This sap sucking insects are the bane of my gardening existence. They gather in plague proportions and will suck all the sap out of a plant if given the chance.
If they're a problem in your garden, use turnips and/or potatoes as decoy crops. Yarrow can also help protect plants.
Because their defence mechanism is to drop from the plant their on, you can also walk around with a bucket of soapy water and gently tap plants so that the beetles drop into the water and drown - chickens also love to eat them.
Garlic and eucalyptus sprays are also good for both killing and deterring them.