Sex-specific responses to aggregation pheromone: Regulation of Colonization Density in the Bark Beetle Ips paraconfusus

Byers, J.A. 1983. Sex-specific responses to aggregation pheromone: Regulation of Colonization Density in the Bark Beetle Ips paraconfusus Journal of Chemical Ecology 9:129-142. pdf

Attack by a male Ips paraconfusus into bark of ponderosa pine showing the "glued" fecal pellets (reddish) around the entrance hole and the xylem sawdust frass (yellow).

Abstract-- About equal numbers of each sex of flying Ips paraconfusus Lanier (Coleoptera: Scolytidae) were caught on traps several meters downwind from a male-infested ponderosa pine log releasing pheromone while a significantly different ratio of over four times more females than males were caught at the pheromone source. Females oriented directly to higher concentrations of colonizing males in a felled tree while males tended to land on the host in adjacent uncolonized areas. The attraction response of walking males to a 1: 1 : 1 mixture of the synthetic pheromone components ipsenol-ipsdienol-cis-verbenol was reduced progressively at higher concentrations while female response continued to increase. These responses may function to regulate density of colonization and limit intraspecific competition.

Key Words- Coleoptera, Scolytidae, Ips paraconfusus, bark beetle, Pinus ponderosa, ipsenol, ipsdienol, cis-verbenol, pheromone, attractant, intra-specific competition.

INTRODUCTION
Most bark beetle species studied, including Ips paraconfusus Lanier (Coleoptera: Scolytidae), use pheromones to attract individuals of both sexes to a suitable host tree for breeding (Borden, 1977). Each species probably has evolved behavioral and physiological mechanisms which attenuate aggregation that would otherwise result in overcolonization and severe competition for food and space. In the bark beetle, Dendroctonus brevicomis (Renwick and Vité, 1970; Bedard et al., 1980a), D. pseudotsugae (Rudinsky et al., 1972, 1974), and Trypodendron lineatum (Nijholt, 1973) inhibitory pheromones may assist in regulating the density of colonization and/or termination of aggregation on host materials. The reduction or inhibition of attraction response in these beetles either affects both sexes or may be limited to one, such as T. lineatum in which effects on females have not been determined. One of the intraspecific inhibitors of attraction response in D. brevicomis, verbenone, also decreases response of its competitor, I. paraconfusus, to its conspecific pheromone (Byers and Wood, 1980). Attractive pheromone components of certain Ips species may, in addition, function to inhibit response of competing bark beetle species to their conspecific pheromone (Birch and Wood, 1975; Birch et al., 1980; Byers and Wood, 1981).

Male I. paraconfusus begin colonizing ponderosa pine, Pinus ponderosa Laws., by excavating a "nuptial chamber" under the bark cortex primarily in the phloem layer. Two attractive pheromone components, ipsenol and ipsdienol (Silverstein et al., 1966), are synthesized only in males from the host terpene, myrcene (Hughes, 1974; Byers et al., 1979; Hendry et al., 1980). A third pheromone component, cis-verbenol, is synthesized from another host compound, alpha-pinene, in both sexes (Renwick et al., 1976; Byers, 1981 a). The three components together are attractive to both males and females in the field (Wood et al., 1968). I. paraconfusus males appear to regulate their attack density since Struble and Hall (1955) reported a maximum of 374 attacks/m² with the majority of samples ranging from 121 to 186/m². The density of females infesting pine is proportional to the density of males which allow entry of up to five females (usually three) into the nuptial chamber. Subsequently, others seeking entry are forcefully rejected (Barr, 1969). A recent study has indicated that the attraction of male I. paraconfusus to logs infested with males was inhibited by volatiles from an additional log infested with mated males and females (Byers, 1981b). This study implicated an olfactory mechanism that may regulate the attack density. The present study reports differences between the sexes in their upwind flight to a male-infested log and in their attraction to a concentration series of pheromone components. In addition, differences between the sexes in landing patterns on a felled tree under colonization reveal a mechanism of male inhibition by high levels of pheromone, which appears to regulate colonization density and intraspecific competition.

METHODS AND MATERIALS

Experimental Grid of sticky-traps, wind vane, and log of 50 males (right of center in foreground).

