The Role of Natural Enemies in the Colour Polymorphism of Philaenus spumarius (L.)

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Abstract
(1) The effect of predators and a parasitoid (Verrallia aucta) on the balanced colour polymorphism of Philaenus spumarius was investigated. (2) Predators were studied by releasing P. spumarius individuals labelled with Phosphorus-32 and monitoring potential predators. (3) This technique was assessed by laboratory and field experiments which showed that the radioisotope would be detectable for up to five days if ingested by a predator. (4) Out of 2000 potential invertebrate predators scanned, only thirteen individuals were found to have preyed on P. spumarius adults. A further nine individual predators had preyed on P. spumarius nymphs. This represents a minimum predation rate of 0.1% and a maximum of 8% (depending on assumptions of dispersal rates, etc.). (5) Predation by vertebrates was not detected on the study area. (6) The parasitoid Verrallia aucta was studied over a ten-year period. Mean rates of attack of Philaenus spumarius were 46% of males and 31% of females. (7) The commonest morph, typica was attacked in proportion to its abundance if all years were grouped. However, in individual years it was attacked more extensively when more abundant relative to other morphs and vice versa. (8) The least common morphs populi and `others' were attacked less than expected by their abundance and the morph trilineata was attacked more than would be expected on a random basis. (9) It is suggested that trilineata has a higher relative survival than other morphs on the study area. (10) A mechanism for the balanced polymorphism is proposed on the basis of these findings.

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