The Chemical ecology of Ips sexdentatua (Boerner) management

Resumo

Favoured by the occasional abundance of breeding substrate, populations of the otherwise secondary European six-toothed pine bark beetle, Ips sexdentatus Boerner (Coleoptera: Curculionidae, Scolytinae), frequently reach outbreak levels, seriously threatening pine stands across Southern Europe. In addition, I. sexdentatus has been shown to be able to carry phytopathogenic fungi, further contributing to their dispersion. Traditional management methods, consisting in the early detection and removal of colonized breeding material and the use of trap trees, together with the application of pesticides, are nowadays complemented or replaced with tools and strategies based on the chemical signals that bark beetles use. While broad-spectrum insecticides used in pest management have many undesirable effects in forest organisms, management through infochemicals can be more specific and definitely cleaner. In this respect, along with the marketing and application of the first specific I. sexdentatus pheromones in Spain, effectiveness of lured traps over traditional trap trees has been recently demonstrated. Still, the applicability of the infochemicals involved in I. sexdentatus for its management through different strategies, and the chemical ecology background behind it has been seldom researched or developed. Within the present thesis, three central topics in bark beetle management by the use of infochemicals have been studied. First, a set of experiments were performed in order to study the possibilities to improve the blend used in pheromone trapping programs for I.¿sexdentatus, while the responses of associated beetles were taken into account. Given that many natural enemies [i. e. Thanasimus formicarius L. (Col.: Cleridae) and Temnochila caerulea Olivier (Col.: Trogossitidae)] eavesdrop on I. sexdentatus pheromonal compounds, and large numbers are consequently by-caught during trapping programs; a second group of experiments tested for trap design modifications that could further improve the specificity of pheromone trapping. During the last part of the thesis, field experiments studying verbenone, one of the better-studied inhibitory compounds for bark beetles, in relation to I. sexdentatus are presented. In addition, and given the inconsistencies detected in the practical implementation of verbenone elsewhere, experimental results from around the globe were retrieved and reviewed quantitatively through meta-analysis. The aggregation power of ipsdienol, the main pheromonal compound of I. sexdentatus, was greatly enhanced when released together with ipsenol at the appropriate ratio. The addition of cis-verbenol helped establishing an improved reference blend. On the other hand, although significant differences could not be detected, a large negative effect was reported for 2-methyl-3-buten-2-ol, while myrtenol had no apparent effect on the aggregation. A deeper insight into the relation between ipsdienol and ipsenol, helped establish an optimal blend, linking it with the natural evolution in the ratio between these compounds through the host colonization sequence. A clear varying response of associated trophic guilds and species assemblage could be reported, depicting the process of habitat foundation by bark beetles. Furthermore, the response peak of I. sexdentatus could be clearly distinguished from that of competitor and predatory guilds. Survival and viability rates of T. formicarius and T. caerulea individuals captured in pheromone traps rapidly decreased after one week in trap containers. Among the tested trap designs, a modified multiple funnel trap incorporating a welded-wire mesh screen and lateral escape windows provided the most promising results. Although conventional slot traps provided a good relationship between the number of I. sexdentatus and T. formicarius captured, T. caerulea individuals were retained, indicating that, in areas where the trogossitid is present, a large number of individuals would be removed from the forest resulting in an expected high impact on their population. Verbenone consistently inhibited the response of I. sexdentatus. Low and high release rates provided ca. 80% reductions in the number of captured beetles in traps baited with ipsdienol, and the number of mass attacked trees also was reduced in an experiment that compared the attack level between pheromone baited trees with and without a device releasing verbenone at a high rate. When tested on log piles, verbenone release rates above 10 mg/day prevented the colonization of I. sexdentatus, but colonization by the associated bark beetle Orthotomicus erosus Wollaston could not be avoided. Results of the meta-analyses on 529 studies dealing with verbenone yielded in an overall large effect on bark beetle response, supporting the hypothesis that it may be signalling the aging condition of hosts. Although significant differences exist between trapping and stand experiments, the effect is still large enough for practical application in the management of bark beetles, both under endemic or epidemic populations.