Effect of photoperiod on growth and development of two color morphs of pea aphid (Acyrthosiphon pisum)

To investigate the effect of photoperiod on growth and development of two color morphs of pea aphid Acyrthosiphon pisum (Harris), red and green color morphs of pea aphid were treated with different photoperiods (10L∶14D, 16L∶8D and 22L∶2D) under different light intensities low (103 μmol·m^-2 s^-1), middle (212 μmol·m^-2 s^-1) and high (313 μmol·m^-2 s^-1). Instars developmental duration, weight difference, mean relative growth rate per day (MRGR) measured. The results showed that at 10L∶14 D photoperiod, the duration of the 4^th instar of green color morph prolonged by 0.21 d compared with that of red color morph. While development duration of green color morphs delayed under low light intensity (P < 0.05), there were no significant differences in weight difference and in MRGR between red and green color pea aphid morphs. At 16L∶8D photoperiod, the duration of the 3^rd instar of red color morph prolonged by 0.52 d and the development duration of red color morphs also delayed under low light intensity (P < 0.05), compared with those of green color morphs. There were also no significant differences in weight difference and MRGR between red and green color morphs under low light intensity (P > 0.05). At 22L∶2D photoperiod, the durations of the 2^nd, and 4^th instar of red color morphs prolonged respectively by 0.12 d and 0.22 d under low light intensity (P < 0.05) compared with those of the green color morphs. There were also no significant differences in development duration, weight difference and MRGR between red and green color morphs of pea aphid under low light intensity (P > 0.05). At 10L∶14D photoperiod, weight difference and MRGR of green color morphs decreased respectively by 6.76% and 14.63% compared with those of red color morphs under middle light intensity (P < 0.05). At 16L∶8D photoperiod, the duration of the 1^st, 2^nd, and 4^th instar of green color morphs prolonged respectively by 0.23 d, 0.60 d and 0.33 d under middle light intensity (P < 0.05). Also green color morphs development duration delayed while weight difference decreased by 28.84% and MRGR decreased by 37.21% compared with those of red color morphs under middle light intensity (P < 0.05). At 22L∶2D photoperiod, the developmental duration of red color morphs delayed while MRGR increased by 12.00% compared with green color morphs under middle light intensity (P < 0.05). At 10L∶14D photoperiod, the duration of the 4th instar of green color morph prolonged by 0.58 d over that of red color morph (P < 0.05), while at 16L∶8D photoperiod, the duration of the 1st instar of green color morphs prolonged by 1.63 d under high light intensity (P < 0.05). At 22L∶2D photoperiod, devel-opmental duration of red color morphs delayed (P < 0.05) while there were no significant differences in weight difference and MRGR between red and green color morphs under high light intensity (P > 0.05). In summary, longer photoperiods had positive effect on growth and development of the two color morphs of pea aphid. Moreover, stronger light intensity and shorter light cycle significantly delayed development of the two color morphs of pea aphids. Therefore the growth and development of pea aphids was influenced by light intensity, photoperiod and color morph pattern. It was therefore recommended to develop more efficient mechanisms to further study the characteristics of color morph of pea aphids.