AgriculturalMeteorology, 11 (1973) 277 283 © Elsevier Scientific PublishingCompany, Amsterdam-Printed in The Netherlands
FROST REDUCES WESTERN SPRUCE BUDWORM POPULATIONS AND DAMAGE IN MONTANA
DAVID G. FELLIN and WYMANC. SCHMIDT U.S.D.A. Forest Service, Inter-mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, Missoula, Mont. (U.S.A.) (Accepted for publication August 9, 1972)
ABSTRACT Fellin, D. G. and Schmidt, W. C., 1973. Frost reduces western spruce budworm populations and damage in Montana. Agrie. Meteorol, 11 : 277 283 Unseasonal temperatures, as low as -6°C, were recorded in western Montana in mid-June 1969, a period when western spruce budworm (Choristoneura oeeidentalis Freeman) larvae were actively feeding on newly developing foliage of coniferous host trees. Studies in progress at that time provided circumstantial evidence that the freeze reduced: (1) budworm populations on Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), western larch (Larix oceidentalis Nutt.), and ponderosa pine (Pinus ponderosa Laws.) over 90%; and (2) budworm damage to young larch 54-71%. INTRODUCTION Unseasonal cold weather has long been recognized as an important factor reducing populations of forest insects. However, the opportunities to measure this phenomenon in the forest are fortuitous and infrequent. This paper presents quantitative data documenting reductions in western spruce budworm (Choristoneura occidentalis Freeman) populations and budworm damage to host trees associated with a mid-June frost in 1969 in western Montana. UNSEASONAL WEATHER IN MONTANA - JUNE 1969 The month of June in western Montana is characterized by average maximum temperatures ranging between 18 ° and 24°C, and minimum temperatures ranging between 2 ° and 5°C. However, in mid-June 1969, most of western Montana, both east and west of the Continental Divide, experienced unseasonably low temperatures. A special weather summary for June reports (U.S. Department of Commerce, 1969): "Temperatures reached very cold levels on the 12th and 13th, following some record snowfall amounts along eastern slopes of the Rockies from Glacier National Park southeastward to the upper Yellowstone v a l l e y . . , at Helena the 30°F ( - I ° C minimum was both the coldest June temperature on record since 1880) and the coldest on record
D.G. FELLIN AND W. C. SCHMIDT
SO late in the spring season. Freeze damage to a wide variety of vegetation was reported by a large proportion of Montana observers on the 1 3 t h ; . . . " At many locations maximum daytime temperatures immediately preceding the freeze ranged between 24 ° and 26°C; at some, they went above 28°C. At more than threefourths of the weather stations, located within or near current budworm outbreak areas, minimum temperatures on June 13 were at or below freezing; some stations reported minimums as low as - 4 ° to - 6 ° C . Between June 9 and June 15, instruments at three weather stations (Table 1) in or near our study areas recorded minimum temperatures of freezing or below for at least TABLE I Maximum and minimum daily temperatures (°C) for three weather stations in western Montana during mid-June 1969* Date
9 10 11 12 13 14 15
Lubrecht Exp. Forest (elev. 1,219 m)
(elev. 1,448 m)
(elev. 1,250 m)
4 -1 -1 -4 -2 0
maximum 26 14 23 7 16 20
9 -1 -1 -4 -1 2
maximum 25 19 25 3 14 21
7 -2 -1 -6 -2 0
maximum 27 21 24 24 14 22
*Temperatures at Lubrecht Experimental Forest and Cottonwood Lakes were recorded with a hygrothermograph; those at Ovando with a maximum-minimum thermometer. four successive days; the lowest temperatures were recorded the morning of June 13. At the Ovando Station, the - 6 ° C minimum on the 13th following a 24°C maximum the day before represents a 30°C temperature drop in about 12 h. The durations o f freezing temperatures for two of the stations (only maximumminimum temperature data were available for Ovando) in a 5-day period are given in Table II. During the hours of freezing temperatures, the relative humidity was 100% at both of these stations. EFFECT OF FROST ON HOST TREES, BUDWORM POPULATIONS, AND BUDWORM DAMAGE The condition of the host trees, budworm population counts, and budworm damage to young western larch offer circumstantial evidence that the unseasonably low temperatures in mid-June 1969 substantially reduced budworm populations. Shortly after mid-June, we first noticed effects of freezing weather on several coniferous tree species in our study areas. By late June, newly developing succulent
REDUCTION OF BUDWORM POPULATIONS AND DAMAGE BY FROST
TABLE Ii Durations of freezing temperatures for Lubrecht Experimental Forest and Cottonwood Lakes. Date
June 11 June12 June 13 June 14 June 15
Freezing hours Lubrecht Exp. Forest
3 7 8 6 1
2 2 9 3 0
shoots were d r o o p e d , had desiccated, gradually faded, and began losing their foliage
(Pseudotsuga menziesii var. glauca (Beissn.) Franco), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannff Parry), and western larch (Larix occidentalis Nutt.), all (Fig. 1). Tree species seriously affected were Douglas-fir
c o m m o n hosts o f the western spruce b u d w o r m . We had three b u d w o r m studies in progress during the freezing period on four areas, located west o f the Continental Divide in the Clearwater and Blackfoot River drainages
Fig.1. Upper crowns of young western larch (A) and subalpine fir (B) damaged by frost. The frost injury occurred on June 11-15, 1969 and the photos were taken on May 20, 1970. Note the persistence (for over 11 months) of the damaged shoots. Dead 1969 shoots of subalpine fir remain pendant on the end of 1968 shoots; 1970 growth has not yet begun. Larch shoots have lost all of their 1969 foliage except for some distal needles; new 1970 needles have begun to develop and can be seen throughout the tree.
