Some time ago, somebody posted an infographic about composting on a worm composting forum. It included a list of things you can put in your compost.
In that list was jams, jellies and preserves. I was surprised to see that in the list. My own experience shows that worms will not eat anything that is high in sugar. There is a reason the are called preserves. The sugar acts as a preservative, much the same way that salt does.
So, just for fun, I decided to run a feeding experiment. I mixed up a bunch of strawberry preserves with shredded cardboard, and put it in my bin in one of the containers I use to do my feeding tests. The container isolates the test food from the rest of the bin while allowing worms to enter into the container to eat whatever is in it, should the choose to do so.
I left this experiment in the bin for 6 weeks, partly because it was winter, and the cold temperatures slowed down the rate at which food was being eaten by the worms, and partly to see if anything would change after a significant period of time.
I was surprised by the results.
There were very few worms in the test container. I think they were after the small amount of horse manure that managed to get into it. But the bedding UNDER the test container was LOADED with worms!
So, what was going on? Well, I don’t know for sure, but I have a guess.
Salt acts as a preservative. If I eat enough of it, I will get quite sick. It might even kill me. Yet salt is a necessary nutrient. I can’t survive without it. What determines the toxicity of salt is the amount of it that I ingest.
Sugar acts as a preservative. But, I suspect, that like salt, its toxicity in the worm bin depends on the density of the sugar in the bin. In large enough amounts, it causes osmotic pressure to draw water out of the microbes and worms, dehydrating them to death, the same way salt does. But in smaller amounts, there is no osmotic pressure, and the sugar actually acts as a food for the microbes, creating a microbe bloom that then becomes food for the worms. So the small amount of sugar that was dissolved in the moisture in the bedding under the test was feeding microbes and attracting worms to feed on them.
This would explain why some people claim that sugar in the bin is beneficial, while others, myself included, advise you not to put it in your bin.
But this is just my best guess. If you have a better explanation, please post it in the comments.