I'm planing to use neem oil as insect repellent on my plants (mostly vegetables) but there is one thing that is puzzling me: If neem oil is efficient insect repellent does it have negative effect on pollinating insects? If so, when is the best time to apply it during flowering period?
Neem oil is not well studied but appears to act in a few different ways, depending on application:
- Via contact with the insect: it is an oil and can block an insects breathing tubes if applied directly to them (i.e. as a spray), it may also be absorbed and disrupt their metabolism. This action is non-specific and could harm beneficial insects. Most beneficial insects are active during the warmer parts of the day, so an early morning/late evening application will reduce the likelihood of this happening.
- As a surface anti-feedant: by coating the surface of the plant with something insects don't want to eat. This is specific to the insects feeding upon the plant and should not affect pollinators. If flowers are coated it may discourage pollinators from visiting, but nectaries tend to buried inside flowers and are unlikely to be affected.
- As a systemic anti-feedant: the plant can absorb some of the neem oil into its tissues, particularly if used a soil drench. I would recommend against this, as it is likely to harm soil organisms and may contaminant nectar sources.
We do use neem oil blended with horticultural soap, predominantly on brassicas and legumes. We try to avoid flowers, and target colonies pest (predominantly aphids) and only apply late at night or very early in the morning. We have not noticed any issues with pollination and have large numbers of predator species such as parasitoid wasps still present.
We have had mixed results, although this combined with encouraging natural predators seems to prevent pests from becoming too problematic. It has been particularly effective on gooseberries at preventing gooseberry sawfly.