Upwind Flight Response to Pheromone. The pattern of flying beetles of each sex orienting to pheromone released by 50 males boring in a ponderosa pine log was determined by intercepting their flight with a grid of sticky-traps placed downwind from the infested log (September 20-28, 1975). The grid of traps (Figure 1) was located in a narrow valley in the Sierra National Forest (Miami Creek drainage at 1200 m elevation, Mariposa County, California). Observations of a wind-vane placed in various positions in the grid indicated a consistent wind direction perpendicular to the trap rows throughout the flight period (time of catch from about 0800 to 2000) probably due to solar heating at lower elevations forcing air upslope in a laminar flow.

Sticky trap surrounding screened log with 50 male Ips paraconfusus that served as pheromone source.

The male-infested log was screened and prepared as described earlier (Byers and Wood, 1980). It was placed inside a tubular sticky-trap (19 cm diam X 30.5 cm high) made of 6.3-mm mesh screen coated with Stickem Special® and 1.2 m above the ground (Bedard and Browne, 1969). Each row of traps had a nylon line perpendicular to the wind direction which suspended 15 flat sticky-trap screens (each 30.5 X 61 cm high) coated with Stickem Special and spaced at 1.5 m intervals 1.2 m above ground. The sticky-traps were monitored for beetles two days prior to introducing the infested log and six days after its removal. The "weighted" mean distances of catch from the pheromone source for each row and sex were calculated from numbers caught on traps 3-13 (and their respective distances) in each row.

Walking Response to Synthetic Pheromone Components. Beetles used for tests in a laboratory olfactometer (Byers et al., 1979, modified from Browne et al., 1974) were collected from naturally infested pine logs obtained from the same area described above. Walking beetles of each sex were tested for their attraction response to a series of increasing concentrations of a 1: 1 : 1 mixture of ipsenol-ipsdienol-cis-verbenol (racemic chemicals) < 97% pure, Chemical Samples Co.). Concentrations of each compound ranged in decimal steps from 10^-11 to 10^-6 g/µl diethyl ether.

Landing on Trunk of Felled Pine during Colonization. Numbers of male and female I. paraconfusus caught on sticky-traps along the trunk of a felled pine were analyzed to determine whether behavioral differences between the sexes existed under conditions of actual colonization. An apparently healthy ponderosa pine tree (0.36 m diam at 1.5 m height) was felled at 08:00 on October 1, 1978, and all branches were removed except those above the 16.3 m height (trap 1).

Felled ponderosa pine with line of sticky-traps for monitoring landing of Ips paraconfusus during colonization.

Eighteen sticky-trap screens, 21 X 21 cm, were placed horizontally on the upper side of the felled tree at intervals of 0.76 m beginning 3.6 m from the "top" (0.1 m diam) and extending along the trunk to 3.3 m from the base. An aggregation of I. paraconfusus near the top of the tree was induced by baiting with a log that had been infested two days earlier with 50 males. The male-infested log was removed 24 hr later. The portion of the tree that was under attack, as indicated by boring dust piles, was noted each day.

The slopes of linear regression of male versus female catch on the 18 traps for successive two-day periods (October 1-8, 1978) were compared to corresponding 95% lower binomial confidence limits (LBCL) of sex ratios of catch (Byers and Wood, 1980) with a t-test method (Mendenhall, 1967). The total male and female catch and the percentage of males in the catch per day during the mass aggregation period were summarized.

RESULTS
Upwind Flight Response to Pheromone. The distribution of catch of I. paraconfusus on the grid of traps appeared as a narrowing pattern focusing to the pheromone source (Figure 1).

FIG. 1. Distribution of male and female Ips paraconfusus caught on sticky-traps spaced in a grid downwind from a log infested with 50 males in the Sierra National Forest, California (September 20-28, 1975). *Mean of distribution of catch ± standard deviation. **Female/male sex ratio was significantly different from sex ratios in either row 1 or 2 (P < 0.001, chi-square).

This pattern is to be expected if beetles are flying upwind toward the source of pheromone. The broad overlaps of the standard deviations of the sexes caught within each row of traps (Figure 1) indicated that both sexes. initially directed their flight upwind toward the pheromone source. The increase in catch on trap rows closer to the source was probably a function of an increase in the proportion of the cross-sectional area of the pheromone "odor plume" that was intercepted by trap area. About equal numbers of male and female beetles appeared to enter the grid area since catches of each sex on rows 3 and 2 were almost identical. However, proportionately fewer males than females were attracted upwind to the source, as the sex ratio caught there was significantly less than on row 1 (Figures 1 and 2), and row 1 was significantly less than rows 2 and 3 (P < 0.05). No beetles were caught after removing the infested log.