D.G. FELLIN AND W. C. SCHMIDT
50 to 80 km northeast of Missoula in western Montana. Measurements of budworm populations and/or damage were taken at least 1 year or for as many as 5 years prior to 1969 and were continued in most cases through 1969 and 1970. Reduction in budworm populations and/or damage as a result of the frost are reflected in data from portions of each of the following studies. Lubrecht Experimental Forest In this study area we investigated the distribution and abundance of overwintering western budworm larvae. In each year we counted the number of larvae emerging from limb and bole samples cut in March and April from 10 trees of each of three species. From 1969 to 1970 the number of budworm larvae decreased an average of 93% (Table III). TABLE II1 Decrease of budworm larvae 1969-1970 Host
Number of larvae spring 1969 spring1970
Ponderosa pine* (Pinus ponderosa Laws.)
*An insignificant number of larvae that emerged from ponderosa pine samples might have been the sugar pine tortrix, C. lambertiana (Busck).
Recorded temperatures on this study area reached - 4 ° C and stayed below freezing a total of 25 h during a 5-day period (Table I and II). Cottonwood Lakes Western budworm began damaging young western larch in this area about 1962. In 1964, we began a study to determine the incidence of terminal shoots severed by budworm on 240 larch saplings over a 7-year period. On other host species, budworm larvae normally feed on buds and newly developing needles, but they display an unusual feeding habit on western larch (Fellin and Schmidt, 1967). On this host, larvae also mine and frequently sever terminal and lateral shoots. Budworm larvae severed an increasing number of terminal shoots from 1964 to 1968 (Fig.2). However, in 1970, following the freeze in June 1969, the incidence reverted back to the 1964 level as shown in Fig.2. Frost damage to the terminal shoots precluded meaningful measurements in 1969.
REDUCTION OF BUDWORMPOPULATIONSAND DAMAGEBY FROST TERMINALS
Fig.2. Incidence of western larch terminals severed by western budworm larvae between 1964 and 1970 at the Cottonwood Lakes study area.
Temperatures recorded in the center of our 6 ha study area reached - 4 ° C and remained below freezing a total of 16 h during a 4-day period (Table I and II). Rice Ridge - Seeley
At these two areas, we studied the trend in budworm larval damage to needle fascicles of western larch. In both 1968 and 1970 we examined a total of 12,000 needle fascicles, 400 on each of 30 trees - 20 at Rice Ridge and 10 at Seeley - recording presence or absence of larval damage to the needles. (See Table IV.) Though we have no temperature records for these two study areas, they were in the vicinity of the Cottonwood Lakes area. Frost damage was obvious on host trees at both sites in late June 1969.
