coffee grounds as a lily beetle repellent: the test

Lots of gardeners add used coffee grounds to their gardens to improve the texture of the soil.  Several readers of this site have suggested that coffee grounds spread around the soil at the base of lilies might also provide an environmentally friendly way to deter lily leaf beetles.  While this sounded entirely plausible (insects using their sense of smell to find their host plants might be repelled by any number of odoriferous  substances), I always wondered whether the gardeners who swore by this method had actually reduced the populations of the beetle by other means, such as diligently hand picking the eggs, larvae and adults.  A couple of days ago, I finally got around to putting the coffee solution to the test.

Because we have so few lily beetles in our test plots (thanks to our very effective biocontrol agent Tetrastichus setifer) I was only able to find eight adult lily beetles to test for their response to coffee.  With so few beetles, I didn't want to risk losing any in an open-garden experiment, so I set up choice tests in plastic bins.  Each bin contained a segment of a lily stem "planted" in about two cups of either soil from my garden or used coffee grounds.  A beetle was placed in the middle, as shown below:

Five beetles headed straight for the coffee; three ended up on the lily planted in garden soil.  When a beetle encountered the coffee, it wasn't in the least deterred, but climbed right up on the mound of grounds and walked across it to the plant.

One reader suggested testing fresh coffee. He has a point. The coffee grounds used in my experiment had already had most of the caffeine extracted to make my morning cuppa joe over the past few weeks. Caffeine may, in fact, act as a natural insecticide, so freshly ground coffee might be a much more effective deterrent against incoming beetles. On the other hand, fresh coffee would be a very expensive way to keep lily beetles out of your garden!