Leaf Litter: Love It and Leave It / March 30, 2021

I have been asked several times in the last two weeks just when it’s safe to get rid of leaf litter without hurting the insects that have spent the winter within it. An urban legend called the 50 degree rule seems to be very popular these days, but it is just that: an urban legend. I am hearing that all insects will have emerged from your leaf litter or the dead stalks of your meadow plants after varying numbers of days when the daytime temperature is above 50 degrees: 5 consecutive days over 50 degrees, 7-10 days consecutive days above 50, or just several days above 50 whether or not they are consecutive are all popular counts.

Insects do become active based on a combination of day length and accumulated degree days. That is, there must be a certain number of days above a given temperature for the insect to complete its development within its chrysalis or pupa so it can emerge as an adult. There are also a number of species that overwinter as adults, as larvae, or as eggs whose activity is triggered by warm temperatures and lengthening days.  The problem is that each insect species requires its own species-specific temperatures to become active and one prediction of some number of days above 50 does not fit all insects. Species of moths, butterflies, bees, beetles, etc. emerge all season long; some in the spring, others in the summer and others still in the fall. For example, at our house in southeast Pennsylvania, beautiful luna moths and Io moths do not emerge from their cocoons nestled within leaf litter until mid-May; various species of oak leaf-miners don’t emerge as adults until mid-July, and the velvet-bean moth doesn’t appear until the end of August at the earliest.

The reason insects appear at different times during the spring, summer and fall is that the food their larvae require isn’t ready all at the same time. The evening primrose moth, for example, doesn’t close until late July because its larvae develop on seed pods of evening primrose, and there are no seed pods until early August. There are a number of moth species called winter moths because they don’t emerge as adults until October; they then remain active on warmer nights all winter. The same story of staggered emergence can be told for native bee species as well; some are active for a few weeks in March, others from June through August and others still are most active in September. So, if you think you can pulverize all of your leaf litter, or cutdown the stalks of last year’s meadow plants after a few days at 50 degrees in the spring without hurting the creatures within, you are mistaken.

This is why we recommend burning or mowing meadows in thirds; one third should be treated each year, so that each section is only mowed or burned once every three years. That assures that the insects in the 2/3rds of your meadow that are left alone can complete their life cycles undisturbed each year, and then colonize the 1/3 that was treated after it regrows.  It is also why it is best to put your leaves somewhere on your property once in the fall and then leave them there forever. The best place by far is under the tree that created them.