Plants employ various defences killing the insect attacker in an early stage.
Oviposition by cabbage white butterflies (Pieris spp.) on brassicaceous
plants, including Brassica nigra, induces a hypersensitive response (HR) -
like leaf necrosis promoting desiccation of eggs. To gain a deeper insight
into the arms race between butterflies and plants, we conducted field and
greenhouse experiments using different B. nigra genotypes. We investigated
variation in HR and consequent survival of P. brassicae egg clusters. Impact
of egg density, distribution type and humidity on HR formation and egg
survival was tested. HR differed among plant genotypes as well as plant
individuals. Egg density per plant did not affect HR formation. Remarkably,
egg survival did not depend on the formation of HR, unless butterflies were
forced to lay single eggs. Larval hatching success from single eggs was lower
on plants expressing HR. This may be due to increased vulnerability of single
eggs to low humidity conditions at necrotic leaf sites. We conclude that
effectiveness of HR-like necrosis in B. nigra varies with plant genotype,
plant individual and the type of egg laying behaviour (singly or clustered).
By clustering eggs, cabbage white butterflies can escape the egg-killing,
direct plant defence trait