Recently, a new pest of grain sorghum that has been causing problems for growers in Texas, Louisiana, Oklahoma, and Mississippi was found in Arkansas. The sugarcane aphid (Melanaphis sacchari) was found in Ashley and Chicot Counties on June 27, in Desha County on July 1, and in Phillips County (Elaine area) on July 11. While the infestation found in Phillips County was quite small (well below a level that would require treatment), it was the first confirmed report of this insect in the major sorghum production region of Arkansas. Robert Goodson, CEA out of Phillips County, reported this morning that he found sugarcane aphids around Helena, so it is on the move. Gus Wilson, CEA out of Chicot County (where this pest has been present a couple of weeks now), reported that most of the limited acreage of milo in Chicot and Ashley Counties was at or would soon reach treatment levels. A publication from Texas AgriLife Extension provides some additional background information, as well as pictures to distinguish it from other aphid species that feed on sorghum.
The sugarcane aphid had previously been observed in the continental United States on sugarcane, but in 2013 it was reported reaching large populations on grain sorghum in Texas, Louisiana, Oklahoma, and Mississippi. Heavy infestations can kill early vegetative sorghum plants and reduce grain yield indirectly due to reductions in plant vigor. However, the major concern from this insect is the potential for harvest difficulties caused by the large amounts of sticky honeydew (the sugary excrement of aphids and other sap-feeding insects) that result from heavy infestations. Reports from previously affected areas indicate that yield losses from harvest difficulties caused by this pest can reach up to 50% in extreme cases. In addition, large amounts of honeydew can result in additional equipment cleaning and repair costs.
Management of this insect could pose a challenge. It has the potential to rapidly multiply—there are limited options for chemical control, and it has only recently become a problem in sorghum in the U.S. A section 18 label exemption has been granted in Arkansas for Transform WG at a rate of 0.75-1.5 oz. of product per acre for control of sugarcane aphid (the section 18 was previously posted to the Arkansas Row Crops blog by Gus Lorenz). This product has provided good control in trials conducted in Texas and Louisiana, and has a favorable pre-harvest interval (PHI) of 14 days. Note that the section 18 label allows a maximum of 2 applications. Efficacy of dimethoate and chlorpyrifos (Lorsban), which are labeled for foliar use against other aphid species in sorghum (greenbug, corn leaf aphid, and yellow sugarcane aphid), has been mixed at best, and the long PHIs of these materials will limit their usefulness in preventing honeydew buildup after heading. The pyrethroids that are often used for midge control do not provide effective control of sugarcane aphid.
Unfortunately, there is no research-based action threshold for sugarcane aphid in sorghum at this time. Be sure to monitor your fields carefully, as this insect can multiply rapidly and is unlikely to be distributed evenly throughout the field. Sugarcane aphids are usually found on the underside of leaves, but reports from our colleagues indicate they will go to other parts of the plant (including the grain head) if populations reach high levels. When making treatment decisions, remember that the major concern is the buildup of honeydew, which needs to be present in large amounts to cause harvest difficulties. If moderate to large colonies of aphids are easily found in your field, and sticky honeydew is starting to appear, it is probably time to spray. If small colonies or individual aphids are found sporadically, wait and continue to monitor the field. Please be on the lookout for more updates as the situation with this insect continues to develop.