Insect Feeding on Sorghum bicolor Pollen and Hymenoptera Attraction to Aphid-Produced Honeydew

Abstract

Pollinators are declining globally, potentially reducing both human food supply and plant diversity. To support pollinator populations, planting of nectar-rich plants with different flowering seasons is encouraged while promoting wind-pollinated plants, including grasses, is rarely recommended. However, many bees and other pollinators collect pollen from grasses which is used as a protein source. In addition to pollen, Hymenoptera may also collect honeydew from plants infested with aphids. In this study, insects consuming or collecting pollen from sweet sorghum, Sorghum bicolor, were recorded while pan traps and yellow sticky card surveys were placed in grain sorghum fields and in areas with Johnsongrass, Sorghum halepense to assess the Hymenoptera response to honeydew excreted by the sorghum aphid (SA), Melanaphis sorghi. Five genera of insects, including bees, hoverflies, and earwigs, were observed feeding on pollen in sweet sorghum, with differences observed by date, but not plant height or panicle length. Nearly 2000 Hymenoptera belonging to 29 families were collected from grain sorghum with 84% associated with aphid infestations. About 4 times as many Hymenoptera were collected in SA infested sorghum with significantly more ants, halictid bees, scelionid, sphecid, encyrtid, mymarid, diapriid and braconid wasps were found in infested sorghum plots. In Johnsongrass plots, 20 times more Hymenoptera were collected from infested plots. Together, the data suggest that sorghum is serving as a pollen food source for hoverflies, earwigs, and bees and sorghum susceptible to SA could provide energy from honeydew. Future research should examine whether planting strips of susceptible sorghum at crop field edges would benefit Hymenoptera and pollinators.

Keywords: agroecosystem; beneficial insects; carbohydrate; conservation biological control; pollen; pollinivore.

Figure 1

Hymenoptera and Diptera attracted to honeydew on grain sorghum infested with sorghum aphid, Melanaphis sorghi.

Figure 2

Top panel, (A) honeybee (Apis mellifera), (B) carpenter bee (Xylocopa micans), and (C) bumblebee (Bombus sp.) collecting sorghum (Sorghum bicolor) pollen. Bottom panel, (D) a hoverfly (Toxomerus politus) and (E) earwig (Doru taeniatum) consuming sorghum pollen.

Figure 3

Insects (log10) observed per panicle for the N109A × PI 257599 F₄ sorghum lines in Tifton, GA 2021. NO5G is PI 257599 (the common name is No. 5 Gambela). Means with the same letter are not different at α = 0.05. Orange, grey, green, and gold shaded lines were phenotyped on 14 September 2021, 21 September 2021, 28 September 2021, and 4 October 2021, respectively. Line F₄-67 was phenotyped on 4 October 2021.