How Bark Beetles Avoid Interference with Squatters: An ESS for Colonization by Ips typographus

Bark beetles aggregate on suitable host trees and their larvae incur severe competition for food from larvae in neighbouring galleries. To maximize their fitness parental bark beetles should therefore balance the bark area claimed for their offspring and the probability of being competitively interfered by squatters. It was shown that the unbeatable colonization strategy is to position the gallery relative to a neighbour so as to prevent newcomers from excavating a gallery in between, provided that the probability of interference by squatters exceeds 0.8. A test of the theory was carried out for the case of Ips typographus attacking spruce trees. It was found that the mean (and the modal) gallery distance was close to the critical threshold distance above which a squatter would be able to raise at least one offspring with sufficient energy reserves to find a suitable host.