TABLE IV The number of fascicles damaged by budworm before and after the frost in 1969 Study area
Number of damaged fascicles 1968 1970
Rice Ridge Seeley
DISCUSSION The western spruce budworm is the most widely distributed and destructive forest defoliator in western North America. Since the mid-1950's, budworm larvae have
D.G. FELLIN AND W. C. SCHMIDT
annually defoliated an average of more than 1 1/2 million ha of coniferous forests in western Montana and north-central Idaho. Data from our studies demonstrate the drastic reduction in western spruce budworm populations and damage to host trees between 1968 and 1970. We recognize that many factors influence fluctuations in budworm populations. However, our contention that unseasonal subfreezing temperatures in June 1969 reduced budworm populations and damage is substantiated by: (1) continuous weather records in the immediate study areas; (2) severe frost damage to host trees; and (3) changes in budworm populations and damage. We suspect, as have others, that the reduction in numbers of larvae likely occurred in one or both of two ways: (1) indirectly by a decrease in available food; and (2) directly by killing larvae outright. Temperatures were low enough to kill newly developing shoots on young subalpine fir, western larch (Fig. 1) and other host trees. Destruction of the food source during a period of rapid larval development likely accounted for a share of the reduction. We know that two conditions presence of food in the alimentary system and contact moisture - will seriously reduce cold-hardiness in some insects (Salt, 1961). During the warm days immediately prior to the freeze, budworm larvae were undoubtedly feeding. Furthermore, snow and high relative humidities accompanied the cold temperatures and exposed many of the larvae to moisture during the time of the freeze. Such conditions certainly could have been fatal to western budworm larvae. We strongly suspect that budworm larvae in many other western Montana forests met the same fate as those in our four study areas. Our deduction is based on the fact that: (1) subfreezing temperatures were reported at many locations in western Montana; (2) the acreage of aerially visible budworm defoliation decreased in the upper Clark Fork and Blackfoot River drainages from roughly 800,000 ha in both 1968 and 1969 to about 600,000 ha in 1970; and (3) the severity of defoliation in 1970 was considerably less pronounced in many budworm-infested forests than it was in years immediately preceding the freeze (H. E. Meyer, personal communication, 1971). On the other hand, many northern Rocky Mountain forests or small pockets of timber infested by the western spruce budworm in 1969 escaped the subfreezing temperatures. In these forests there was neither noticeable reduction in budworm populations nor was there visible frost damage to coniferous hosts. Although this paper presents quantitative data documenting reductions in western spruce budworm populations associated with unseasonal frost in western Montana, it is likely that this is not an uncommon phenomenon in the northern Rocky Mountains and elsewhere. Reductions of Choristoneura spp. populations attributable to unseasonal frosts - either directly through freezing of larvae and/or indirectly through destruction of their food supply - have been reported in Idaho (Klein, 1967), Colorado (Buchanan, 1947), Ontario (Prebble, 1945), Manitoba (Warren, 1954), and Quebec (Blais, 1957, 1958). Though unseasonal fall and spring frosts can substantially reduce Choristoneura spp. populations, larvae are normally unaffected by subfreezing temperatures during diapause - an overwintering "resting" state for second instar larvae. For example, from
REDUCTION OF BUDWORM POPULATIONSAND DAMAGEBY FROST
November 11 to 17 during the fall of 1959 an unseasonal cold wave covered most of Montana. Low temperatures of - 4 3 ° C and - 4 7 ° C were recorded at a study area in the Blackfoot River Valley, northeast of Missoula, but had little effect on survival of overwintering western spruce budworm larvae (Terrell, 1960). ACKNOWLEDGEMENTS We appreciate the use of study areas on Lubrecht Experimental Forest and the Lolo National Forest and weather data provided by the University of Montana School of Forestry. We are particularly indebted to Barbara H. Honkala, formerly Biological Research Technician, at the Forestry Sciences Laboratory, Missoula, Montana, for her assistance in data collection. REFERENCES Blais, J. R., 1957. Some relationships of the spruce budworm, Choristoneura fumiferana (Clem.) to black spruce, Picea mariana (Moench) Voss. Forestry Chron., 33:364 372. Blais, J. R., 1958. Effects of 1956 spring and summer temperatures on spruce budworm populations (Choristoneura fumiferana (Clem.)) in the Gaspd Peninsula. Can. Entomologist, 90(6): 354-361. Buchanan, W. D., 1947. Spruce Budworm Progress Report 1947. Rept. of the Forest Insect Laboratory, Bur. of Entomology and Plant Quar., U.S. Dept. Agric. Res. Adm., Fort Collins, Colo., 9 pp. Fellin, D. G. and W. C. Schmidt, 1967. Spruce budworm larvae sever stems of western larch shoots in Montana. J. Forestry, 65 (4): 258-260. Klein, W. H., 1967. Forest Insect Conditions in the Intermountain States During 1966. Rept. of the Branch of Forest Insect and Disease Control, Division of Timber Management, U.S. Dept. Agric. Forest Serv., Region Four, Ogden, Utah, 17 pp. Prebble, M. L., 1945. Spruce budworm. Domin. Dept. Agric., ScL Serv., Div. Entornol., Forest Insect Investigations, Progr. Rept., 1(4): 2. Salt, R. W., 1961. Principles of insect cold-hardiness. Ann. Rev. Entomol., 6:5 5-74. Terrell, T. T., 1960. Survival o f Spruce Budworm Larvae in Unseasonal Low Temperatures in Montana. Rept. Missoula Forest Insect Laboratory, Intermountain Forest and Range Exp. Sta., U.S.D.A., Forest Service, Missoula, Montana, March 1960, 2 pp. U.S. Dept. of Commerce, ESSA, 1969. Climatologicaldata-Montana, June 1969, 72(6): 103-128. Warren, G. L., 1954. The spruce needleworm, Dioryctria reniculella Grt. as a predator of spruce budworm. Can. Dept. Agric., Sci. Serv. Forest Biol. Div., Progr. Rept., 10(3): 2-3.