Oecologia (1997) 112:48±54 Ó Springer-Verlag 1997 John D. Reeve Predation and bark beetle dynamics Received: 23 January 1997 / Accepted: 5 April 1997 Abstract Bark beetle populations may undergo dra- aging, reduces the ability of the tree to resist beetle at- matic ¯uctuations and are often important pests in tack through its oleoresin system (Rudinsky 1962; Ber- coniferous forests. Their dynamics are thought to be ryman 1972, 1973; Raa and Berryman 1983; Berryman primarily driven by factors aecting the resistance of the and Ferrell 1988; Christiansen and Bakke 1988; Raa host tree to attack, i.e., bottom-up forces, while natural 1988), although there is evidence that the relationship enemies are usually assigned a minor role in these sys- between resistance and water stress is not simple (Lorio tems. I present behavioral experiments that suggest that 1988; Lorio et al. 1990). As their densities increase, the clerid beetle Thanasimus dubius may be an important beetles are often able to overcome even resistant trees source of mortality for the bark beetle Dendroctonus through mass attack, and the population may only col- frontalis during attack of the host tree, and determine lapse when the supply of host trees has been depleted. In the nature of the functional response of T. dubius under contrast, natural enemies are usually assigned a minor conditions close to natural. I also examine the numerical role in bark beetle dynamics, at best only accelerating response of T. dubius to large-scale ¯uctuations in the collapse of the beetle population (Berryman 1973; D. frontalis density, and the relationship between bark Berryman and Ferrell 1988; Gre goire 1988). Bark beetle beetle population trends and predator density, and ®nd dynamics would appear to be primarily driven by factors that beetle populations tend to decline when predator aecting the resistance of the host tree to attack, and are densities are high. Combined with the eects of clerid thus dominated by bottom-up forces (Hunter and Price larvae on bark beetle broods, these results suggest that 1992; Power 1992). top-down forces generated by natural enemies could also There is increasing evidence, however, that natural be an important component of bark beetle dynamics. enemies may be important factors in the dynamics of at The implications of these results for bark beetle least some bark beetles. Studies using exclusion cages dynamics are discussed in relation to the prolonged have shown that the natural enemy complex as a whole life-cycle of clerid beetles. can in¯ict signi®cant mortality on the brood (immature stages) of bark beetles within an infested tree (Linit and Key words Scolytidae á Cleridae á Predation á Ratio Stephen 1983; Miller 1984, 1986; Riley and Goyer 1986, dependence Weslien 1992). Clerid beetles (Coleoptera: Cleridae) in particular appear to be major natural enemies in several systems, preying on both bark beetle adults and their Introduction brood. Studies of Thanasimus formicarius (L.) have shown that its larvae can generate high levels of brood Populations of bark beetles (Coleoptera: Scolytidae) mortality in Ips typographus (L.) (Mills 1985; Weslien may undergo dramatic ¯uctuations and are often and Regnander 1992; Weslien 1994) and Tomicus pin- important pests in coniferous forests. Outbreaks are iperda (L.) (Schroeder and Weslien 1994) in Eurasia, and believed to occur when some factor, such as drought or Thanasimus dubius (F.) larvae are thought to be impor- tant predators on Ips grandicollis (Eichh.) and Dendr- octonus frontalis Zimm. in North America and Australia (Thatcher and Pickard 1966; Mignot and Anderson 1969; Moore 1972; Linit and Stephen 1983; Lawson and J.D. Reeve Southern Research Station, USDA Forest Service, Morgan 1992). There is less quantitative information on 2500 Shreveport Hwy., Pineville, LA 71360, USA the impact of adult clerids on adult bark beetles. Fiske Fax: (318) 473-7222; e-mail: [email protected] (1908) found numerous fragments of adult D. frontalis 49 lodged in the bark crevices of trees where adult T. dubius Estimation of adult predator densities were abundant, which suggests a high predation rate. Thatcher and Pickard (1966) released D. frontalis and As a precursor to laboratory studies of T. dubius and D. frontalis T. dubius adults into a room containing a freshly cut behavior (see below), I estimated the density of adult predators on trees undergoing mass attack by D. frontalis. Trees currently under pine log and found that predation reduced the number attack were ®rst located within D. frontalis infestations, and traps of bark beetles successfully attacking the log by about were then attached to the bark surface to sample the resident 50%. In contrast, some studies have reported very low population of adult T. dubius. The traps were cylindrical enclosures densities of adult clerids (Schmid 1970), or few