Do we need to be concerned about bees feeding on honeydew produced by sugarcane aphids in sorghum fields? This is a question heard on repeated occasions when visiting sorghum fields infested by sugarcane aphids, and it was recently e-mailed to me by a county Extension agent. Bees, wasps, flies, butterflies, and many other insect species are being observed in these fields because they consume honeydew (Figure 1).
Figure 1. (a) Bumble bee, honeybee and cuckoo wasp; (b) five-banded tiphiid wasps; (c) a blow fly (Calliphoridae) and a horse fly (Tabanidae); (d) moth; (e) a red legged leaf footed bug (Coreidae); and (f) a bush cricket or katydid (Tettigoniidae) preying upon a wasp in sorghum plants. All these insects are attracted to honeydew secretion produced by sugarcane aphids. (Photo: Raul T. Villanueva, UK).
Honeydew is a secretion produced by aphids and many other homopteran insects (whiteflies, scales, leafhoppers). Honeydew is a sugar-rich liquid that is forced out of the anus of aphids. This liquid falls on upper surfaces of lower leaves where it accumulates. Initially, leaves have the appearance of being wet, shiny, or drizzly. In many cases, sooty mold fungi colonize and grow over the honeydew, covering leaves with a black powdery coating (Figure 2). However, when honeydew is produced in copious amounts – such as the case of sugarcane aphids – it attracts many other insects, including honey bees.
Figure 2. View of a sorghum plant heavily infested by sugarcane aphids. Notice that lower leaves are completely damaged by sugarcane aphid feeding; middle leaves have sooty mold on upper surfaces; and upper leaves have a drizzly appearance. Also, notice the abundant numbers of honey bees, bumble bees, and lady bugs on the entire plant. (Photo: Raul T. Villanueva, UK).
Sorghum is a wind-pollinated plant, and, on occasion, bees can visit these plants. However, their numbers are usually low compared to other crops, such as apples or soybeans. The high number of bees, wasps, or other pollinators observed in sorghum fields is (as explained above) due to plentiful quantities of honeydew produced by sugarcane aphids. The sugarcane aphid is an invasive species that produces population outbreaks in a very short time. Their natural enemies (ladybugs, parasitoids, lacewings, syrphid flies) cannot keep up with sugarcane aphid populations in spite of the great numbers of natural enemies observed in fields.
Although the two insecticides (flupyradifurone and sulfoxaflor) that are available to control sugarcane aphids in sorghum (information on their proper use in grain, forage, and sweet sorghum here) can potentially affect natural enemies and pollinators for a short time, there is no study on the effects of honeydew on bees in sorghum fields treated with or without the application of these pesticides. However, if sorghum is not sprayed with insecticides, then honeydew becomes a food source for bees, wasps, and butterflies.
- Sugarcane Aphid: Occurrence in August 2017, Misidentification, and Insecticides Registered for Grain, Forage and Sweet Sorghum (KPN)
By Raul T. Villanueva, Extension Entomologist