 1985 were 4 individuals in the balsamea) and red spruce (Picea rubens). In a laboratory study, Burrows Trail plots and 5 individuals in the transect plots. Adult young to mid instar larvae fed extensively on and in some cases catches in 1986 were 833 individuals in the Burrows Trail plots completely consumed the root systems of seedlings of balsam fir and 753 individuals in the transect plots. The 1984 and 1986 adult (Abies balsamea), red spruce (Picea rubens), white spruce (Picea catches and 1985-1986 larval sampling indicates that H. gracilis glauca), mountain ash (Sorbus americana), and white birch has a two-year life cycle on Camels Hump. In 1986 the highest (Betula papyrifera var. cordifolia) (Table 3). In mid summer 1985 trap cacthes were in the spruce-fir zone between the elevations on Camels Hump between the elevations of 950 m and 1100 m, of 950 m and 1100 m (Table 1). H. gracilis adults were caught we found a large number of dead seedlings which when examined, at every plot along the transect line but in very low numbers below showed root girdling and damage similar to the seedling injury the spruce-fir/hardwoods transition zone. we observed in our controlled feeding studies in the laboratory. Peak adult emergence in 1984 and 1986 was during mid-July. The As the larvae mature they move to the larger roots (lst, 2nd, and period of adult activity extended from the last week of June 3rd order) of balsam fir and red spruce. They do not bore within through the first week of August; adults were most numerous the roots but cause extensive damage to inner bark, phloem, and between July 7 and July 21. We found H. gracilis to have two exterior xylem. Entire roots may be girdled. One large balsam daily flights; one at dusk and the other at dawn. The dusk flights fir had 32 major wounds on the lrst and 2nd order roots out occurred from sunset to roughly one-half hour past. These flights to 50 cm. We have observed similar extensive root damage to appeared to be dispersal and/or mating flights although we red spruce both on Camels Hump and Mount Abraham in observed no coupling at this time. Flight behavior was fast and Lincoln Vermont. Late instar H. gracifis larvae are seldom found erratic at heights up at 3 m. Towards the end of the dusk flights tunneling in the roots; a trait common with many other Hepialids. females could be seen flying slower than the males while However, we did observe them tunneling in the petiole bases of broadcasting eggs singly onto the ground. The dawn flights are the mountain wood fern. short, taking place during the 30 minutes preceeding sunrise. This flight consisted mainly of slow-flying, egg laying females The root feeding by H. gracifis is considered a contributing factor broadcasting eggs in a apparently non-discriminating fashion. to the decline of high elevation balsam fir and red spruce. We have determined that H. gracilis is an abundant component of H. gracilis adult and larval activity was also found in 19 out of high elevation conifer stands, particularly those in the process 20 other permanent plots throughout Vermont within the Green of decline. Tests have proven that even a single larva is capable Mountain National Forest (Table 2). Five Yi m2soil excavations of killing a seedling. This could be an impact upon stand were dug and two sticky board traps were monitored at each of regeneration. The feeding of the full grown larvae on the root these sites. Numbers of H. gracilis were found to be positively systems of apparently healthy spruce and fir trees produces injury correlated with site elevation with more individuals at the highest which could serve as infection courts for various wood decay fungi elevations.