EFFECT OF TEMPERATURE ON THE DEVELOPMENT OF SUGARCANE APHID, Melanaphis sacchari, ON SORGHUM
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The sugarcane aphid, Melanaphis sacchari (Zehntner), is a pest of sorghum, Sorghum bicolor (L.) Moench, and sugarcane, Saccharum officinarum L., in tropical and subtropical regions of the world. Sugarcane aphids have severely damaged sorghum in the United States since 2013. Abiotic factors, especially temperature, affect development of insects including aphids. Understanding aphid developmental rates at different temperatures is important for evaluating and developing sorghums with durable resistance. Fecundity and longevity of sugarcane aphids were assessed in clip cages on susceptible ‘ATx399 x RTx430’ sorghum at four alternating temperatures of 11:24, 16:29, 21:34, and 26:39 oC (night:day) and a relative humidity of 65% in an incubator. A photoperiod of 11:13 dark:light hours corresponded with daily cool and warm temperatures. In total, 48 sugarcane aphids in clip cages were used for each temperature. The birth date of the first nymph per clip cage was recorded. The nymph was retained and allowed to mature. Temperature greatly affected the lengths of the pre-reproductive, reproductive, and post- reproductive periods, as well as total longevity, fecundity, reproductive rate, intrinsic rate of increase, and mean generation time of the sugarcane aphids. The pre-reproductive period was 2.4, 1.3, 1.0 and 1. 5 days at 11:24, 16:29, 21:34 and 26:39 oC, respectively, and more than twice as long at 11:24 as at 21:34 oC. The maximum number of reproductive days was about 20 at the cool temperature of 11:24 oC but decreased slowly to about 15 days at the warm temperature of 21:34 oC. At the hot temperature of 26:39 oC, the duration of the reproductive period was only 0.12 that at the cool temperature of 11:24 oC. Increasing temperature from cool to hot decreased the length of the post-reproductive period from about 23 days to 1 day. The aphids lived as long as 45 days at the cool temperature of 11:24 oC, but survived only 5 days at the hot temperature of 26:39 oC. From cool to moderate temperature of 16:29 oC, the lifespan of the aphid decreased by 27.6 %. However, from the warm (21:34 oC) to hot temperature (26:39 oC), the percentage of decrease in the lifetime of the aphids was as much as 79.8%. The aphid produced a maximum number of nymphs 3 days earlier at the moderate (16:29 oC) than at the cool temperature (11:24 oC). However, maximum average daily fecundity occurred 1 day earlier at the warm temperature of 21:34 oC than at the moderate temperature of 16:29 oC. Total fecundity per was greatest at 16:29 oC, with fewest nymphs produced at 26:39 oC. The ‘intrinsic rate of increase” of the population was greatest (33%) at the warm temperature of 21:34 oC and least (1%) at the hot temperature of 26:39 oC. The cool (11:24 oC) and hot (26:39 oC) temperatures decreased the growth rate of the sugarcane aphid populations on sorghum. The mean generation time was 3.2, 1.8, 1.3 and 2.1 days for sugarcane aphids at the cool, moderate, warm and hot temperatures, respectively. The most favorable temperature for development of sugarcane aphids on sorghum was 16:29 ºC (moderate temperature), demonstrated by the comparatively large total fecundity and reproductive rate.