Bark Beetle Mass Attack Literature Review

Transcription

1 Bark Beetle Mass Attack Literature Review Tara Costanzo 19 April 2011 Insect Behavior BSPM 507 Prof. Lou Bjostad Abstract A bark beetle s life cycle begins with hatching from an egg and ends with migration to and colonization of a new host tree. The absence of pheromones provides the pioneer beetles (first attackers) with the advantage of dispersing into the forest and spreading randomly. The four stages in the sequence of bark beetle mass attacking behavior are dispersal, selection, concentration and establishment. Bark beetles (Coleoptera: Curculionidae: Scolytinae) are relatively host specific pests of trees. However, the beetles can and will deviate to non- host species in unusual circumstances. Large populations are capable of overcoming tree defenses by means of a synchronous attack of multiple individual beetles; this density is regulated by the production of aggregation and anti- aggregation pheromones. An in- depth discussion of the mass attack phases, bark beetles, and pheromones and tree defenses are introduced, as well as modeling technologies provided as references for developing management plans. Spatial modeling in recent studies has provided simulation outputs in order to define analysis and predictions of spatial patterns to assist the improvement of controlling bark beetle disturbances. Concluding this review are suggestions for management techniques and methods. Introduction Bark beetles are one of the most common disturbance agents in pine forest ecosystems. They utilize a complex chemical based communication system to locate host 1

2 trees, which provide a place for food, mating, and reproducing. During these processes the beetles have evolved to secrete a pheromone that results in population aggregation. According to Wood (1982), population aggregation must be both timely and of sufficient magnitude to exploit the new resource. The phases of bark beetle mass attack, colonization and tree defenses, and aggregation and anti- aggregation pheromones will be discussed as a means to understanding non- social insect interactions and behaviors, concentrating on bark beetles that attack coniferous trees. A single bark beetle cannot overcome a tree s defense mechanisms and therefore when a first attacker or pioneer beetle finds a suitable host, the process of burrowing into the bark produces aggregation pheromones, which attract mates and other beetles to colonize a tree. There are two objectives of these pheromones: (1) to induce colonization and (2) to terminate the colonization process once the threshold of attack is reached. Soon after colonization is established and the tree s defenses are overcome, brood development commences and ultimately tree mortality is reached. As a forester, management is necessary for forest structure, predicting how the beetles will spread is unknown but recent studies have attempted to resolve this issue. In an effort to plan for future management strategies, new studies in spatial patterns will aid in an increased understanding of beetle dispersal. Discussion Phases of Mass Attack A bark beetle s life cycle begins with hatching from an egg and ends with migration to and colonization of a new host tree. The four phases of mass attack are dispersal, selection, concentration, and establishment (Wood 1982). Some articles list mass attack as 2

3 having three phases where dispersal and selection are combined into one phase. For the purposes of this article, it will be discussed as four phases. The first stage of a mass attack is dispersal (Wood 1982). Female beetles tend to emerge before male beetles. The absence of pheromones provides the pioneer beetles with the advantage of dispersing into the forest and spread randomly (Perez and Dragicevic 2011). Little is known about this phase due to the extreme difficulty of experimenting and observing beetles in flight, a better understanding is necessary. However, it is known that mortality plays a key role during this phase (Wood 1982). The next phase is selection. It is either a random or direct process when landing on host or non- host trees. Zhang and Schlyter (2004) suggest that natural selection has caused bark beetles to evolve several olfactory mechanisms to aid in finding their hosts. Zhang and Schlyter (2004) also suggest that non- host leaf and bark volatiles are important negative olfactory signals used by bark beetles in host detection and selection. The insects are equipped with multiple sensory receptors for visual, mechanical, gustatory, and olfactory stimuli and cues (Stadler 1976, Zhang and Schlyter 2004). Non- host volatiles (non- host stimuli, such as odors) and unsuitable host signals provide protective qualities for host trees in mixed forest cover types as well as providing potential use in protecting other trees from attack. Feeding or boring initiate pheromone production and thus commences the concentration phase. Concentration is characterized by a mass attack of thousands of beetles on a living tree over a period of time (Coster et al., 1977, Bunt 1980). Fixed action pattern sequences were studied and observed in Ips beetles by Paynter et al. (1990). The sequence of behaviors began with landing on the bark of a host tree, followed by searching for a suitable boring site, then inspection of a crevice under bark scales during the 3

