Burn old stems at the end of the year to destroy overwintering beetles. Hand pick the beetles and larvae from plants when seen from late spring onwards.
Large plantings, where hand removal is not feasible, can be sprayed with the organic insecticide pyrethrum (e.g. Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg, Pyrol Bug & Larvae Killer or Growing Success Fruit & Veg Bug Killer). No synthetic insecticides are available to the amateur gardener for control pests on asparagus. Plants in flower should not be sprayed to avoid harming bees and other pollinating insects.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)