SIGNIFICANCE OF PHOTOPERIOD ON THE FITNESS OF THE SUGARCANE APHID AND APPLICABILITY OF BRIX REFRACTOMETRY AS A POTENTIAL METHOD TO PREDICT AND DETECT RESISTANCE OF SORGHUM

Little is known of the biology of the sugarcane aphid, Melanaphis sacchari (Zehntner), that became a persistent insect pest of sorghum, Sorghum bicolor (L.) Moench, in North America in 2013. Understanding the fitness of sugarcane aphid in relation to environmental conditions and developing a method to predict resistance would aid in understanding how sugarcane aphids develop and evaluating sorghum for resistance to this major insect pest. Effects of three photoperiods of 14:10, 13:11, and 12:12 light:dark hours were evaluated on sugarcane aphids on susceptible ‘ATx399 x RTx430’ sorghum at daily temperatures of 30 (light or day) and 20°C (dark or night), respectively, in an incubator. A sugarcane aphid from a pure colony was placed individually into a clip cage, with two clip cages per each of four plants in six pots. When each aphid produced a nymph, the mother aphid was discarded and the nymph was retained and allowed to mature and produce offspring until it died in the clip cage. The nymphs produced were counted and removed each day. The average pre-reproductive period for the sugarcane aphid was approximately 53% (6.6 days) longer at a photoperiod of 14:10 light:dark hours than at the two other photoperiods. The reproductive period of each aphid at the 14:10 photoperiod was 13.9 days, 2 days longer than the 11.8 days at a photoperiod of 13:11 light:dark hours. The post-reproductive period for each sugarcane aphid averaged 2.4 days at a photoperiod of 12:12, 4.7 days shorter than the 7.1 days at a photoperiod of 13:11 light:dark hours. Total fecundity (87.7 aphids) at the 13:11 photoperiod was 24 aphids more than the 63 aphids at 12:12 and 31 aphids more than the 56.3 aphids at 14:10 light:dark hours. Developmental times and fitness of sugarcane aphids differed significantly at the different photoperiods. Efficacy of Brix refractometry as a potential method for determining the resistance of sorghum against sugarcane aphids was evaluated in a greenhouse and field. Resistant sorghum line ‘RTx2783’ was compared to susceptible sorghum line ‘RTx430’ at the boot stage. The ºBrix values were collected by crushing a mature sorghum leaf and subsequently pipetting the liquid into a Brix reader. The average ºBrix of the resistant sorghum did not differ from the susceptible sorghum line in the greenhouse (F1,38 = 0.0016; P = 0.8985) nor in the field (F1,18 = 0.3424; P = 0.5656). The difference between the ºBrix of the two sorghum lines in the greenhouse was only 0.01% and merely 0.03%in the field. Brix refractometry was not an applicable method to use for determining resistant sorghums to sugarcane aphids. A Brix refractometer, although a common tool in plant breeding, was incapable of detecting resistance and susceptibly in two common sorghum lines. The ºBrix values were too similar; therefore, a plant breeder should not use ºBrix to define resistance or susceptibility of the two sorghum lines. The mechanism of resistance in ‘RTx2783’ was not related to the total sugar content and was undetectable by Brix refractometry.