Pristiphora abietina (lesser spruce sawfly)

Silvicultural Control

Tree species are alternated and pure spruce plantations should not be established on risky sites (Netherer and Führer, 1999). If they are already there, the stands should be converted to another tree composition with a maximum of 30% spruce and a minimum of 70% of other tree species (Wiener, 1995).

Amelioration of a Site

Measures are taken to decompose the raw humus layer by high-thinning of the stand. This raises the pH-value of the soil, regenerates a sufficient nutrient supply, deteriorates the conditions for the sawfly and improves the trees' resistance against attack (Büttner, 1956; Merker and Niechziol, 1957; Niechziol, 1958; Merker, 1962).

Genotype Selection of Trees

Oviposition usually coincides with mid- and later flushing spruce genotypes (Ohnesorge, 1957). However, by growing early flushing trees the stands will only be exposed to a late spring frost.

Chemical Control

This requires a precise forecast of damage in the following spring. Prognosis is carried out in the late autumn or early spring prior to pupation, when the pupal eyes of the pronymphs are visible and maximum cocoon mortality has occurred. The litter and humus horizon of 16 test plots (32 if possible) per stand (each plot is 25 cm x 25 cm), situated in the crown projections of as many trees as possible, are stripped off. Each sample is quantitatively searched for live cocoons. The number of live cocoons, the percentage of live female cocoons (larger than 5.8 mm) and the rate of pronymphs are counted. The critical number (the number of females ready for hatching per m²) varies according to the age class of the spruce stand. Stands that are 40- to 60-years-old have a critical number of 50 to 60 females per m²; stands that are 15- to 25-years-old have a critical number of 15 to 25 females per m²; and younger stands have a critical number of 0.5 to 2 females per m².

In the past, chlorinated hydrocarbons and organophosphorous compounds were used to control defoliators. At present, pyrethroids (e.g. deltamethrin) and chitin-blockers (e.g. diflubenzuron) are used to control P. abietina. As the treatment must coincide with early larval stages, the exact timing of application is necessary. This means that sometimes a second treatment is unavoidable, if oviposition is delayed by unfavourable weather conditions. Non-target organisms are usually spared with the application of chitin-blockers. Insecticide treatments can be performed regardless of birds because they disregard the larvae of P. abietina. The insecticide must hit the upper part of the crowns, therefore application by aircraft is necessary in older stands.

Biological Control

The use of micro-organisms has not been successful. The only organisms used against P. abietina are in the Formica rufa group, which predate on the larvae (Egger, 1989). However, problems with the durability of settling Formica nests within spruce stands make this method questionable and gives it only limited success.