Possibilities of biological control using entomopathogenic nematodes against Leptinotarsa decemlineata L. and Athalia rosae L. larvae

Possibilities of applying entomopathogenic nematodes against Colorado beetles under field conditions were examined. In the course of the experiments Steinernema carpocapsae (S.C.) All and Heterorhabditis spp. (HP 88) strains were used as infectors. In the preliminary laboratory experiments the nematodes were tested for effectiveness on foliage and in soil, respectively, under optimized conditions. Both strains were found to be much more effective and to maintain effectivity much longer in moist soil than on the foliage. The S.C. was remarkably more effective and persistent than the HP. In three small plot field experiments infective nematode juveniles (IJ3) were sprayed onto the foliage and at the same time over the soil surface. Their effectiveness was determined in the first step by the mortality per cent of the insects treated, then due to more severe criteria by the numerical proportion of those destroyed insects whose carcasses contained living or dead nematodes. In a hot droughty July-August period IJ3 of the S.C. strain sprayed in the evening hours infected maximum 54 more or less 6 per cent, while when sprayed in the morning hours only 34 more or less 7 per cent of the L2-L4 Colorado beetle larvae present on the foliage of potato plants. The effectiveness of the HP strain was much lower (characterized by a maximum mortality of 29 more or less 8 per cent on evening spraying). Although within the range of dose used (25-10,000 IJ3 per plant) no correlation between dose and effect could be detected, the best effect was attained in all the three cases with a dose of about 50,000 IJ3 per plant. On the soil surface spraying (in the evening) of late L4 larvae in the stage before pupation resulted in 89 per cent mortality with the S.C. and 60 per cent mortality with the HP strain (dose: 3-6 milliards per square metre). Spraying Steinernema carpocapsae nematode suspension against Athalia rosae larvae living on leaves of rape plants resulted in nearly 40 per cent mortality. S. carpocapsae suspension sprayed onto the soil destroyed 67 per cent of the developmental stages of A. rosae living under the soil surface. In the course of laboratory infection of A. rosae larvae moving into the soil by Steinernema feltiae and S. caropcapsae nematodes, the latter nematode species was found to be more suitable for the control of pests