We raise dahlias on our deck (7500') in about 100 pots. One large pot has white grubs that have eaten the tubers of one plant so far. We have removed about 30 grubs 1" long from this pot. Is there a way to kill the grubs that are certain to remain? Are our other pots in danger of being contaminated? If so, woulld we be ahead to remove this pot from our deck and throw away that soil so as not to allow these grubs near the other pots?
Thank you, Bill Fisher
La Plata County Colorado
These are probably grubs of the bumble flower beetle. A sheet on this is at: http://bspm.agsci.colostate.edu/files/2013/03/Bumble-Flower-Beetle1.pdf
This grub develops in organic matter rich material and often is found in manure or compost. They feed primarily on the decaying organic matter and associated microbes. However, if confined with plants - such as your case - they will also chew belowground parts of the plant.
This should be an isolated situation - they will not spread. The adults that emerge will feed on ooze at trees or on pollen in certain flowers (e.g., thistles) and should not produce another generation this season (no more egg laying).
I suspect this is an isolated situation, one beetle found a pot with some nice decomposing compost and dumped a bunch of eggs a couple of months ago. If a similar situation is present next year (e.g., planting in decomposing compost) it may happen again, but it is rare.
Using soil that has decomposed organic matter, with well rotted compost or similar material, is not attractive to egg laying.
Controls at this time are probably not warranted. It should be near the end of the cycle in that pot. Furthermore there are no insecticides that are labeled/effective for this problem. (Insect parasitic nematodes of the genus Heterorhabditis may be effective, but these need to be mail ordered.)