Mild Abiotic Stress Affects Development and Stimulates Hormesis of Hemp Aphid Phorodon cannabis

SIMPLE SUMMARY: For centuries, hemp has been used by humans as a source of natural fiber. Nowadays, it is a multipurpose crop with many uses, but the most valuable are hemp metabolites including terpenes and cannabinoids. During the cultivation of hemp, farmers encounter wilting plants, inhabited by the hemp aphid—a persistent plant-damaging insect. The species lives mainly on the undersides of leaves and on flower stalks and feeds on the phloem sap. We studied the effects of temperature and the herbicide used by farmers in routine hemp cultivation on the population biology of hemp aphids. Hemp aphids thrived best at moderate temperatures between 20 and 25 °C. At this temperature range, they lived for about 25 days, during which they reproduced for 15 days, producing between 54.5 and 111.6 nymphs in total per female. At 28 °C, aphid survival and reproductive capacity were much lower. Treatment of plants with herbicide caused mild stress in aphids and resulted in increased aphid reproduction and a change in their behavior; aphids settled on lower parts of the plant rather than on the top part of the plant (growing point), which is normal in untreated plants. This new knowledge may help to manage the hemp aphid and reduce damage to hemp crops in the future. ABSTRACT: The hemp aphid Phorodon cannabis Passerini is a well- known (Asia, Europe) or newly emerging (North America) insect. It is a monophagous insect pest causing considerable damage in field and glasshouse cultivations. The aim of this work was to study the effects of meteorological (temperature) and agronomical (herbicide) factors on the biology of the hemp aphid. In one experiment, hemp plants were kept at constant temperatures ranging from 20 to 30 °C, and aphid survival and fecundity were measured. In a related experiment conducted at 20 °C, plants were treated with field-appropriate rates of a selective graminicide containing quizalofop-P-tefuryl (40 gL(−1), 4.38%, HRAC group 1), commonly used to control weeds in hemp, and aphid enzyme activity was measured in addition to population parameters. We found that hemp aphids could live, feed and reproduce within the whole studied range of temperatures, demonstrating its great evolutionary plasticity. However, the optimal temperature for development was 25 °C, at which the insect lived and reproduced for 25 and 15 days, respectively, with an average fecundity of 7.5 nymphs per reproduction day. The herbicide treatment increased the activity of superoxide dismutase (SOD), catalase (CAT), β-glucosidase, S-glutathione transferase (GST), oxidoreductive peroxidase (POD), and polyphenol oxidase (PPO) in the aphids, but only on certain days after treatment, which indicates a mild stress in aphid tissues, related to a higher reproduction and changed feeding behavior; aphids moved from the actively growing tips compared to untreated plants. The results of these experiments are discussed in terms of the impact on the future management of this pest.