4 investigation of the bark. At this point, the pioneer attacker stops and either flies away or continues with the search. Next a push response occurred when there was an interaction between males and females at an entrance. This interaction generally involved a female butting up against a male at the entrance site and then inspecting the hole. Touching often followed the inspection, although contact with another beetle usually turned out to be accidental. Ultimately the establishment phase occurs and mycangia play a role in causing death, by producing population- aggregation pheromones (Wood 1982). Mycangia are the pockets, on the surface of the beetle s body, that contain pathogenic fungi. These fungi essentially clog tree cells blocking essential nutrient and water translocation. During this phase brood production and development is dependent on the population- aggregating pheromones and vital to the success of the species, and mortality of the host tree ensues. Bark Beetles Bark beetles, recently sub- classified within Coleoptera: Curculionidae: Scolytinae, are relatively host- specific and act as individuals (Pureswaran et al., 2006). They are not social insects, like ants, wasps, termites or bees; however, bark beetles must act as a group in order to overcome a tree s defense response and the ability to aggregate is essential. Pioneer beetles are the first to arrive and initiate a mass attack. Swarm intelligence is a theory based on vertebrate behaviors and biological examples of swarming, flocking, and herding. A group of agents that communicate with each other by acting on their local environment is defined as a swarm (Hoffmeyer 1995, Perez and Dragicevic 2011). In the Perez and Dragicevic (2011) study, swarm intelligence is used as a means to approach and simulate the decision- making and aggregation behavior of mountain pine 4

5 beetles. Social insects benefit from the cooperative efforts of the colony and many different algorithms have been derived from these interactions. Swarm intelligence approach is used to study non- social insects and behavioral aspects of attack aggregation. Swarms of non- social insects differ from social insect characteristics. Social insects are characterized by overlapping generations, cooperative care, and reproductive division of labor, which are not characteristics of bark beetles. However, the purpose of aggregating in bark beetles is for several reasons. In this case, a large group of beetles is more efficient and effective in colonizing a host tree and overcoming the tree s defenses (Perez and Dragicevic 2011). Pioneer beetles are guided to host trees by visual response to dark silhouettes followed by the detection of the correct feeding stimulus (Perez and Dragicevic 2011). When pioneer beetles arrive at a tree they produce an aggregation pheromone to attract more beetles to the tree. There must be a sufficient amount of this pheromone produced to attract enough beetles in order to kill a tree. When the mass attack is under way, an anti- aggregation pheromone terminates the attack in order to prevent overcrowding. Overcrowding can be detrimental to a colony, causing competition for resources for larvae (Raffa and Berryman 1983, Raffa and Berryman 1987, Pureswaran 2006). This is known as intraspecific competition. Dendroctonus or tree killer species share the ability to kill healthy trees. When population numbers are high and at epidemic levels, beetle colonization can occur and overwhelm a healthy tree s defense mechanisms. The strain on defenses arises when resin flows are drained and pathogenic fungi are introduced. However if numbers are low then the tree s resin flow may overcome the beetles and the attackers are killed in the process. Alternately, if there are too many beetles attacking a tree intraspecific competition will 5

6 result in a reduction in the number of offspring per female (Anderbrant 1985, Paynter 1990). Bark beetles develop in a fairly protected environment. Under the bark, beetles are protected from most predation and climate and they use the wood as a food source, which is unpalatable to most other organisms. However, this habitat is temporary because after one generation a host tree s nutritional value is exhausted and adults must find new hosts (Raffa and Berryman 1983). Pheromones Bark beetles are able to colonize healthy trees and the aggregation pheromones produced attract both males and females. These pheromones are necessary to lure enough beetles to the tree to overcome the tree s defenses (Aukema and Raffa 2004). The pheromone systems involved in the aggregation of bark beetles has been the subject of intense investigation with primary emphasis on identification, biological activity, and biosynthesis of various chemical components (Raffa and Berryman 1983). Pureswaran and Borden (2003) propose that after pairing the males take over the role of pheromone production. This appears to be an important factor in intraspecific competition and resource partitioning, population regulation and reproductive success. Pureswaran and Borden (2003) provide a more in- depth understanding of the aggregation and anti- aggregation pheromone production by male and female beetles. For the Dendroctonus species, there are five different pheromones responsible for mediating initiation and termination of aggregation on host trees (Figure 1). This was also found to be true in D. valens. There were five volatile compounds identified from extracts of 6

