MECHANISMS OF RESISTANCE TO THE FLEA BEETLE, PHYLLOTRETA-CRUCIFERAE (GOEZE), IN MUSTARD SEEDLINGS, SINAPIS-ALBA L

Antixenosis (nonpreference) and tolerance were identified as two mechanisms of resistance in seedlings of mustard, Sinapis alba 'Ochre', that likely account for the high level of resistance that this plant shows to flea beetles, Phyllotreta cruciferae Goeze, in the field. Sinapis alba showed antixenosis in no-choice and choice feeding tests with a susceptible species, Brassica napus 'Westar'. The beetles consumed about twice as much of the B. napus foliage as compared with S. alba. The highest level of antixenosis occurred in younger seedlings, indicating that S. alba puts forth an optimum defense when it is most vulnerable to flea beetle herbivory. Seedlings of S. alba were tolerant of low levels of flea beetle feeding damage to their cotyledons and continued to grow at almost the same rate as undamaged seedlings, whereas the growth of B. napus declined continuously with the level of feeding damage. The level of tolerance shown by S. alba combined with antixenosis probably accounts for the resistance reported for this crop in the field. In comparison with S. alba, B. napus seedlings, which show no tolerance and are preferred hosts, succumb rapidly to flea beetle attack unless protected with an insecticide.