Rose chafer on my marigolds

(Question)

I have this strange bug over my marigolds (20-30 on a plant). They appear to be eating the plant and mating. I am wondering if this is a pest (to be destroyed) or somehow beneficial to something. I live in the thousand Island region (zone 7 I believe). It was found on my marigolds among my vegetable plants. So far just on the marigolds. The conditions are mostly dry (unless it rains) full sun. Topsoil. Thank you

(Answer)

 

The strange bug on your marigolds is Macrodactylus subspinosus, also know as rose chafer.  Although it commonly considered a pest on roses and certain types of fruit it will also attack many other plants, including marigolds.

Rose chafers have one generation each year.  The adult beetles, feeding on your marigolds, develop in early spring and will continue to feed and mate until  late June.  They live about three weeks, so your infestation should be over soon. Adults emerge from the ground in late May and early June. Because the rose chafer prefers sandy soil to lay eggs, plants located on sandy sites are most likely to be attacked. Adult beetles feed on plants for three or four weeks, generally until late June.  The females then lay their eggs in the the soil.  Grubs emerge in a couple of weeks and the larvae feed on plant roots until early fall, when they burrow into the soil for the winter.

You can control the adult beetle by hand picking, or you might try a beetle trap.  You can control the larvae, by cultivation of the soil and by applying parasitic nematodes to the soil.

Below are links to information on the Rose Chafer from the University of Minnesota:

www.oardc.ohio-state.edu/grapeipm/rose_chafer.htm

and a link to a Toronto Master Gardener response to a question on controlling Japanese beetle as the techniques for Rose Chafer are the same www.torontomastergardeners.ca/index.php/askagardener/japanese-beetles/.