7 both sexes hindguts. These compounds are cis- and trans- verbenol, myrtenal, mertenol, and verbenone (Shi and Sun 2010). Figure 1. According to the Pheromone of Scolyti(n)ae table 1, there are 12 genera and 27 species of bark beetle accounted for that attack coniferous trees and the different pheromones produced by males and females of the different species (Wood 1982). When females burrow into the bark, oxidation of the host monoterpenes α- pinene produces the aggregation pheromone trans- verbenol, which is the male attractant. After males arrive, they release exo- bervicomin (a female attractant at low levels). Mass- attack is launched when host monoterpenes like α- pinene and mycrene synergize aggregation pheromones. Male beetles produce the pheromones that regulate attack densities. These are exo- brevicomin and frontalin. Both sexes can produce an attack terminating pheromone called verbenone. 2- Phenylalanine is produced by males and is attributed to have anti- aggregation properties in traps and on trees baited with aggregation 7

8 pheromones. These compounds signal the unavailability of that host and cause the attack to switch to other neighboring trees (Pureswaran and Borden 2003). Bark beetles use defense secretions as precursors and synergists for their aggregation pheromones (Raffa and Berryman1983), whereby they maximize response from other flying beetles. This process also ensures that when a tree is made suitable for brood production that the attack will terminate, thereby reducing the chances of intraspecific competition (Raffa and Berryman 1983). However, inhibitory pheromones are only effective within the specific tree rather than among neighboring trees (Perez and Dragicevic 2011). Colonization and Tree Defenses The substrate for most bark beetles consists of recently killed trees, but some are able to colonize living trees. Ecological classification of the Scoliti(n)ae have emphasized either (a) location of colonization or (b) the condition of the host as determined by physiological parameters, according to Wood (1982). Tree strength and defenses dictate the number of beetles for successful infestation; when numerous individual beetles deploy pheromone- mediated synchronous attacks, overcoming a healthy tree s defenses can be accomplished (Perez and Dragicevic 2011). Trees that are successfully colonized are always attacked by large numbers of beetles where as trees that were unsuccessfully attacked contain few beetles (Raffa and Berryman 1983). In a study by Latty and Reid (2010) it was noted that first attackers (pioneers) on live trees suffer higher mortality rates than beetles that join aggregations. Successfully entering a tree and body condition are positively correlated. Beetles that are in poor condition (starved beetles for the study) were noted to be in a state of desperation and 8

9 immediately began tunneling construction (within 2 hours) where as beetles in better condition (non- starved beetles in the study) took longer to initiate tunneling (34.7 hours), suggesting that pioneering is a nonlinear behavioral response i.e. beetles with the highest and lowest energy stores are both likely to avoid pioneering. It is also noted in this study that pioneering was more likely to occur in favorable conditions with smaller diameter trees, earlier in the season, and when the environment favored success. These findings suggest that internal and external variables are incorporated in decision making to attack an uncolonized tree (Latty and Reid 2010). A tree s defense system is adequate when attack densities are low. The tree s response is to confine beetles and associated fungi with toxic and inhibitory compounds within necrotic lesions found on the bark surface. However, surpassing this threshold of attack, (~40 galleries/m^2) can exhaust a tree s defenses and mortality may occur. The concentrated attack is directly related to the depletion of host s defenses and reproductive success. The bark beetles depend on the mortality of the tree for reproductive success. However, beyond the threshold of attack, at >80 galleries/m^2, density numbers may suppress brood development. An increase in attack density may cause an exponential decline in brood production due to intraspecific competition. In the study, an optimal density for reproduction and survival is ~62 galleries/m^2 (Raffa and Berryman1983). Conifer trees are able to interfere with the communication efforts of the bark beetles. As Raffa and Berryman (1983) point out, Pheromone synthesis within the hindgut and emission from the entrance site are distinct events, and beetles contained within a copious flow of resin seem unable to elicit attraction. This interruption of communication is directly related to the number of pioneer beetles during the initial attack. In addition to the pheromones, bark beetles disseminate an associated pathogenic fungi while boring into 9

