Effect of timing of natural enemy arrival on suppression of the sorghum aphid, Melanaphis sorghi

Sorghum aphid, Melanaphis sorghi (Theobald) (Hemiptera: Aphididae) is an invasive pest of sorghum (Sorghum bicolor) in North America. First observed in South Texas in 2013, it rapidly spread across the continent. Although causing great economic harm the first four years, suppression by a diverse guild of natural enemies (enemies), including predators (coccinellids, syrphids, and lacewings) and parasitoids, lessened its impact in later years. Based on field observations and predator-prey theory, suppression was likely mediated by the timing of arrival of enemies after annual infestation of sorghum by sorghum aphid. An experiment using exclusion cages to manipulate predation exposure was devised to test this hypothesis. Three cage treatments were used: no exclusion (sham) cages (ripped fabric with access for all enemies), partial exclusion cages (coarse mesh fabric with access for parasitoids only) and total exclusion cages (fine mesh fabric excluding all enemies). Timing of enemy arrival was manipulated by ‘closing’ and ‘opening’ the cage treatments on sorghum aphid infested sorghum plants over a two week period by switching total exclusion cages during one of the two weeks with partial exclusion or sham cages, simulating early and late enemy arrival. The experiment was repeated four times during the growing season. The expectation was that aphid suppression would be greatest in cages allowing early access of enemies. If these predictions hold true, data from this experiment would support the ecological theory that posits early enemy arrival is key to successful pest suppression.