Assessing the optimal frequency of early parasitoid releases in an apple orchard to control Dysaphis plantaginea: a proof-of-concept study

Abstract

ABSTRACT Alternatives to pesticides to control the rosy apple aphid, Dysaphis plantaginea, are being developed. Natural enemies usually arrive too late after the first aphid infestation to provide effective control. This study tested the effects of mass release of two parasitoid species, Aphidius matricariae and Ephedrus cerasicola, on the numbers of aphid colonies and aphids per tree. In a trial undertaken in two years in an organic apple orchard, parasitoids were released three times early in the season to match the presence of aphid fundatrix (sensitive stages), on a weekly or bi-weekly frequency. The degree-days necessary for parasitoid emergence in the field after release were calculated. A sufficient level of aphid control by parasitoids was reached during the first month of the survey (March), but for both release treatments the control mostly failed during the second part of the monitoring session (April to late May) compared with the control treatment with neem oil. The relative effects of release frequencies were different between the two years, probably because of interannual differences in aphid population dynamics and initial infestation in orchards. The field survey suggested that parasitoid releases, at either frequency, were promising candidates for biological control of the rosy aphid, although the method still needs further calibration. This case study laid the first empirical basis to help develop future control methods of aphids by parasitoid releases in apple orchards. Releases should be done one to two weeks before first aphid detection to account for long development times of parasitoids at relatively low temperatures.