10 the selected host trees. These fungi quickly invade ray cells, causing desiccation and disruption of transpiration processes. After the phloem and xylem tissues are killed by the fungi, the tree has no defense mechanisms to impair brood development (Raffa and Berryman 1983). Computer Based Modeling In an effort to aid in management planning, computer based models and algorithms have provided information: from this modeling endeavor, we have observed that event starting with a completely homogenous environment, the positive and negative feedback generated by attacking beetles soon results in a rich, spatially dependent chemical landscape that tends to modify future events Powell et al. (1998). The dispersal from and aggregation on a host provide an overriding importance in bark beetle ecology that model representations provide essential ecological credibility. The spatial and quantitative interpretations of the ecosystem and bark beetle interaction models play a key role in studying the least understood, dispersal stage of the mass attack sequence (Powell et al., 1998). Spatial modeling in recent studies has provided simulation outputs in order to define analysis and predictions of spatial patterns to assist the improvement of controlling bark beetle disturbances. As well as provide methods for prevention of these disturbances. The aggregation behavior of non- social insects can be simulated in mathematical models to describe the decision making of swarms in the forest ecosystem. Perez and Dragicevic (2011) offer a swarming intelligence and agent based modeling program called ForestSimMPB for mountain pine beetles specifically. This program integrates agent based modeling and geographic information systems utilizing swarming intelligence 10

11 mathematical algorithms to evaluate, scenarios of the forest landscape structure including changes, to showcase the spatial reasoning and interactions of bark beetles, and to characterize the extent and spatial patterns of beetle outbreaks (Perez and Dragicevic 2011). Management Options Ultimately mortality ensues and mass attacked trees die due to the combined efforts of tunneling adult beetles, pathogenic fungi, and feeding larvae, when translocation (of essential nutrients and water) and transpiration functions are disrupted. As a beetle progresses through a tree s tissues, it releases aggregation pheromones, which attract potential mates and other colonizers. In the Perez and Dragicevic (2010) study simulation of spatial patterns of beetle outbreaks utilizing a swarm intelligence algorithm, the complexities of aggregation behaviors can be analyzed and these models can serve as reference tools for management strategies. This program is based on mountain pine beetle (Dendroctonus ponderosae Hopkins) interactions. It effectively simulates infestations that mimic aggregation behavior allowing the complexities of infestations to be analyzed. The ForestSimMPB program integrates agent based modeling and swarming intelligence algorithms and GIS to determine emergent spatial patterns of infested forested lands. The study sites were located in British Columbia, Canada, and explore space- time dynamics and visualizing forest environment and structure changes and emphasize ecological processes where individual agents interact. These interactions include communication, organization, and micro- population behavior patterns (Perez and Dragicevic 2010). 11

12 Current management techniques include sanitation cuts and large clear- cut areas. Alternative management possibilities include the use of anti- aggregation pheromones in pre- harvest pheromone baiting applications (Borden et al., 1993, Huber and Borden 2001). Huber and Borden (2001) offers that application of a systemic arsenical, monosodium methanearsonate to kill parent and brood beetles (Maclauchlan et al., 1988) and the proposed pheromone verbenone to prevent attack on susceptible trees, (Amman et al., 1989; Lindgren et al., 1989) may be appropriate pre- harvest treatments. Verbenone is not always effective in repelling beetles. For example, when it is exposed to ultraviolet light, it is photoisomerized to an inactive form, chrysanthenone (Huber 2001). Conclusion Although many of the articles discussed in this review are based on western studies of coniferous forests, it is important to realize that bark beetles are neither just a western issue nor do they only affect coniferous forests. Since the late 1990 s, bark beetles, namely Dendroctonus valens (red turpentine beetle), have been destroyed millions of forested acres in China (Shi and Sun 2010). Bark beetles, like Agrilus planipennis Fairmaire (emerald ash borer) and Pityopthorus juglandis Blackman (walnut twig beetle), affect many broadleaf tree species as well, such as Fraxinus species (true ash) and Juglans species (walnut) causing multi- million dollar losses in commercial industries of lumber, fruit and nut production. Bark beetles are a major pest of forests and one of the most common disturbance agents in pine forest ecosystems. New technology and simulation models of spatial patterns of beetle outbreaks utilizing a swarm intelligence algorithm, aid in the determination of the complexities of aggregation behaviors can be analyzed and these models can serve as 12

