In late summer there is an intense saga playing out between monarch butterflies and the milkweeds in our hay fields. Although monarchs are in a general decline, we are seeing reasonable numbers at our yard flowers and in the fields in late summer. I began an experiment some years ago in an attempt to improve the production of monarch caterpillars from hundreds of common milkweeds, Asclepias syriaca, growing in our 40 acres of grasslands. I found that the natural crop of milkweeds apparently matured too early for the southward migrating monarchs and few caterpillars were produced. The older and tougher leaves appealed more to milkweed tussock moths than monarchs. I did recently find one last instar monarch cat on Sept. 1 getting ready to pupate on a mature milkweed seed pod, but clearly monarchs are not using many of these naturally maturing milkweeds. If many of the fields (about 25 acres) were mowed in middle to late June, milkweeds quickly regrew from roots and were ideal for egg laying and growth of monarch caterpillars in late summer and early fall. I should note that I hold back about 15 acres of grasslands which are not cut at all to provide a diversity of habitats.
This year I was not able to get the fields mowed as early as I wanted, due to weather and scheduling issues with our local farmer who leases the fields for hay; they were eventually cut about July 11. By the end of August a bumper crop of tender young milkweeds was available and I noticed some female monarchs laying many eggs on them. Now the question is whether there is enough time for the cats to mature before the first frost which was late last year (Oct. 20).
The morale of this story is that it is paradoxically possible to produce more monarchs by cutting milkweeds in mid-to late June in SW VA, spurring re-growth and production of new foliage which is more palatable to monarch cats than the dried and mature leaves normally available to them in late summer.