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Singly infested trees have brown needles in May/June, or the shoots at the top and on sunny parts of the crown are defoliated. Repeated infestations cause dieback of the leaders and a bushy deformation of the crown.
The widespread establishment of spruce plantations in risky sites has led to several control trials of P. abietina. The short-term control effects are achieved by the use of insecticides, but these effects are often insufficient for a pest that recovers rapidly. The long-term effects of biological control, i.e. settlement of Formica rufa, were not really successful. Ant colonies need a lot of light to persist for many years and an adequate prey supply besides P. abietina larvae, and these conditions are difficult to maintain in pure spruce stands. The best long-term effects were made with the following methods.
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.
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.
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.
Short incidences of outbreaks can be tolerated from the point of view of forestry because they only cause low increment losses. However, single heavy infestation can be critical for plantations of Christmas trees. The damage caused in longer epidemics results in considerable deformations of the tree crowns causing a bushy shape and/or long, spear-formed leaders without ramification. The height growth is reduced to zero after some years. Average height increment losses from 1917 to 1934 amounted to 33-40% in the Swiss epidemic area. Losses of volume increment increased to 60-68% within 10 years in the Swiss area (Nägeli, 1936), to 60% in Freiburg, Germany (Niechziol, 1958), and amounted to 7.5 m³/ha after 4 years injury in Carinthia, Austria (Schedl, 1953).