Studies on population dynamics of Diprion pini L. (Hym., Diprionidae) in Lower Saxony 1991/92
1994
Altenkirch, W. (Niedersaechsische Forstliche Versuchsanstalt, Goettingen (Germany). Abt. Waldschutz)
The paper reports on the gradation of Diprion pini in small areas of Lower Saxony (North-Western Germany) in 1991/92. The second generation of larvae in autumn 1991 had caused obvious damage on about 20 ha of pine stands. During the winter of 1991/92 the number of cocoons reached 4000/square meters max. around the trunk bases of damaged trees. At these high cocoon-density rates the proportion of females ready to hatch was significantly lower than at the lower density rates more distant from the trunk. The first strong flight (first wave) happened between 5th and 10th May 1992. The checks of oviposition resulted in a correspondingly high degree of danger. The parasitization of the eggs was low, but fluctuated strongly. No measures of chemical control were introduced, because we were able to assume that the young shoots and needles growing in May would remain largely unaffected. Feeding-tests confirmed similar statements found in older literature. As predicted, the feeding-damage remained within acceptable limits. Further waves of wasps (in June and July) remained on a very low level. A second generation did not emerge in 1992. The remaining cocoons of autumn 1991 were affected by antagonists - parasites and predators. During the winter of 1992/93 D. pini was found again in the outbreak-area in the state of latency. The results show that it is not necessary to stop outbreaks of D. pini in all cases by means of chemical control. But we had to take into account the danger caused by trunk-inhabitant beetles (Pissodes pini, Pissodes piniphilus, Phaenops cyanea) as secondary pests. Due to the current extreme weather conditions throughout the whole pine area of North-Eastern Germany, this danger was particularly high
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