Compact plants reduce biological control of Myzus persicae by Aphidius colemani

• Paclobutrazol-treated plants had a higher proportion of concealed feeding locations.
• Proportionally more aphids fed in concealed locations when parasitoids were present.
• We observed lower aphid suppression by Aphidius colemani on paclobutrazol treated plants.

Common horticultural practices, such as the use of plant growth regulators, may negatively influence the outcome of biological control programs. Plant growth regulators are applied to many ornamental and agricultural crops and can result in compact plants that have more branches and are bushier than untreated plants. Since plant architectural complexity can have strong effects on natural enemy foraging efficiency and pest suppression, our hypothesis was that the use of plant growth regulators would reduce aphid suppression by the parasitoid Aphidius colemani. In this study we investigated how the plant growth regulator paclobutrazol and the parasitic wasp A. colemani interact to affect the abundance and behavior of Myzus persicae. We found that paclobutrazol alone reduced aphid abundance compared to untreated plants. However, when parasitoids were present, paclobutrazol and associated changes in plant architecture reduced parasitism and increased aphid abundance compared to untreated plants. A likely mechanism for this result is that significantly more M. persicae fed in concealed locations on paclobutrazol-treated plants than on untreated plants. This study demonstrates that paclobutrazol reduced the efficacy of biological control by A. colemani and suggests that plant growth regulators could also affect biological control of other organisms.

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