13 reference tools for management strategies. By introducing a program like ForestSimMPB to forest managers, simulation models that are produced can provide a means to recognizing dispersal patterns with the hope to identify and manage areas that are not infested. With new developments like ForestSimMPB, dispersal can be studied more in- depth. This new program can be useful in the improvement of prevention and control of bark beetle disturbances. At the current rate of which bark beetles, namely mountain pine beetle, have affected the pine forests in the western United States and Canada, it is important to consider the complexities of infestations. Understanding and learning the spatiotemporal behaviors and spatial distribution patterns in which these pests spread is essential for land managers, especially when it pertains to determining management approaches for prevention and control of these major pests. It is imperative for natural resource managers and land stewards that management strategies are developed in an effort to prevent and control devastating disasters like wildfires that can destroy the environment, interfering with possible regeneration sites for the new generation of forests. Regeneration is a major concern for recreationists and land managers alike. With issues like climate change, carbon sinks and losses regeneration is essential for the reduction of carbon emissions. The carbon cycle can only remain neutral if regeneration occurs to store carbon. Current funding for land management is mostly allocated to firefighting. However, if these new programs can successfully predict spatial patterns of dispersal, then funding can be re- allocated to proactive prevention and disaster preparedness efforts rather than spending tax dollars on after- the- fact disaster clean- up efforts that could have been prevented. Also, by incorporating these new programs within agency offices, new jobs may 13

14 be generated to manage the program analyses and ultimately boost the current job market and economy. WORKS CITED/REFERENCES Anderbrant, O., Schlyter, F., and Birgersson, G Intraspecific competition affecting parents and offspring in the bark beetle Ips typographus. Oikos. 45: Aukema, B., and Raffa, K "Gender- and sequence- dependent predation within group colonizers of defended plants: a constraint on cheating among bark beetles?." Oecologia 138.2: Borden, J., Ryker, L., Chong, L., Pierce Jr., H., Johnston, B., and Oehlschlager, A Response of the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae), to five semiochemicals in British Columbia lodegepole pine forests. Canadian Journal of Forest Research 17: Bunt, W., Coster, J., and Johnson, P "Behavior of the Southern Pine- Beetle (Coleoptera, Scolytidae) on the bark of host trees during mass attack." Annals of the Entomological Society of America 73.6: Coster, J., Payne, T., Hart, E., and Edson, L Aggression of the southern pine beetle in response to attractive host trees. Environ. Entomol. 6: Hoffmeyer, J The swarming body. In: Rauch, I., Carr, G. (Eds.). Proceedings of the Fifth Congress of the International Association for Semiotic Studies. IASS, Berkley, California Huber, D., and Borden, J "Protection of lodgepole pines from mass attack by mountain pine beetle, Dendroctonus ponderosae, with nonhost angiosperm volatiles and verbenone." Entomologia experimentalis et applicata. 99.2: Latty, T., and Reid, M "Who goes first? Condition and danger dependent pioneering in a group- living bark beetle (Dendroctonus ponderosae)." Behavioral Ecology and Sociobiology 64.4: Paynter, Q., Anderbrant, O., and Schlyter, F "Behavior of male and female spruce bark beetles, Ips typographus, on the bark of host trees during mass attack." Journal of Insect Behavior 3.4: Perez, L., and Dragicevic, S "ForestSimMPB: A swarming intelligence and agent- based modeling approach for mountain pine beetle outbreaks." Ecological Informatics 6.1:

15 Powell, J., Tams, J., Bentz, B., and Logan, J "Theoretical analysis of "switching" in a localized model for mountain pine beetle mass attack." Journal of Theoretical Biology : Pureswaran, D., and Borden, J "Is bigger better? Size and pheromone production in the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera : Scolytidae)." Journal of Insect Behavior 16.6: Pureswaran, D., Sullivan, B., and Ayres, M "Fitness consequences of pheromone production and host selection strategies in a tree- killing bark beetle (Coleoptera : Curculionidae : Scolytinae)." Oecologia 148.4: Raffa, K. F. and Berryman, A. A The role of host plant- resistance in the colonization behavior and ecology of bark beetles (Coleoptera, Scolytidae). Ecological Monographs 53.1: Raffa, K., and Berryman, A Interacting selective pressures in conifer- bark beetle systems: a basis for reciprocal adaptations? Am. Nat. 129: Shi, Z., and Sun, J "Quantitative variation and biosynthesis of hindgut volatiles associated with the red turpentine beetle, Dendroctonus valens LeConte, at different attack phases." Bulletin of Entomological Research : Stadler, E Sensory aspects of insect plant interactions. Proceedings of XV International Congress of Entomology. Washington, D.C Wood, D "The role of pheromones, kairomones, and allomones in the host selection and colonization behavior of bark beetles." Annual Review of Entomology 27: Zhang, Q., and Schlyter, F "Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer- inhabiting bark beetles." Agricultural and Forest Entomology 6.1: