The effect of Phorodon cannabis on plant growth and photosynthetic potential in two industrial hemp varieties

Plants have evolved ways to defend themselves against herbivory, yet these strategies can be costly and may lead to trade-offs which are particularly important to understand in cropping systems. Industrial hemp (Cannabis sativa) hosts many arthropod herbivores. Understanding how herbivores alter plant performance and plant defenses in industrial hemp is important to minimize crop losses and expand knowledge of insect-plant interactions in this system. We evaluated how a specialist aphid, Phorodon cannabis, affects plant growth and photosynthetic potential in hemp. Laboratory experiments were conducted on two varieties of industrial hemp. On each variety, plants in vegetative stage and in flowering stage were infested with aphids for two weeks and non-infested plants were used as a control. To assess photosynthetic potential, yield of photosystem II (YII) and electron transport rate (ETR) were measured with a chlorophyll fluorometer. Dry weight and number of aphids were also taken to assess aphid reproductive success and aphid impact on plant biomass. The YII and ETR values were not different between infested and non-infested plants at each phenological stage. In flowering plants, aphid herbivory significantly reduced plant biomass even though the number of aphids were significantly lower compared to vegetative plants. These results indicate that aphid herbivory is not impacting the initial stage of photosynthesis, but aphids are impacting plant biomass – especially when aphids are introduced during the flowering stage. Future work will determine which plant traits affect aphid success during the flowering stage and if aphid herbivory influences plant metabolite profiles