Bekæmpelsesmiddelforskning fra Miljøstyrelsen, 100 – Skade- og nyttedyrfaunaen på økologiske og konventionelle nordmannsgranarealer

Skade- og nyttedyrfaunaen på økologiske og konventionelle nordmannsgranarealer

Summary and conclusions

During two seasons in 20 Christmas tree plantations (Abies nordmanniana) and the adjacent hedgerow samples of arthropod fauna was collected monthly by means of a beating screen. Half of the plantations were organic the other half were with conventional use of pesticides. In total about 850 samples were collected and 151.834 specimens of arthropods (and snails) were identified. Analyses were made of relationships between the cultural method, flora and fauna.

Results indicate a potential for exploiting natural regulating factors of pests depending of choice of tree species composition in the hedgerows.

Organic growing does not in general give an increase in the ratio of beneficials/pests or herbivores/predators. More taxonomic groups (among these beneficial insect groups) are found in organic plantations. However, the most severe pest, woolly aphids, Dreyfusia nordmannianae, are most numerous and densities significantly higher in organic compared to conventional plantations. The variation in pest density was very prominent among the organic plantations.

An effect of hedgerows on the abundance of aphids was shown. Aphid density is significantly less close to the hedge. Correspondingly some of the potential natural enemies show a higher density close to the hedges (e.g. staphylinids). Most groups of organisms decrease with increasing distance to the hedges. Only for a few groups it is the other way around.

PCA-analysis shows in general a weak relationship between fauna of the hedges and fauna on the Nordmann fir. When hedgerows consist of conifers the fauna is more like the fauna on the Christmas trees whereas fauna of deciduous tree hedge rows and forests have the least similarities.

The increase in aphid density from the hedge to the centre of the plantation is weak but significant.

Diversity analysis shows no significant differences between the fauna on the organic Christmas trees compared to the conventional ones. On the other hand, there is a significant difference in the epigaeic flora between the two types of plantations. Plant diversity is significantly higher in the organic plantations.

Quality of trees was assessed. Conventionally grown trees had a tendency for being more open in the top and organic trees had a tendency for yellowing discoloration of the needles.

The most noticeable result was perhaps the remarkable variation in occurrence of insect pest among the organic areas. An analysis of this variation may explain which factors are most important in regulation and occurrence of insect pest in Nordmann fir.