In summer of 2016, several reports from the southern San Joaquin Valley came in about aphids in forage sorghum that were difficult or impossible to control with typical applications of dimethoate and chlorpyrifos. By the end of August, CDFA identified the species of this newly emerged pest as the sugarcane aphid – Melanaphis sacchari – now an invasive species. Previously, the sugarcane aphid (SCA) was a pest only in the south of the USA and Mexico, then more recently moved to the southwest. SCA are distinguishable from the greenbug aphid – Schizaphis graminum – by the lighter color from pale green to orange, and by the shorter cornicles with black tips on the rear of the insect (Figure).
Sugarcane aphid is a rapid reproducer with a new generation in less than one day. This insect is a parthenogenic (reproduces asexually), viviparous (gives live birth) reproducer with telescoping generations (the newly birthed nymph is already pregnant with the next generation). It’s not surprising then that populations in host fields can spike from 50 to 500 aphids per leaf in two weeks (United Sorghum Checkoff, 2016). Hosts include Sorghum spp., including forage and grain sorghums, Sudangrass, sorghum-Sudan hybrids, and the weed Johnsongrass. SCA will feed and reproduce readily on these species, whereas it may feed on corn, crabgrass, and Barnyardgrass but neither reproduce nor survive long. Good weed control is a start at curbing influxes of SCA into crops. Visit the UC Integrated Weed Management Page and consult your PCA to help develop a rigorous weed control program.
Populations of SCA have been reported to be distributed throughout a sorghum field, but early infestations may be localized on field edges. Infestation starts in the lower canopy and moves upward. SCA tends to congregate on the underside of leaves or in shaded areas of the canopy, but can be observed on stalks of the upper canopy in heavily infested areas. Feeding damage comes both from sap sucking on leaves and stalks as well as from the deposit of honeydew – excrement – on leaves. Sap sucking reduces translocation of photosynthates from leaves to plant sinks such as newly formed leaves and grain heads. Deposition of honeydew supports growth of sooty mold on leaf surfaces, which reduces the amount of light reaching chloroplasts in leaves, thus reducing photosynthesis. Both of these phenomena have the ultimate effects of early leaf senescence, delayed maturity, and reduced grain fill in grain and forage sorghums. A possible problem with heavy honeydew in a field may be the gumming of chopping equipment at harvest. Early control of SCA can protect yields and quality and make harvesting less difficult.
Control recommendations are based almost exclusively on studies performed on sorghum grown for grain production, so the following are not recommendations, but are probably guidelines for how we can begin to develop pest management tools for SCA in California produced sorghum for silage. Scouting should be frequent and early. Yield loss greatly increases when infestation occurs at earlier stages. Currently, the United Sorghum Checkoff recommends a treatment threshold of 50 aphids/leaf on 25% of the plants grown for grain. Currently, flupyradifurone (registered) and sulfloxafor (not registered for SCA on sorghum in CA) have been shown to provide good knockdown plus the greatest residual control of SCA in sorghum, although dimethoate and chlorpyrifos knock populations down quickly as well. Flupyradifurone and sulfloxafor are more selective materials and are softer on beneficials that predate or parasitize SCA, whereas chlorpyrifos and dimethoate are not. Tank mixes should be explored more, but spray timing studies have shown that when flupyradifuron and/or sulfloxafor are the first sprays in the season, SCA control and yield protection are better than when the first treatments are chlorpyrifos, even when followed by the more selective insecticides. Neonicotinoid seed treatments were shown to provide effective control for 40 days, and may be more important for late planted sorghum which will mature more slowly toward the end of the season. Other tactics of effective control may include spot treatment to knockdown and suppress early infestations on field edges, early harvest of field edges (if a late stage infestation), or early harvest of the crop (if the whole field is infested). The economic decision of early harvest should consider the cost savings from avoiding an insecticide treatment as well as the added costs of supplementing starch in the total mixed ration of the animals. These considerations are important since early harvest or non-treatment of an infested field is highly likely to result in undeveloped grain.
SCA should be expected as a repeat offender in summer-autumn of 2017 in CA. Until then, good weed control of Johnsongrass, Barnyardgrass, and crabgrass are pertinent. With the control tools currently available, early scouting and proper identification will be key in effective management of this pest. Still, more research is needed to evaluate effectiveness of insecticide control methods, economic thresholds, and timing of insecticide applications to develop CA forage sorghum specific information particularly useful to local production.