Aggregation Behavior of Dendroctonus ponderosae and. other Bark Beetles

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1 Colorado State University Aggregation Behavior of Dendroctonus ponderosae and other Bark Beetles Mary F Adams, MA NSF GK12 Fellow Mary.Adams@colostate.edu Telegoddess@hotmail.com Abstract The Mountain Pine Beetle (Dendroctonus ponderosae) causes extensive mortality in Ponderosa Pine (Pinus ponderosa Lawson) along the Colorado Front Range and is of great concern to ski and tourism business in the National Parks, US Forest Service and Colorado State forest service lands. Hypothesis-driven research centers on the aggregation of bark beetles. Does aggregation benefit bark beetles by diluting predation? The hypothesis is that decreased risk of predation is a potential benefit to aggregation behavior that attracts mates and procures resources when colonizing well defended or poorly defended trees. Temperature relationships affect mountain pine beetle behavior. North side bark temperatures are cooler and less variable and diurnal and nocturnal temperatures differ by 8 degrees F. Difference in stand temperature influence beetle behavior directly or

2 indirectly through the dispersion and activity of mountain pine beetle aggregation pheromones (Schmid et al 1991). Outbreaks of mountain pine beetle are related to colonization patterns in thinned and unthinned plots of lodge pole pine (Pinus contorta Douglas). Beetles tend to colonize trees with large diameters. If trees with smaller diameter were attacked it was only because they were close to trees with larger diameter. Older trees tended to resist attack in spite of diameter, vigor or position relative to other attacked trees (Preisler and Mitchell 1993). Bark beetle species (Coleoptera: Scolytidae) have eruptive outbreaks triggered by immigration and characterized by endemic and epidemic outbreaks in previously stable populations. Host mortality during extensive outbreaks can continue after environmental stresses have eased. This is attributed to the ability of beetles to rapidly aggregate on selected trees in response to attractive pheromones (Raffa and Berryman 1986). Other factors of bark beetle aggregation include; 1) interaction of bark beetle aggregation and conifer defense rates; 2) pre-aggregation landing and gallery initiation on lodgepole pine and 3) gallery initiation during aggregation. Introduction Mountain Pine beetles kill millions of trees in Canadian and United States forests during epidemics. Mountain pine beetle affects the lodgepole pine ecosystem by causing large areas to suffer extreme losses of forest cover. The probability of beetle attacks can be calculated. The number of square units that must be attacked was determined by one or more pioneer beetles inducing aggregation. It was possible to calculate if a tree would 2

3 become an aggregator. Gallery starts are greater at the start of the epidemic decreasing in midyears and increasing in latter years. Acceleration and deceleration are phases of gallery initiation. Acceleration is highly variable. Deceleration is observed by a decreasing rate of gallery initiation. Pioneer beetles release pheromones to initiate the aggregation phase. Switching means all the beetles are aggregating on one tree and a pheromone cue is sent to indicate that density is great, move on to the next tree. Switching caused by changes in beetle density happens after the focus (first) tree has been attacked. The possibility that beetles use strategies that are flexible and density dependent merits attention. Plant defenses cannot hold up during beetle attacks. An ideal should include direct resistance and indirect mechanisms that inhibit chemical communication. Predator dilution may be a benefit to aggregation because predators decreased the number of beetle attackers in a density-dependent manner. Aggregations of herbivorous insects have benefits such as location of mates, defense or escape from predators. Temperature relationships affect mountain pine beetle behavior. North side bark temperatures are cooler and less variable and diurnal and nocturnal temperatures differ by 8 º F at the time of this study( Schmid et al. 1991), mountain pine beetle aggregation behavior was not known to be affected by air and bark temperatures. Difference in stand temperature influence beetle behavior directly or indirectly through the dispersion and activity of mountain pine beetle aggregation pheromones. Beetle attacks are related to tree diameter and spacing among trees. Beetles prefer trees with large diameters. If a tree with a small diameter was close to other trees being attacked, then it stands a chance of being attacked also. 3

