Colour polymorphism of the invasive paropsine leaf beetle Paropsisterna cloelia (Stål) (Coleoptera: Chrysomelidae) depends on adult maturity and age

The Eucalyptus variegated beetle, Paropsisterna cloelia (Stål) (Coleoptera: Chrysomelidae), a paropsine leaf beetle from Australia, has been invasive in New Zealand since 2016. The species’ colour polymorphism (red, green, brown, or black elytra) has historically caused uncertainty about its identity. The study investigated whether true colour morphs exist in New Zealand populations of P. cloelia or whether elytral colour changes as beetles mature. Over two consecutive sampling seasons (spring to autumn), adult P. cloelia were collected from eucalypt trees at a field site in Marlborough, counted, and the elytral colour and rigidity recorded. We found that the abundance of adults with different elytral colour and rigidity was temporally segregated, starting with a high abundance of brown/hard adults early in the sampling season (after overwintering), followed by a decline in mid-season that was simultaneous to an increase in red/soft and red-green/hard beetles (new generation). As red/soft and red-green/hard beetles declined, brown/hard beetles increased again and the cycle was repeated in the second half of the sampling season. We infer from our field observations that elytral colour and rigidity change as beetles mature, emerging from pupae as teneral stage with red/soft elytra, progressing to red/hard and green/hard, and finally turning brown. However, these interpretations should be confirmed experimentally by following the development of caged adults in the field or in laboratory rearings.