Early-season predation impacts the establishment of aphids and spread of beet yellows virus in sugar beet

The potential of predators to impact the establishment of aphid vectors and the spread of beet yellows virus in sugar beet was examined. Myzus persicae carrying beet yellows virus (BYV) were released on six interior sites and six edge sites in each of four fields at the end of May. Aphids established at low densities and BYV was spread in circular patches around the infested plants at all sites. The number of diseased plants per patch at the end of September ranged from a field-average of 130 to 210 in the four fields. There was a weak tendency towards better aphid establishment and greater virus spread in fields in less complex landscapes. Edge sites had less virus spread than interior sites in one field, more virus spread in two other fields, and there was no statistically significant difference in the fourth field. In the field where virus spread was lowest at edge sites, we used predator exclosure and direct observation to manipulate and quantify the effects of early season predation. On a warm day in early June, 81% ofAphis fabae exposed to predators on young beet plants disappeared during a 24 h period, compared to 10% of aphids protected by clipcages. Intermediate levels of predator exclusion, allowing aphids to walk away but restricting predator access, showed that predation was responsible for aphid disappearance.Cantharis lateralis L. (Coleoptera: Cantharidae) was the most frequently observed foliar predator (>90%). It was found eating aphids on several occasions. The incidence of predators was 1.8 per plant per h in the field interior and 3.8 per plant per h. near the edge. In the same field, aphids and virus were released in six edge and six interior sites, that were surrounded by 0.5 m high plastic open-top barriers (‘exclosures’). Pitfall trapping inside the barriers reduced potential soil predator densities to ca. one-tenth of the open field level and arrivals of flying predators were reduced. Inside the exclosures, aphid establishment was enhanced, and virus spread at exclosure sites was increased by about 50% compared to open sites. Foliar and pitfall sampling yielded the following predators:Cantharis lateralis, C. rufa L. (Coleoptera: Cantharidae),Coccinella septempunctata L.,C. undecimpunctata L. (Coleoptera: Coccinellidae),Pterostichus cupreus (L.),Harpalus rufipes (de Geer),Patrobus atrorufus (Strom),Trechus quadristriatus (Schrk.),Bembidion lampros (Herbst) (Coleoptera: Carabidae). In a laboratory no-choice trial (with 10M. persicae /day offered), each of these species ate aphids with consumption rates varying from 1.7 to 9.2 aphids/day. The results show that early predation substantially impacted aphid establishment in one field, and resulted in reduced virus spread. Results in the other fields show that these results cannot be easily generalized.