Artificial diet is fruitful pre-storage nutrition for long-term cold storage of laboratory-reared Harmonia axyridis (Pallas) adults

Abstract

Cold storage of laboratory-reared natural enemy is important for developing feasible biological control program. Harmonia axyridis, a ‘chill intolerant’ insect, is an important biological control agent in Asia. Previous studies related to cold storage of H. axyridis mostly focused on the field-collected adults, while these techniques were not applicable for laboratory-reared individuals. Pre-storage nutrition has been confirmed to be a critical factor that affects cold tolerance of a given species. In this study, Acyrthosiphon pisum-reared newly emerged adults of H. axyridis were provided with artificial diet or A. pisum as pre-storage diet (named Ha-artificial diet and Ha-aphid, respectively) for 4 days, with sex separated. Before storage, the adults were acclimated at 15 °C for another two days. After that, the six-day old unmated adults were stored at 6 °C for 15, 30, 45, and 60 days. After storage, the adults were transferred back to 25 °C and evaluated for fitness. We found that: (1) adult survival decreased with the increasing of days of storage, and Ha-artificial diet had relatively higher survival rate compared to Ha-aphid; (2) weight loss during storage increased with prolonged storage duration, and Ha-artificial diet had significantly lower weight loss compared to Ha-aphid following the four storage periods; (3) both the pre-oviposition period and egg hatch rates were neither affected by pre-storage diet nor storage duration. While at 60 days, Ha-aphid had significantly lower egg hatch rates compared to unstored control; (4) egg production of Ha-artificial diet following 30 days of storage was significantly higher than that at 60 days or that of Ha-aphid at 30 days, but they all did not significantly differ with unstored control. In summary, artificial diet was suitable for serving as pre-storage diet for laboratory-reared H. axyridis adults, and Ha-artificial diet could be stored for at least 30 days without any detrimental effects. Our results highlight the positive effects of artificial diet serving as pre-storage nutrition and might encourage more studies to develop effective storage techniques for predatory natural enemies.