Tritrophic effects on survival and development in a Coccidophagous coccinellid.

Chilocorus nigritus is currently considered one of the most successful biological control agents of armoured scale insects. However, establishment of this beetle in crop pest situations has not always been successful and there are still gaps in our knowledge of its ecology and behaviour. The research involved an examination of tritrophic effects on the survival and development of this common diaspid predator. The effect of a forced change in host plant on the developmental time of the juvenile stages was also examined. The prey and host plants used were the armoured scales Aspidiotus nerii Bouché Homoptera: Diaspididae and Abgrallaspis cyanophylli (Signoret) Homoptera: Diaspididae, on potatoes (Solanum tuberosum L.) and Butternut squashes (Cucurbita moschata Duchesne ex Lamarck). C. nigritus eggs were incubated on four treatments of scales on potatoes or squashes for ten days, half the second instar larvae were then switched to the same scales on the other host plant. Daily observations were made during development to adult emergence. C. nigritus larvae survived and completed development on two species of diaspid scales and the two host plants examined with varying levels of success. Larvae were able to switch from feeding on A. nerii on potatoes to A. nerii on squashes or A. cyanophylli on potatoes to A. cyanophylli on squashes and vice versa with little or no deleterious effects when compared to those beetles reared on one prey and host plant throughout. There were significant differences in survival of larvae reared to the adult stage on both A. nerii and A. cyanophylli on potatoes when compared to larvae reared on these scales on squashes. Squash appears to be a less desirable and potato a more favourable host plant for survival and development. The results have implications for rearing programmes, and the release and establishment of C. nigritus, in fields and glasshouses where scale pests may be present on a variety of host plants, or may be on host plants different to those used in the insectary.

Searching behaviour in Chilocorus nigritus (F.) (Coleoptera: Coccinellidae).

Chilocorus spp are generalist predators of armoured scale insects. C. nigritus is a common predator; however there are several gaps in the literature regarding aspects of its behaviour and general ecology. The study was conducted in order to examine the foraging behaviour of this coccidophagous species on the biparental strain of the diaspid, Aspidiotus nerii Bouché (Homoptera: Diaspididae), a widely distributed polyphagous pest. Unlike many other Coccoidea, diaspids do not produce honeydew and the main aim of the experiment was to establish whether C. nigritus use stimuli other than visual cues in prey location. Twenty to twenty eight day old female C. nigritus maintained on a culture of A nerii at 26 +/- 1 degrees C and R.H. of 45 +/- 10 % were offered five treatments with twenty minutes between treatments. The treatments offered were; host plant only, host plant with scale insect wash, overlapping generations of scales, host plant with scales removed, and plain filter paper. Beetle search behaviours were observed for ten minutes, videotaped and analysed using video analysis software. Searching behaviour in the absence of the scale or its food plant (plain filter paper) served as control. Tests of within-subject contrasts of the various parameters examined showed that turning rate was higher and beetles arrested more on host plant material when scales were present and on host plants from which scales had been removed than on all other treatments. Analyses of behaviours suggest that beetles use either olfactory or contact chemoreception cues when locating their prey at plant surface level.