Tomicus minor (lesser pine shoot beetle)

The typical egg galleries under the bark of pine logs or trees are the best indication of the presence of T. minor.

As pine shoot beetles, and T. minor in particular, are dependent on a continuous supply of suitable host material for their survival, the focus in control should be on preventive forest protection measures, often referred to as 'forest hygiene' in the older literature. Silvicultural and logging practices should be organized in such a way as to minimise the availability of brood material during the spring flight. There are no specific control measures used against T. minor that differ from the general control measures against pine shoot beetles, and are described for T. piniperda. For T. minor, the main focus should be on preventing damage to sawlogs by timely transportation and debarking, storage under water or, as the last resort when none of the previous options are available, spraying with insecticides. It is noteworthy that T. minor mainly breeds in storm-felled trees (Långström, 1984), seldom occurs in log piles (Långström et al., 1984) and never attacks stumps (Hellqvist, 1984). Less attention therefore needs to be paid to preventive forest protection measures for T. minor when planning silvicultural and logging operations than for T. piniperda.

In principle, the economic impact of T minor is threefold: firstly, there may be growth losses caused by extensive shoot-feeding in the pine crowns; secondly, tree mortality may be caused by stem attacks; and thirdly, deterioration of timber quality may occur as a result of beetle-vectored blue-staining of sawlogs and pulpwood.There are no studies dealing with growth losses due to shoot feeding by T. minor alone. However, Greese (1926) described a case where pine trees lost, on average, 215 shoots due to T. minor shoot damage, which should be enough to result in measurable growth loss (cf. data given for T. piniperda). Little is known about the productivity of T. minor during the breeding phase. It mainly attacks storm-felled trees and logging waste of mature trees after clear felling, but attack densities vary considerably between sites (Annila and Petäistö, 1978; Långström, 1984). Attack density is often similar to that of T. piniperda but brood production is more variable and under favourable conditions (the underside of large storm-felled trees) exceeds 5000 beetles/m² and 10,000 beetles per tree (Långström, 1983b, 1984). Thus, large numbers of beetles may be produced in wind-throws or clear felling (more than 400,000 per ha in one case; Lekander, 1974) and surrounding trees are therefore in danger of shoot damage and subsequent growth losses. As many fallen trees escape attack by T. minor (due to lack of beetles or residual resistance via root contacts), damage patterns are difficult to generalize. T. minor was originally considered more aggressive than T. piniperda (Escherich, 1923), but later studies in Fennoscandia showed that it was seldom, if ever, found alone on 'beetle-killed' trees (Långström and Hellqvist, 1993; Annila et al., 1999). Furthermore, the blue-stain fungus Ophiostoma canum, which is the main fungal associate of T. minor, was clearly less aggressive than the fungi associated with T. piniperda (Solheim et al., 2001). This finding supports the idea that T. minor lacks the capacity to overcome the resistance of live trees. A similar situation seems to occur in Yunnan, China, where the two Tomicus species co-exist, but T. piniperda is the principal tree killer (Ye and Ding, 1999; Långström et al., 2002).Thus, the economic importance of T. minor mainly relates to the third type of damage, i.e. the transmission of blue-stain into the pine timber. The presence of any blue-staining in conifer sawlogs reduces the value to a fraction (ca 20%) of the price of prime timber on the market. On the other hand, T. minor seldom attacks the most valuable butt logs, which somewhat reduces its importance as a timber pest. No estimates of the economic losses due to blue-staining caused by T. minor are available, but there is no doubt that these would be considerable if not properly counteracted. At least in Fennoscandia, saw mills have developed timber handling practices that minimise insect damage. Although some blue-staining is acceptable in pulpwood destined for sulphate processing, there is a cost connected with the processing of blue-stained timber, as more chemicals are needed for bleaching (Löyttyniemi et al., 1978).