Zusammenhänge zwischen Insektenfraß, Witterungsfaktoren und Eichenschäden

Freising

1999

89 S.

zahlr. Lit. Ang.

Bandaufführung

13466

72236

Since the early eighties, extensive decline of oak is observed in most European countries. This disease has to be put down to several causes like extreme climatic conditions, damage by insect pests, changing fungi pathogenicity and anthropogene influences. The importance of every single parameter as initiator, predisposing or only additional factor is not yet clear and has to be judged regionally different, but there seems to be a significant coincidence of the centres of oak diseases with areas of mass propagation of pest insects, especially of the gypsy moth, Lymantria dispar. Between 1992 - 94 there was a pandemic outbreak of gypsy moth over several German regions with an infested area of 30.000 ha in Frankonia, where in 1994 the number of visibly damaged sessile and penduculate oaks increased to a portion of 84%. This event was the starting point of our research project. Defoliation by insects as a primary cause and climatic conditions were considered responsible for oak decline in the observation area. The aims of this project were to show the acute and longlasting effects of insect damage and the contribution of climate to the disease symptoms. Another important aspect was the part of sendary nocious organisms like fungi (Microsphaera alphitoides) and wood-or barkbreeding beetles (especially Agrilus) in oak decline. 12 observation stands were chosen representing different insect damages and evaluated periodically for 5 years: - stands periodically defoliated by the early feeding (mai, june) Tortrix viridana - stands with only defoliation by Lymantria dispar (feeding later from june to juli/august) - the mainly found constellation: combined defoliation by Tortrix viridana and Lymantria dispar. Different degrees of defoliation, climatic extrema and secondary nocious organisms could be investigated in regard to their effects on oak health and decline symptoms. However, abiotic and biotic effectors occuring at the same time do not only act seperately but have synergistic or antagonistic effects. In these cases, we always look at the results of several effecting parameters which we only know partly (an example: the correlation between temperature, humidity, insect defoliation and the propagation and pathogenicity of mildew). However, we found some significant results from these extended investigations: We found a significant correlation between insect defoliation and its resulting disease symptoms. The role of insects in oak decline strongly depends on time and degree of defoliation, the period of time trees stay without foliage and the number of defoliation events during a time period. - Defoliation by Tortrix viridana is not harmful over a long-term period in case oaks are of good state of health. Trees that show already decline symptoms are stronger affected. It may even result in an accumulating effect of every loss of foliage. Symptoms of such a damage are sparce foliage, extreme production of water sprouts, less radial increment, dieback of twigs and branches and mucous exsudation. In the case of sequential defoliation by Tortrix, may be combined with other stress factors (like dry years), the predisposition for an attack by Agrilus increases. - The effects of defoliation by Lymantria dispar are very controversely discussed. Our experience shows that gypsy moth has to be considered a more harmful pest than Tortrix viridana. In healthy oaks stands, a single defoliation by Lymantria dispar may be judged as predisposing component for the attack of secondary nocious organisms. But there is - besides a higher mortality - a chronical effect which is very difficult to estimate. These stands are more susceptible to stress, in return climatic conditions or secondary factors like mildew become more important for the proceeding of the disease. - In case of combined defoliation by Tortrix viridana and Lymantria dispar, oaks lost their complete foliage for the whole vegetation period, because after the feeding time new sprouts were infected and killed by oak mildew Microsphaera alphitoides. This constellation caused enormous damage and high mortality (mostly more than 50% of the oaks of these stands died). Despite following optimal conditions regeneration of oak stands is not to be expected in this case. Because of high mortality, the opening of these stands favours the propagation of Agrilus. From these results, practitians may draw some important conclusions concerning the need of control measures against insect pests in oak stands: 1. Early feeding insects like Tortrix viridana only have to be controlled if stands already show signs of lower vitality. Control measures may help to recover and to reduce the risk of attacking secondary nocious organisms. To recognize early disease symptoms is very important in this case. 2. Defoliation by gypsy moth may be tolerated, but causes increased mortality, chronical weakening of the stand and thus a higher predisposition to climatic stress and secondarx pests. In case of mass propagation of Lymantria dispar, monitoring of Tortrix viridana has to be carried out consciously. Simultaneous mass propagations will damage oak stands or even cause death without intervention.