FIG. 2. Total number of male and female Ips paraconfusus caught at the log infested with 50 males and on sticky-traps (nos. 3-13) in rows of increasing distance downwind from the pheromone source, Sierra National Forest, California (September 20-28, 1975). The distance from the pheromone source for each row is based on the average trap distance weighted by the catch. Brackets represent 95% asymmetric binomial confidence limits.

Walking Response to Synthetic Pheromone Components. Male and female response was similar at lower release rates (10^-11 - 10^-9 g/µl) of ipsenol - cis-verbenol - ipsdienol. However, attraction of males, but not females, was inhibited at higher release rates (10^-8 - 10^-6 g/µl, Figure 3).

FIG. 3. Response of male and female Ips paraconfusus to a 1:1:1 mixture (separate weights) of the pheromone components ipsenol (I), cis-verbenol (II), and ipsdienol (IlI) in the laboratory olfactometer. Means compared within each sex between concentrations followed by the same letter were not significantly different at alpha = 0.05, and means compared at the same concentration between sexes followed by star symbols were significantly different at alpha = 0.01 (chi-square). Brackets represent 95% binomial confidence limits, N = 90-120.

The actual release rate (g/min) was estimated to be about 2.2 times the g/µl concentration, which assumes that the compounds were released in proportion to the volume reduction of solvent (Byers and Wood, 1981). These results indicated that response of males increased with an increase in pheromone concentration to a threshold above which a reduction in attraction occurred. On the other hand, the attraction of females continued to increase in proportion to the logarithm of the concentration of pheromone components.

Landing on Trunk of Felled Pine during Colonization. Traps progressively further from the origin of colonization at the "top" of the felled tree caught increasingly higher proportions of males on the first, second, and third days of aggregation (Figure 4).

FIG. 4. Trends in the proportion of male Ips paraconfusus caught on sticky-traps distributed distally from the origin of colonization on the trunk of a felled ponderosa pine in the Sierra National Forest, California (October 1-5, 1978). An aggregation of I. paraconfusus near the top of the tree was induced by placing a 50-male-infested log there for one day only. Proportions within each day followed by the same letter were not significantly different at alpha = 0.05 (chi-square).

In order to observe these trends in sex ratio it was necessary to combine the catches of each three successive traps for a total of six groups. Numerous piles of boring dust were observed only at the "top" of the tree beginning on the second day. On the third day piles appeared further along the tree corresponding to the highest catches. By the fourth day, the entire surface that was covered with traps appeared to be relatively uniformly attacked (about 110-150 attacks/m²) and no trends in sex ratio then were observed (Figure 4).

Males responded differently from females to areas of colonization corresponding to the 18 trap locations on all four two-day periods since the slopes of the regression lines of male against female catch were significantly less than the corresponding LBCL (95%) sex ratios of catch on all traps (Table 1).
TABLE I. Comparison of sex ratio of total catch of Ips paraconfusus and linear regression of male and female catch on 18 sticky-traps along trunk of felled ponderosa pine tree, Sierra National Forest, California (October 1-8, 1978)

Date	Sex (males/females)	LBCL (95%)^a sex ratio	Regression slope (x = female vs. y = male)	Difference between LBCL and regression slope (P value)^b

Oct. 1-2	0.660	0.511	0.107	µ 0.001
Oct. 3-4	0.346	0.274	0.093	0.034
Oct. 5-6	0.418	0.297	0.141	0.030
Oct. 7-8	0.300	0.185	-0.078	0.027

^aLower binomial confidence limit (95%) of thc sex ratio of total catch.
^bA P value less than 0.05 indicates the regression slope was significantly less than the LBCL of the sex ratio.

Similarly, compared to the first and second days of attack, the proportion of males that landed on the fourth day significantly decreased (each P < 0.001, chi-square) when female aggregation was the most intense (Figure 5).

FIG. 5. Relationship between proportion of male Ips paraconfusus caught and the number of each sex caught daily on sticky-traps on the trunk of a felled ponderosa pine during colonization in the Sierra National Forest, California (October 1-8, 1978).

