I had a query from a gentleman asking whether bees would protect his tri-color birch from aphids, or whether it would be safe to use an insecticide to get rid of the aphids without harming the bees.
I hope the gentleman concerned sees this post - I tried to respond, but the email came back 'undeliverable'.
Still, I publish my answer here, in case it helps others.
it's fairly common that bees are seen feeding on what is known as 'honeydew' - which is a type of sweet excretion produced by aphids and scale insects that feed on plant sap.
Ants also like honeydew, and even 'protect' aphids in a form of symbiotic relationship that benefits ant and aphid - sometimes referred to as 'farming'!
However, in the case described by the gentleman, I'm afraid the bees are neither eating nor killing off the aphids, so the presence of the bees will not protect the tree from aphid attack.
Insecticides act on insects, and that includes beneficial ones, so spraying or treating with a toxin is a bad idea.
This is a bad idea too! Systemic insecticides (like neonicotinoids) basically create a toxic plant. Indeed, systemic insecticides are designed to ensure sap sucking insects are poisoned by the chemical.
Research suggests that flowers produced by a neonicotinoid treated tree or plant, will in turn present that poison to pollinators or other beneficial insects in their nectar (Bonmatin et al 2004 is one example). Systemic insecticides may also be persistent in soil, and leach to other areas, thus contaminating other plants.
Presumably, aphids that ingested the sap will then be toxic for other creatures (beneficial predators) that might feed on them.
had aphids on my plums several years running (though no bees feeding on the aphid honeydew).
The plum tree recovered each time, and I was relieved because the plums are excellent.
In the case of aphid infestation, I think it would be a good idea to act quickly. This is what I did:
I have had no aphid problems this year.
Some people also advocate growing onions and alliums nearby (and I have heard of garlic spray being used, but have no experience of this for aphid control, though I have used it to deter slugs from my lupines).
If you are concerned about aphids, you could also invest in lacewing and ladybeetle larvae to protect your plants from aphids ahead of aphid season.
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