4 Beetle Dispersion and Aggregation Models Mountain Pine beetles kill millions of trees in Canadian and United States forests during epidemics. Mountain pine beetle affects the lodgepole pine ecosystem by causing large areas to suffer extreme losses of forest cover. These outbreaks cause a high level of concern, from the USFS to the mountain resorts, impacting stewardship of the land and the economy. Walter Cole s et al. (1985) research for the US forest service studied the relationship between the insect and environmental factors and the quantification of these relationships based relative to beetle behavior. Mathematical modeling was used. Using data from sampling of life tables showed insect density trends over years correlating life state and size of infested trees. Sampling schemes are based on the behavior of the insect. Impact modeling gives clues to modes of behavioral interactions. Raffa and Berryman (1986) developed a computer simulation model of population behavior in relation to population density with population growth to show how host and beetle factors can interact to affect population behavior. The most important factor to emerge was how host vigor is affected by stresses such as drought and defoliation. Mountain pine beetles breed in weakened hosts and aggregate in response to attractive pheromones. The host loses its defense mechanisms. Long term protection from beetle outbreaks is assisted by thinning stands. Newly developed mountain pine beetles emerge in late July and early August. When one of a few mountain pine beetle attack a green tree, the aggregation pheromone is released and many beetles are attracted for a mass attack of the tree. 4

5 Cooperative group attacks involve communication through aggregation pheromones. Attack density data can be collected from hosts that have survived and those that have died. A model for the benefits and costs of group attack gives insight into animals that hunt in groups (Berryman et al 1985). Negron and Popp (2003) developed models to guide silvicultural treatments and restoration efforts by establishing stocking levels in Colorado, which is dry and has poor growing conditions. Ponderosa pine stands are more susceptible to mountain pine beetle attack based on forest conditions. Infested trees are larger in diameter and breast height. One infested plot demonstrated increasing stand density index which increased the probability of infestation. Dispersion Aggregation Model Burnell (1977) devised a model that according to WE Cole et al (1985), used three assumptions in dispersal/ aggregation. 1. Pioneer beetles attack with random distribution over the available bark surface. 2. A tree has a threshold of aggregation that is required to induce aggregation. 3. Any tree becoming an aggregator will be mass attacked and killed. The probability of beetle attacks can be calculated. The number of square units that must be attacked was determined by one of more pioneer beetles inducing aggregation. It became possible to calculate if a tree would become an aggregator. At the start of an aggregation by a pioneer beetle, larger trees are attacked because they provide more food in the phloem for the larva. Pioneer beetle density rises and the 5

6 beetles move to smaller trees because the larger trees are depleted, correspondingly, the epidemic collapses as pioneer beetle density decreases (Cole et al 1985). Gallery starts are greater at the start of the epidemic, decreasing in midyears, and increasing in latter years. Acceleration and deceleration are phases of gallery initiation. Acceleration is highly variable. Deceleration is observed by a decreasing rate of gallery initiation. Pioneer beetles release pheromones to initiate the aggregation phase. Pre-aggregation is a period of landing and gallery starts by pioneer beetles during which host selection takes place. Some of the pioneer beetles release aggregation pheromones to start the aggregation (Hynum and Berryman 1981). Bioassays indicate that there may be gallery initiation stimulants in the bark. Beetles do not distinguish between dead hosts, nonhosts or hosts during landing. After landing, a beetle walks on the bark and decides whether to attack or leave. Olfactory and gustatory stimuli have been suggested as the mitigating factor (Hynum and Berryman 1980). Attacking in large numbers like an army has its advantages for the beetles and disadvantages for the host. Bark beetles can reproduce in healthy trees because their numbers overwhelm the host defenses. Aggregation and antiaggregation pheromones dictate the process. Beetles that arrive early must deal with host defenses, so they send out aggregation pheromones to attract more beetles. The early arrivals can start gallery initiation and egg laying sooner. Beetles that arrive late have to compete and the phloem is less available, but do not have to battle tree defenses. Pureswaran et al, (2006) investigated relationships between timing of arrival, body size, pheromone production 6

