There is a tree outside my window that is frequented by bees. Any time I look out there are at least a few bees flying from leaf to leaf, and stopping to walk around the leaves they land on (see Photo). I鈥檝e seen individual bees continue this behaviour for a while. Why are they doing this?
鈥� The insect pictured appears to be a social wasp (or yellow jacket as it is known in the US) rather than a bee. The foliage the wasp is sitting on is that of the , Liriodendron tulipifera.
I suspect this particular tree is infested with a pale green aphid, , which is a common pest on that plant. Heavy infestations build up during the summer and the foliage becomes increasingly sticky as a fine drizzle of sugary honeydew, excreted by the aphids, accumulates on it. Honeydew is eaten by a wide range of insects, including wasps, flies and sometimes bees. The wasps you saw will have been tucking into the honeydew.
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The tulip-tree aphid is a North American species that has recently become established in the UK. Since it was first found here in 2004 in the London area, it has turned up in a number of places in southern England and south Wales.
Andrew Halstead, Principal Entomologist, Royal Horticultural Society, Wisley, Surrey, UK
鈥� This photograph is of a wasp, not a bee. The most likely explanation is that the wasps are attracted by honeydew, though it is conceivable that they also come to drink droplets of water on the leaves.
Wasps are normally carnivorous, but in the autumn the colonies break up and the nests are abandoned, leaving the wasps to fend for themselves. It is at this time especially that they are attracted by sugary liquids, such as those found in fruit, jam and honeydew.
Honeydew is excreted by aphids and plant hoppers whose piercing mouthparts enable them to feed on sap from the leaves. The sap is rich in sugars but poor in protein. What protein it does contain is extracted by the insect鈥檚 gut, but most of the rest of the sap is excreted, coating the leaves on which the insects feed with honeydew.
Some ant species actually farm aphids for their honeydew, collecting and carrying the aphids to suitable plants. When stimulated, the aphids excrete a droplet of honeydew which the ants feed on. The relationship is mutually beneficial: in return, the ants keep any aphid predators at bay.
鈥淎nts farm aphids for honeydew, collecting and carrying aphids to suitable plants鈥�
Terence Hollingworth, Blagnac, France