BR. J. ENT. nai. HIST., <> 1993 



Exochomus 4-pustulatus L. (pine ladybird) 



This small ladybird has a rim around its elytra so that it fits very tightly against 

 a flat substrate when elamped down. It is then almost impregnable. It clamped down 

 very readily. 



Myzia oblongoguttata L. (striped ladybird) 



One of the two used successfully ate an aphid. Both stopped still when attacked 

 and waited until the ants gave up rather than running away. 



Non-carnivorous species 



Halyzia 16-guttata L. (orange ladybird) 



Very active and mobile. Made no attempt to clamp down when attacked, but ran 

 away immediately and tried to fly. Although primarily a mildew feeder, the orange 

 ladybird may eat small aphids when food is scarce (Majerus & Kearns, 1989). One 

 of our specimens did grab hold of an aphid. 



Micraspis 16-punctata L. (16-spot ladybird) and 

 Psyllobora 22-punctata L. (22-spot ladybird) 



Neither of these species encountered aphids in the trials as they went straight up 

 the nearest leaf each time, presumably as a result of different food searching behaviour 

 associated with mildew feeding. Ants encountering these species treated them in the 

 same way as carnivorous species. 



Discussion 



In our experiments, the ants are clearly vigorously defending the aphid colony. 

 There is definitely more than just accidental disturbance of aphid predators. Nixon's 

 (1951) conclusion of incidental protection of the aphids, is not borne out by our 

 experiments. 



Way (1963) summarized three reasons why ants may attack other insects: (1) if 

 the ant is a predatory species which would be expected to attack most insects in their 

 foraging territories; (2) if other insects are hostile to the ants themselves, and (3) if 

 the other insect intrudes on the nest or on a food source which the ant is monopolizing. 



The attacks in our experiments are clearly not a predatory effect as the ladybird 

 is rarely physically injured, let alone killed. Also, if this were the case, one would 

 expect an equal likelihood of attacks at any point on the plant. However, our ladybirds 

 were often ignored when on leaves away from the colony, but attacked when near 

 or on the aphid colony. The results for the ant response data also show this — the 

 ants were far more likely to attack after the ladybird had reached the colony than 

 before. 



The ladybirds did not appear to be hostile to the defending ants, only to their 

 attended Homoptera. We conclude that this is a case of ownership behaviour. 



Variation in the assiduousness of the ants' tending of colonies may explain in part 

 the ladybirds' strategy. There would at first appear to be little reason for the ladybird 

 remaining on a plant after first encountering the ants. Their feeding success rate was 

 minimal and they were liable to continual attacks from the ants. However, in view 

 of the fact that the ladybirds are relatively immune to attack due to their protective 