7 and fitness in Dendroctonus frontalis. Most beetles arrived at the middle of the attack and produced longer galleries. The number of offspring was also higher. Stabilizing selection favored beetles that attacked in the middle of the sequence. They found no differences in body size or pheromone amounts between early and late arrivers. Aggregation/Susceptibility Model Switching means all the beetles are aggregating on one tree and a pheromone cue is sent to indicate that density is great, move on to the next tree. This switching in changes to beetle density happens after the focus (first) tree has been attacked. The recipient tree is mass attacked and killed faster than the focus tree. Larger trees have thick phloem which enhances the beetles survival. When switching occurs, beetle move to trees with larger diameters. Many conclude that if switching can be disrupted by thinning stands, there will be less attack in new areas (Cole et al. 1985). Bark Beetle Chemical Communication Systems Beetles use strategies that are flexible and density dependent. Plant defenses cannot hold up during beetle attacks. Forest protection should include direct resistance and indirect mechanisms that inhibit chemical communication. Pheromones are consistent, so variations of environmental factors dictate behavior. Fluid ecological interactions directly impact pheromone signals (Raffa 2001). Bark beetles respond to pheromones and colonize trees in aggregations. Do aggregation pheromones that start landing behavior also affect beetle entry behavior? This is unknown. In beetle populations that are low, they avoid healthy trees and seem to have a preference for weakened trees, such as those struck by lightening. When population densities are high, the beetles attack vigorous trees. Entry and gallery 7

8 construction assays were conducted to determine if bark beetles response to various concentrations of phloem monoterpenes would determine if they will accept or reject, or accept host coniferous trees. Diluting Predation Hypothesis driven research centers on the aggregation of bark beetles. Does aggregation benefit bark beetles by diluting predation? The hypothesis is that decreased risk of predation is a benefit to aggregation behavior.predator dilution may be a benefit to aggregation because predators decreased the number of beetle attackers in a densitydependent manner. Aggregations of herbivorous insects have benefits such as location of mates, defense or escape from predators. (Aukema and Raffa 2004). Beetle populations tend to decline when predator densities are high. Host trees may also develop resistance to attack (Reeve 1997). Integrated pest management has many combinant factors to develop strategies against the bark beetle. Temperature Variation Temperature relationships affect mountain pine beetle behavior(schmid et al. 1991). North side bark temperatures are cooler and less variable and diurnal and nocturnal temperatures differ by 8º F. Stand temperature directly affects beetle behavior and the attendant dispersion of pheromones. Beetle behavior is observed as beetles avoid thinned stands of trees because they are warmer. The mountain pine beetle aggregating pheromone is influenced by warmer temperatures in its dispersal and activity. In populations of mountain pine beetle, as adult size increases, and the male: female ratio decreases, stress-induced mortality follows in the larval and (or) adult stages. An experiment was conducted in cold storage to determine if fat content of newly 8

9 emerged beetles is correlated with their size, which may be an indication of their flight capacity and motivations of beetles to enter the host. Overwintering pine beetle larvae are subjected to low temperatures, which may affect their sex ratio. Their minimum supercooling point is about -29 º F. Adverse weather conditions can affect the larval instars and the sex ratio of adult beetles (Safranyik 1976). Colonization Patterns in Thinned and Unthinned Lodgepole Pine Stands Beetle attacks are related to tree diameter and spacing among trees. Beetles prefer trees with large diameters. Smaller nearby trees can also be attacked. Thin plots were unattractive to beetles unless an attack was started. Switching still occurred, even though the trees were farther apart. If the stand is very vigorous in a thinned plot, it was observed that only one tree was colonized. Old trees on thinned plots appeared to be resistant to attacks, which was a surprise because their resistance was not related to vigor, diameter, or position in relation to other attacked trees (Preisler and Mitchell 1993). The host remains a suitable habitat for only one generation (Berryman et al 1989). Emerging adults will not reproduce unless they can locate and successfully colonize new hosts. This process has placed pressure on hosts for rapid and complex defense mechanisms. Host death must occur for successful beetle reproduction. To repel the attack, hosts have developed allelochemicals and induced biochemical responses. Each outcome is different. The proposition lies in the ratio of host defense to attack virility and cooperative beetle behavior. Tree by tree, host defenses cannot withstand a large, sustained attack. Bark beetles are considered to be the most damaging of all forest insects 9