DISCUSSION
Previous reports indicated that males were less responsive to pheromone than females based on their attraction in the laboratory to naturally produced pheromone (Wood, 1962) or to gut extracts of fed males (Pitman et al., 1965). The attraction of attacking males to pheromone has been described as an "area orientation" (higher number of turns per unit distance traveled) compared to the female's "straight-line orientation" (Wood and Bushing, 1963). It was further speculated that males would be attracted to host substrates under colonization but that after landing thigmotactic or gustatory stimulants might induce attacks on the bark before the source of pheromone, a male entrance tunnel, was reached. The present study indicates that these earlier observations were the result of an inhibition of male response to higher concentrations of ipsenol, cis-verbenol, and ipsdienol. Furthermore, my laboratory bioassays indicate that the differences in sex ratio of catch in the grid trap experiment were probably due to inhibition of male response by higher concentrations of these pheromone components encountered when the male-infested log was approached. Colonizing males produce the highest amounts of ipsenol and ipsdienol during the first eight days of an attack (Byers, 1981b), so these levels of pheromone were available to inhibit the response of males. In the first experiment, females were not allowed to join males at the pheromone source so the possibility of release of inhibitors by females, or female-induced release of inhibitors by males, cannot account for the sex-specific differences in attraction.

Sex-specific responses were clearly shown in the experiment that monitored beetles landing on the felled ponderosa pine tree being colonized. The highest proportion of males in the trap catches occurred farthest from the origin of attack during the first three days. By the fourth day male attacks had occurred throughout the trunk, and there were no significant trends in sex ratio along the trunk, probably because pheromone was being released at all trap locations. The regression slope of male to female catch was significantly less than the 95% lower binomial confidence limit of the overall sex ratio of the catch, a fact which may be explained by inhibition of male response, while female response increased at those areas of high colonization density and pheromone concentration. As the colonization of the tree progressed during the week, the proportion of males in the catch significantly decreased, and it was lowest at the peak of the aggregation when pheromone release was presumably the highest (Byers, 1981b). The responding population would probably arise from several brood locations not necessarily of the same age that each contributed a sex ratio of approximately 1:1 over the period (Struble and Hall, 1955). Furthermore, based on the beetle's longevity in the laboratory, it appears that the absolute sex ratio of the population in nature would be composed of several days' emergence as well as reemerged parent adults (Struble and Hall, 1955). Thus, the significant change in the sex ratio of catch observed during the first four days of colonization is probably a result of differential response of the sexes to varying pheromone release rates rather than a significant change in the sex ratio of the available adults.

Many investigators of bark beetle biology have wondered about the discrepancy between the approximate male-female ratio of emerging beetles (1:1) to the sex ratio caught on traps with male-infested logs (1:3) or the sex ratio in nuptial chambers (1:3) (Struble and Hall, 1955; Wood, 1962; Gara, 1963; Cameron and Borden, 1967). One previous hypothesis suggested that I. paraconfusus males sustain higher levels of mortality than females because of greater exposure to predation and host tree resin during the initial attack (Gara, 1963). The higher male mortality might result in absolute numbers of males of about 1/3 those of females in the responding population. However, the 1:0.86 male-female sex ratio of beetles believed to be entering the grid area, and the 1:1.35 male-female ratio landing during the first day on the felled tree, suggests that differences in mortality between the sexes because of such differences in predation and death during initial burrowing does not appear to significantly change the sex ratio of flying beetles from that at emergence.

In this study, males and females appeared to be equally responsive to low concentrations of pheromone, but many of the males deviated from the path taken by females when flying to infested hosts to land on areas adjacent to those with higher densities of males which are releasing higher amounts of pheromone. This behavioral mechanism would function to regulate density of colonization and intraspecific competition for food and space. The sexual difference in response was shown to occur even before beetles landed, and it is probable that the effect intensifies after landing. Once males are on the host, other factors may contribute to density regulation and prevent new attacks in fully infested substrates. If no new attacks occur, the colonized areas would become nonattractive since ipsenol and ipsdienol quantities in mated males decrease to negligible levels after 7-10 days (Byers, 1981b).

In contrast to I. paraconfusus, both sexes of the bark beetle Scolytus mutistriatus have been shown to respond in about equal numbers to sources of natural attraction (Peacock et al., 1971). Tilden et al. (1979) reported that more males than females of the monogamous bark beetle, D. brevicomis, were trapped close to the source of synthetic pheromones than on surrounding unbaited traps further away. However, the sex ratio was different only between the source and two comparisons on outlying traps while six other comparisons at similar distances were not significantly different. In other studies, the sex ratio of D. brevicomis was found not to be significantly different from unity during attraction to a trap releasing synthetic pheromones (Vité and Pitman, 1969; Wood, 1972; Wood et al., 1976; Byers and Wood, 1980; Bedard et al., 1980b) or, more importantly, to traps containing naturally infested host logs (Byers and Wood, 1980). Neither sex of D. brevicomis appeared to be inhibited by increasing the concentrations of its synthetic pheromone components in the same laboratory walking bioassay (Byers and Wood, 1981) that demonstrated inhibition of male I. paraconfusus in the present study. The responses of the sexes of D. brevicomis to individual components of its pheromone (exo-brevicomin and frontalin) were shown to be different (Vité and Pitman, 1969), but this may be an artifact since these compounds are never released alone during mass colonization, except by the first female (Byers and Wood, 1980). However, certain ratios of these pheromone components might function to regulate "close-range" behavior after landing such as mating and avoidance of competition. Verbenone, produced by male D. brevicomis (Renwick, 1967), inhibits the response of both sexes to their attractants in the laboratory (Hughes and Pitman, 1971) or field (Bedard et al., 1980a) and may function to regulate attack density (Byers and Wood, 1980) or terminate aggregation (Renwick and Vité, 1970). D. frontalis males also release verbenone which inhibits male response (females were not tested; Rudinsky, 1973), and synthetic verbenone released at "higher concentrations" inhibited both sexes (Vité and Renwick, 1970; Payne et al., 1978).

The attack density and termination of mass attack in D. pseudotsugae may be controlled in part by release of 3-methyl-2-cyclohexene-1-one (MCH) from females when a male arrives and stridulates at the entrance hole (Rudinsky and Michael, 1972). On the other hand, Pitman and Vité (1974) found that males contain considerably more MCH than females, and they believe males are primarily responsible for release of the inhibitor. The responses to pheromone in both sexes are inhibited (Rudinsky et al., 1974), but males appear to be more strongly affected (Rudinsky et al., 1972). The density of D. pseudotsugae attack on a tree under colonization has been reduced by MCH released at multiple sources, and the distance of nearest attack from MCH elution points increased with the concentration of MCH (Furniss et at., 1974). Surprisingly, at the highest concentrations of MCH release, the attack density increased (Hedden and Pitman, 1978). Nijholt (1973) reported that males of Trypodendron lineatum released an unidentified inhibitory pheromone after joining the female. Unfortunately, the possibility of changes in the sex ratio of the responding beetles after inhibitors were released was not investigated. However, a recent study has shown that female T. lineatum decrease their release of the attractive pheromone, lineatin, after the male arrives (Klimetzek et al., 1981). Klimetzek et al. found evidence of a male-induced inhibition of response which appeared to operate only at close range and may "involve a communication system other than olfaction" (possibly acoustic).

The theory that sex-specific differences in response to intraspecific semiochemicals can function to regulate density of colonization has not been suggested elsewhere for Ips species. However, Lanier et al. (1972) found sex-specific differences in I. pini in which females, but not males, from Idaho could discriminate between New York and Idaho male-infested logs, while both sexes of New York beetles could differentiate Idaho and New York pheromone. These differences appear to be the result of differential response of the geographical populations to enantiomers of ipsdienol (Lanier et al., 1980). Mustaparta et al. (1980) showed that both Idaho and New York females (males not tested) had two receptor cells each specialized for a specific enantiomer of ipsdienol. Since the receptor systems for ipsdienol of the two populations are virtually identical, the response differences appear to be governed by the central nervous system rather than the peripheral receptors (Lanier et al., 1980). Indicative of the present study, Lanier et al. (1980) found in the laboratory that the responses of male I. pini from New York or Idaho to odor from male-infested logs were always less than those for the same type of female. Electrophysiological studies may reveal whether receptors on the antenna of a male I. paraconfusus (and I. pini ) adapt at lower concentrations of pheromone than the receptors of females, or whether the sexual difference in response is a consequence of the central nervous system.

In all Ips species distributed throughout the northern hemisphere, the males initiate the attack, are polygamous, and in many species studied, the males produce various ratios and amounts of ipsenol, cis-verbenol, and ipsdienol (Vité et al., 1972). Therefore, olfactory mechanisms similar to that described here may operate in reducing intraspecific competition in other species of the genus Ips.

Acknowledgements- I thank D.L. Wood, Department of Entomological Sciences, University of California at Berkeley, for support and use of his research facilities; I also thank W.D. Bedard and P.E. Tilden of the USDA Forest Service for use of their research facilities at Oakhurst, California. I am grateful to R.P. Akers, A.M. Liebhold, D.R. Owen, and D.L. Wood, Department of Entomological Sciences, University of California at Berkeley, and J.S. Elkinton, Department of Entomology, University of Massachusetts, for helpful reviews of the manuscript. The work was supported in part by grants from the Rockefeller Foundation, USDA Forest Service, and Regional Research Project W-110, SEA/USDA (to D.L.W.)

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