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In spring, egg clusters of D. pini can be detected on the previous year's needles; during mid-summer, eggs may eventually also be detected on needles of the current year. Eggs are laid in rows on adjacent needles and are covered by a pale-yellow spumous coating (see Pictures). Young foliage of Scots pine is toxic to larvae of D. pini (Geri, 1988). Oviposition and initial feeding usually takes place on older needles and the needles of the current year are untouched until late summer. Young larvae exhibit a partial feeding on the needles, leaving the vascular bundles behind. Older larvae consume the entire needles, so that only the sheath is left over. At first, needles close to the oviposition sites are consumed. Later, larvae move to adjacent branches for feeding. Larvae live in large colonies (60 to 100 larvae), which can split into several groups during development (see Pictures). Their faeces have a characteristic rhombic shape and are green. The ovoid brown cocoons with the pupating larvae can be found on twigs or on overstorey vegetation during summer and in the forest litter close to the tree trunk during autumn and winter.
Monitoring and Action Thresholds
Pine sawfly populations can be monitored by visual inspection of pine stands for frass damage, by sampling of egg clusters from cut pine twigs or by collection of cocoons from vegetation or forest litter. In most areas, where outbreaks of D. pini frequently occurred in the past (Germany, France, Finland, Poland), the search for hibernating pine sawfly cocoons in the forest litter is included in routine surveys of pine pests during winter ('winter prognosis'; Schwerdtfeger, 1981; Feemers, 1997). Sticky traps, placed under the canopy of pine trees, can also be used for collecting faecal pellets (Simandl, 1989). Since the identification of the sex pheromone (Bergström et al., 1995), efforts were made to use pheromone traps for monitoring of pine sawflies (Anderbrant, 1993, 1998). Pheromone-baited traps proved to be a reliable tool for detection of the different flight waves of D. pini, even at very low populations, and the capture rates of males reflected results from the routine winter surveys (Herz et al., 2000).
The design for the winter cocoon sampling varies from country to country. In Germany for instance, a particular area (1 to 5 m²) of forest litter extending from the trunk of a pine tree is examined in susceptible forest stands. Collected cocoons are checked for vitality, parasitism, development stage, sex etc. The threshold for further surveillance is 20 vital cocoons/m² or six vital females/m², ready for emergence. Further measures include sampling of cocoons and eggs in early spring and visual inspection of pine stands for larval colonies. In the case of more than one larval colony/tree, control measures may be necessary (Feemers, 1997).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Outbreaks of D. pini were observed in all parts of Europe (Escherich, 1942; Pschorn-Walcher, 1982; Geri, 1988; Sharov, 1993), occurring at irregular intervals (Kangas, 1963; Klimitzek, 1990). Defoliation by D. pini can reduce increment and timber yield of Scots pine, but usually does not cause the death of the tree. However, after heavy infestation, the weakened tree may be more susceptible to subsequent attack by bark beetles, buprestids and pine weevils and be eventually killed (Annila et al., 1999). Repeated defoliation during consecutive years of an outbreak may also kill the pines (Langstrom et al., 2001). During outbreaks in East Germany and the Netherlands, a tree mortality of 14 and 24%, respectively, was recorded after a heavy defoliation of mature pine forests. Loss in increments ranged from 16% at a moderate defoliation of 20%, to 63% at a nearly complete defoliation of 85% (Pschorn-Walcher, 1982). In a detailed study on a local outbreak in western Finland, heavy defoliation reduced the volume growth by 94% and caused a tree mortality of 30%. The average economic value of the losses due to reduced growth and tree mortality was estimated as US$310 per hectare (Lyytikainen-Saarenmaa and Tomppo, 2002).
One difficulty in surveillance of D. pini is the eruptive outbreak pattern of this species with a rapid population build-up after long periods of latency (Geri, 1988; Sharov, 1993). It is assumed that the infestation spreads from epicentres into the surroundings (Geri, 1988). The spatial extension of an outbreak is therefore favoured by the presence of large areas of pure Scots pine forests, as they occur in the lowlands of Central and Northern Europe. Such serious extended outbreaks, covering several tens to hundreds of thousands of hectares, occurred between 1990 and 1995 in Poland and the eastern part of Germany (Majunke et al., 1992).