10 Conclusion We are faced with a dilemma in forest management with the current bark beetle epidemic. Research must be increased, and government funding is not enough. Colorado needs to develop a timber industry to handle all the downed lodgepole and ponderosa pine. Private industry such as mountain resorts, ski areas, timber operations, mountain home developments, golf courses, planned mountain retirement communities may be willing to invest money for research to solve the problem of loss of beauty of pristine forests and possible future impacts or imbalances of the forest ecosystem. This literature review prioritized thinning of forests to control present outbreaks and prevent future outbreaks. A cold, harsh winter with temperatures below -29 º F. for three to five days would kill the larvae that are overwintering in the trees. Another method that could slash beetle numbers would be wildfires. Coloradoans would rather avoid that because we have already recently experienced wildfire danger and destruction of mountain property. Ecologists say the long term drought in Colorado and an aging lodgepole forest puts the forests in a beetle vulnerable stage. Kim Vogel, a spokeswoman for the forest service says, It s the right time for it to be happening at this scale, given these warm winters and hotter summers. It s prime conditions for beetles to thrive and grow. Foresters say the beetle epidemic is a natural process. It s nature s way of recycling the forest in an era when humans living amid all the trees have demanded wildfire suppression (Hartmant.Rocky Mountain News 2007) Any attempt manage the beetle devastation will require surefire results at a cost acceptable rate. Much is still to be done. 10

11 References Aukema, BH. and Raffa, K Does aggregation benefit bark beetles by diluting predation? Links between a group-colonization strategy and the absence of emergent multiple predator effects. Ecological Entomology. 29: Berryman, AA, Raffa KF, Millstein, JA and Stenseth, NC Interaction Dynamics of Bark Beetle Aggregation and Conifer Defense Rates. Oikos. 56, No 2: Berryman, AA, Raffa KF, Millstein, JA and Stenseth, NC.1985.Evolution of optimal group attack, with particular reference to bark beetles ( Coleoptera, Scolytidae). Ecology. 66 (3): Cole, W.E., Amman, G.D. and Jensen, C Mountain Pine beetle Dynamics in Lodgepole Pine Forests Part III: Sampling and Modeling of Mountain Pine Beetle Populations.USDA Forest Service General Technical Report INT-188: Hynum, BG and Berryman, AA Dendroctonus ponderosae Coleoptera:Scolytidae: Pre-aggregation Landing and Gallery Initiation on Lodgepole Pine. Canadian Entomologist. 112: Hynum, BG Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae) Gallery initiation on lodgepole pine during aggregation. Environmental Entomology. 10(6): Negron, J, Popp, JB Probabiltiy of ponderosa pine infestation by mountain pine beetle in the Colorado Front Range. Forest Ecology and Mangement. 191: no Preisler, H.K., Mitchell, R.G Colonization patterns of the mountain pine beetle in thinned and unthinned lodgepole pine stands. Forest Science. 39 (3): Pureswaran, DS, Sullivan BT and Ayres MP Fitness consequences of pheromone production and host selection strategies in a tree-killing bark beetle (Coleoptera: Curculionidae: Scolytidae). Oecologia. 148 (4): Raffa, KF, Berryman, AA A mechanistic computer model of mountain pine beetle populations interacting with lodgepole pine stands and its implications for forest managers. Forest Science. 32(3): Raffa, KF Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems. Chemoecology. 11(2): Reeve JD Predation and bark beetle dynamics. Oecologia. 112(1):

12 Schmid, J.M, Mata, S.A and Schmidt, R.A Bark Temperature patterns in ponderosa pine stands and their possible effects on mountain pine-beetle behavior. Canadian Journal of Forest Research. 21: Wallin, KF and Raffa, KF Density-mediated responses of bark beetles to host allelochemicals: a link between individual behavior and population dynamics. Ecological Entomology 27: