Effects of photoperiod and diet on diapause tendency, maturation and fecundity in Harmonia axyridis (Coleoptera: Coccinellidae)

The effects of photoperiodic conditions of larval development and adult maturation (L:D=12:12 vs. 18:6) and different diets (sugar solution, frozen eggs of Sitotroga cerealella, different numbers of aphids Myzus persicae, and their combinations) on survival, reproductive maturation and fecundity of Harmonia axyridis were studied in laboratory conditions. The fundamental aim of the work was to distinguish between cue effect of diet (neurohormonal triggering of reproduction) and direct effect of diet (nutritional maintenance of reproduction). When adults were kept under short-day conditions, the proportion of ovipositing females decreased and the duration of the pre-oviposition period increased. Moreover, a strong reaction to the direction of changes in the day length was demonstrated: when larvae and pupae developed at long day and adults were transferred to short day, the proportion of ovipositing females was much lower than in individuals that were permanently kept under short-day conditions. The percentage of ovipositing females, the rate of their reproductive maturation and the average daily fecundity gradually increased in the following succession of diets: sugar+5 aphids per day <sugar+eggs<sugar+eggs+5 aphids per day<sugar+100 aphids per day'. However, dissection showed that most of the non-laying females fed on these diets (particularly those kept under long-day conditions) have started reproductive maturation, while even first stages of oogenesis were not found in females fed on sugar solution alone. We conclude that cue effect of diet (reproductive activation) can be achieved almost independently of the number of prey consumed, while nutritional effects (the rate of reproductive maturation and fecundity) are sensitive both to the quality and quantity of food.