Use of Rapeseed Meal to Control Black Vine Weevil Larvae Infesting Potted Rhododendron

Abstract

Abstract Defatted rapeseed meal (RSM) from Brassica napus L. was applied as a mulch to potted rhododendron and strawberry (companion plants) infested with black vine weevil larvae to evaluate its potential as a natural soil fumigant. In the first year of study, three potting media (Sunshine® mix #1, 9:1 bark:sand, and 1:1 bark:peat [by vol]) were tested for their effects on larval survival. In the second year, 0, 50, 100, or 200 g (0, 1.8, 3.5, or 7 oz) of RSM were added to potted plants of rhododendron cv. ‘Ignatius Sargent’ grown in 1:1 bark:peat media in #1 (3-liter) containers. Meal was added approximately 16 weeks after planting rhododendron and strawberry plants. In the first study potting media type was found to have a significant effect on insecticidal activity of the meal resulting in a higher rate of larvae surviving in pots with Sunshine® mix than those with custom mixes of bark and sand or bark and peat. In the second study the highest rate of RSM, 200 g (7 oz)/l gal pot, reduced larval survival by 70% compared to the lowest rate, 50 g (1.8 oz) RSM, and the control. Shoot growth of rhododendron was unaffected by all rates of RSM, however strawberry leaf margins became necrotic with RSM rates of 100 (3.5 oz) and 200 g (7 oz). Rapeseed meal is toxic to black vine weevil larvae, but rates necessary for effective control in a nursery application may be too high for practical use.