Sugarcane aphid – A new pest on sorghum
An emerging research focus of the entomology program at ARCH is the sugarcane aphid, Melanaphis sacchari, a newly invasive pest of sorghum that is spreading rapidly across the High Plains. In addition to killing plants in vegetative stages, large populations of M. sacchari may persist on plants all the way through grain fill, resulting in reduced seed weight and grain quality, as well as harvesting difficulties associated with heavy accumulations of honeydew. Forage sorghums have also suffered economic impact.
Kansas sorghum producers were fortunate in 2014, as large flights of sugarcane aphid from source populations along the Gulf Coast were carried northeast into Missouri, Alabama, Mississippi and Tennessee, rather than directly northward into Kansas. By November 2014, only two counties in Kansas were infested (Sumner and Sedgwick) and the crop was mature enough to escape damage. Kansas sorghum is unlikely to be so fortunate in 2015, as source populations to the south are now far more widely distributed to the west in the Texas and Oklahoma panhandle region (see map). It is not yet known how far north the aphid will survive the winter, but without sexual reproduction or an egg stage, it is unlikely to survive in Kansas or anywhere a hard freeze occurs. This suggests that management of the aphid to reduce overwintering populations along the southern limits of its range will be critically important in limiting the extent and intensity of migrations of winged aphids next spring and summer. The problem in southern latitudes is that sorghum tends to resprout after harvest and together with Johnsongrass, an alternate host plant, can potentially sustain aphid populations all winter long. The peak reproductive rate of M. sacharri is double that of greenbugs (8-10 nymphs per female, per day) and development from birth to adult is one day faster at 74 ¿F. Thus, these aphids will pose a greater challenge for our aphid predator populations to control, at least initially. Historically, infestations by newly invasive aphid species are worst in the first few years following their appearance, become gradually more sporadic in occurrence, and eventually, inconsequential. This pattern occurs because our native complex of aphid natural enemies must change some of their habits and behaviors as they adapt to exploit this new food source, a process that can require many generations. Unfortunately, this evolutionary process can be delayed and sirupted by the heavy insecticide use that is required to protect crop yields during aphid outbreaks.
Documentation on management of sugarcane aphid on grain sorghum can be found here: http://sorghumcheckoff.com/wp-content/uploads/2013/12/2014SorghumU_Robstown_MBrewerAphids.pdf
Note that the above document was prepared in 2013 and the worst case scenario projected for 2014 was largely realized, with extensive geographic expansion and > $100 million in losses / control costs in the region. Insecticidal control is difficult and requires ground application of relatively large volumes of material via drop nozzles to obtain good coverage of lower leaf surfaces. Conventional insecticides, even chlorpyrifos, have not provided satisfactory control and emergency registrations are being sought, and granted, on a state-by-state basis for use of Transform (sulfoxaflor), a Dow Agrosciences product which has provided the best results to date.