Help for the Home Gardener from the Contra Costa Master Gardeners' Help Desk
Reply from the CCMG Help Desk: The insect you brought to share with us is indeed a garden problem. It is a Western Spotted Cucumber Beetle (Diabrotica undecimpunctata). The adults of this beetle feed on the leaves of melons, squash and other cucurbits, as well as corn, potatoes, tomatoes, eggplant, beans, peas, beets, asparagus, cabbage, lettuce, and other vegetables...almost all the popular vegetables. They also feed on ripening peaches and apricots and other soft fruit. Management of this pest can be difficult if not caught early on. Small seedlings can be destroyed quickly; however, established plants are more likely to survive an attack.
The least toxic method of extermination is hand picking and dropping into a bucket of soapy water to drown them. Since the beetles are now attacking your Calendulas, you may want to remove or cut the plants down and bag the remains for disposal in your garbage in order to get rid of any potential eggs or larvae that may be on them. This should help reduce the population of the beetle in your garden.
Young seedlings and other small plants can be protected from damage by coverings such as screens, protective cloth, or individual cups or cones until they are large enough to tolerate damage.
Here is a link to some more information on this pest, along with pictures to help you with identification: ttp://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/cucumberbeet.html
Good luck managing these destructive insects! Fortunately you covered your vegetables before the beetles moved in to your garden! Keep your protection in place. You probably should keep the covers until plants are at leasatlarge enough to withstand damage from the beetles.
Contra Costa Master Gardeners Help Desk
Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: firstname.lastname@example.org, or on the web at http://ccmg.ucanr.edu/Ask_Us/