Great news - you can now use excess honeydew to increase the population of your forest!! Each purchased slot increases the maximum population of your forest by 1 butterfly up to a maximum population of 160. The cost for each slot will increase with each purchase, much like forest expansions. You can only increase your population using honeydew once you have reached maximum expansion.
Note: this does not increase the size of your forest itself, just the amount of butterflies you can have flying around