Influence of companion planting on damson hop aphid Phorodon humuli, two spotted spider mite Tetranychus urticae, and their antagonists in low trellis hops

Abstract

The effects on population development of damson-hop aphid Phorodon humuli and two-spotted spider mite Tetranychus urticae from planting drive rows with grass (2002-3 only), Brassica juncea, Phacelia tanacetifolia (2004-6 only), a meadow-mix of grass and flowering plants were compared with a bare soil control on aphid-susceptible low trellis hop (Humulus lupulus L.) cultivars First Gold (FGO) and Herald (HER) in 2002-4 and on partially aphid-resistant cultivar Boadicea (BOA) in 2004-6. Aphid and mite natural enemies were monitored from beat and foliage samples. Irrespective of ground cover treatment, generalist predators prevented damaging aphid populations from developing on BOA each year, and a combination of aphid–specific and generalists on FGO/HER in two of the three years experiments. Fewest migrant aphids settled, and lower aphid populations ensued, on FGO/HER with meadow-mix than other treatments, but the difference was insufficient to prevent peak populations of ca 1000 per leaf in 2004. Numbers of aphidiid parasitoids reflected aphid population densities on leaves with the highest numbers on FGO/HER in 2004, and none on BOA in any year. Seven species of primary parasitoids were recorded, dominated by Aphidius matricariae and A. picipes (95%), and eight species of hyperparasitoids which became increasingly prevalent as the season progressed. Spider mite population densities remained below ten actives per leaf in all but one year. They were regulated by a combination of phytoseiid mites (six species dominated by Typhlodromus pyri) and insect predators. Hops with meadow-mix had the highest population densities of spider mites, but also the highest numbers of phytoseiids. Although companion plants depressed average cone weight by ca 18% compared with the bare soil control, that loss may be preventable by using additional fertilizers, in which case permanent meadow-mix would provide a suitable companion plant treatment for biological control of both P. humuli and T. urticae on aphid-resistant low trellis cultivars such as BOA, but used alone is unreliable on aphid-susceptible cultivars.