en- made of ®ne polyethylene screening, 1.5 m in length, with two collecting funnels at the bottom. They were attached near the counters between predator and prey resulting in preda- center of the vertical distribution of D. frontalis attacks, at a height tion (Bunt et al. 1980). Studies in simple laboratory of 4 m on the bole (Fargo et al. 1978). To prevent any insects from arenas provide further indications that adult clerids can escaping the sample area, the bottom of the trap was attached ®rst, potentially consume large numbers of bark beetles, and and the screening of the trap was then raised quickly up the bole and sealed. The traps were left in place for 1 week, an interval have also provided detailed information on their be- sucient for the majority of T. dubius present to fall into the col- havior (Berryman 1966; Turnbow et al. 1978; Nebeker lecting funnels (J.D. Reeve, unpublished data). A total of 21 trees and Mizell 1980; Turnbow and Franklin 1980; Frazier et (loblolly or shortleaf pines) in eight infestations were sampled across a 2-year period, with a maximum of three trees sampled per al. 1981). 2 In this paper, I present behavioral experiments that infestation. The density of T. dubius was expressed as number/dm of bole surface, calculated by dividing the total predators caught by manipulate the densities of adult D. frontalis and the surface area enclosed by the trap. T. dubius on caged pine logs (``bolts'') in an eort to estimate the mortality in¯icted by adult T. dubius under conditions close to natural, and investigate its functional Laboratory studies of predation by T. dubius response. While it would be desirable to estimate pre- I used the fact that D. frontalis will readily attack fresh pine bolts in dation rates in the ®eld, this is impractical because the laboratory (Thatcher and Pickard 1966) to estimate predation overlapping bark scales conceal the insects much of the rates by T. dubius under conditions resembling those in the ®eld, time, making it dicult to determine their densities. but where densities can be controlled and the fate of the prey Much of the evidence of predation, detached D. frontalis readily determined. I used bolts cut from mature loblolly pines (80 cm long and 20 cm in diameter) as arenas for these studies, with body parts, would also be lost under ®eld conditions. I 50 cm of the bolt enclosed by a spherical screen cage 60 cm in examine also the numerical response of T. dubius to diameter. The cage was attached at the top and bottom using duct large-scale ¯uctuations in D. frontalis density, and the tape, after smoothing the bark at the attachment points to obtain a relationship between bark beetle population trends and close ®t. Four dierent predator densities were used in the experi- ments (0, 10, 20, and 40 adult T. dubius) and crossed with three prey predator density, using data from a long-term survey densities (100, 200, and 400 adult D. frontalis), yielding a total of 12 that spans the most recent outbreak in central Louisi- treatment combinations. The predator treatments were chosen to ana. The results indicate that T. dubius may be an represent a range of predator densities similar to that observed in important source of bark beetle mortality during attack the ®eld, from 0 to 1.25 predators/dm2 (see Results). Equal pro- of the host tree, and that D. frontalis populations tend to portions of males and females were used in the predator treatments. The prey densities were selected to create a range of attack densities decline when predator densities are high. The implica- representative of a 1-day period (the experimental time interval) in tions of these results for the population dynamics of nature, given what is known about overall attack densities and the D. frontalis are discussed in relation to the prolonged duration of mass attack (Fargo et al. 1978; Dixon and Payne 1979a; life-cycle of T. dubius. Coulson 1980). The experiments were conducted in an environ- mental chamber set at a constant 23°C and illuminated with ¯uo- rescent light from 0700 to 1900 hours. The overall experiment was divided into three temporal blocks, each 2 months long; within each Materials and methods temporal block, every treatment combination was replicated once, with the order randomly determined. Adult T. dubius were collected for the experiments using baited Natural history multiple-funnel traps (see description below) and were stored in a refrigerator for no longer than 2 weeks. Adult D. frontalis were In common with a number of other bark beetles, D. frontalis can reared from naturally infested bark or logs, and were collected mass attack and kill living trees. Favored hosts in the southern daily and immediately used in the experiments. The predators were United States are loblolly pine, Pinus taeda L., and shortleaf pine, starved for 1 day at room temperature (approximately 23°C) P.