ON THE DEVELOPMENT, FEEDING-ACTIVITY AND PREY PREFERENCE OF HIPPODAMIA-CONVERGENS GUER.-MEN. (COL., COCCINELLIDAE) PREYING ON THRIPS-TABACILIND. (THYS., THRIPIDAE) AND 2 SPECIES OF APHIDAE

Thrips tabaci Lind. is very difficult to control and thus one of the m ost important pests of leek in Europe. As an alternative to insecticid es biological control may be carried out by making use of beneficial i nsects, the Coccinellidae being one of the most important predatory fa milies. In the present study laboratory trials were carried out with H ippodamia convergens Guer.-Men, one of the polyphagous representatives of the lady birds, to figure out the feeding activity, development an d prey preference of H. convergens against the onion thrips in compari son with two aphid species. H. convergens developed fully when fed exc lusively T. tabaci larvae, although mortality was considerably higher and development took much longer. When fed thrips the complete develop ment lasted 29,8 days on an average, the mortality reaching 88%. When offered Acyrthosiphon pisum (Harris) H. convergens took only 24 days f or development with a mortality of only 16%. Above all, the fourth lar val instar and the pupa suffered from a very high mortality, reaching 45 and 50% respectively. With A. pisum as a prey mortality decreased t o only 6%. The duration of the pupal development was not affected by t he prey species. The feeding activity of H. convergens increased signi ficantly during the development. The larvae of the second and fourth i nstar predated 28 and 170 thrips larvae respectively per day as a maxi mum. The adults achieved the highest feeding activity, predating more than 300 thrips larvae in 24 hours. When adding up the number of prey individuals per day for each developmental instar, the L-2 larvae pred ated more than 90 and the L-4 larvae more than 1000 thrips larvae. Whe n offered thrips larvae and Myzus persicae (Sulz.) simultaneously, the re was an increasing prey preference during the development of H, conv ergens from first larval instar to adult. As to the last instars there was 20-25% less predation of thrips larvae compared to the aphids dur ing 24 hours.