Influence of hop yard ground flora on invertebrate pests of hops and their natural enemies

Abstract

A 3-year study in Washington State, USA examined the influence of hop yard ground flora both on the invertebrate pests of hops [including hop aphid (Phorodon humuli Schrank), two-spotted spider mite (Tetranychus urticae Koch) and hop looper (Hypena humuli Harris)] and on their natural enemies. Each year half of the experimental plots were sown with a mixture of ‘insectary’ plants [including California poppy (Eschscholzia californica von Chamisso), dwarf cornflower (Centaurea cyanus L.) and buckwheat (Fagopyrum esculentum Moench)]. Season-long average cover provided by the flowering mixture ranged from 2% to 26%, with some blooms being present for most of the season in each year. In terms of vegetation, the other main differences between cover-cropped and control plots were in the proportions of bare soil (which was always significantly higher in control plots) and the percentage of Chenopodium album L., which was significantly higher in the cover-cropped plots in two of the three seasons. Populations of spider mites on hop foliage were significantly lower in cover-cropped plots than in control plots in 2 of 3 years, while mite numbers in the cones at harvest were generally low and usually did not differ between treatments. Responses of hop aphids were much more variable, with no consistent treatment effect on either leaf or cone infestations. Larval populations of hop looper tended to be higher in cover-cropped plots in the first two generations, but usually not in the third (final) generation. Of the beneficial invertebrates, spiders and nabids consistently reached higher average densities in the ground flora of cover-cropped plots than in control plots in all 3 years, while anthocorids, geocorids and parasitic hymenoptera reached higher mean densities in the former plots in 2 of 3 years. Adult coccinellids were higher in cover-cropped plots only in 2006.