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Beneficial Bugs in Fall

By: Sierra Laverty, Assistant Horticulture Director | 10/15/2018

In the spring and summer, any garden is awash with wonder. Annual and perennial flower buds open to reveal magnetic colors. The sound of crickets and cicadas ring as the days and nights get hotter. Buzzing and fluttering insects of all sizes forage on nectar and pollen, or sometimes each other.

Autumn is a quieter time. The garden’s wild nature isn’t as visible or audible. We know to provide pollinators with food (in the form of flowers) in spring and summer because they beg us to, as they search our yards for signs of color. Most of us don’t know to provide them with shelter as winter approaches, or simply don’t know how!

Pollinators and other beneficial insects overwinter as an egg, larvae, pupae (juvenile), or adult, and need habitat to do so. The pupae stage of the butterfly is a chrysalis, made from silk the caterpillar produces and then encapsulates in a harder shell. The charismatic Western Tiger Swallowtail (Papilio rutulus) forms a brown chrysalis in fall that is designed to blend in with dull twigs and bark. With strategies like this, it’s easy to understand why many creatures go unnoticed through the colder months.

Another charming pollinator that needs a home for the winter is the mason bee (Osmia spp.). Mason bees are notorious for their use of mud while building their nests, which are found in hollow stems and other circular openings The blue orchard mason bee (Osmia lignaria), an important insect to agriculture, actually spends most of its tiny life inside the nest, from July to the following March.

Mason bees are an anomaly in the “bee world” however; 70% of bees in North America nest underground! These bees include the fuzzy, social bumblebee (Bombus), as well as the solitary mining bee (Andrena), which can burrow down 9 feet deep. They either use their legs or mandibles to dig like a dog, or repurpose old rodent burrows to build their nests.

Ground-nesting bees may prefer bare earth to establish their homes, but many other insects favor leaf litter. Pest predators such as lacewings, ground beetles, spiders, big-eyed bugs, and hoverflies use leaf litter to survive the winter. Bumblebee queens can also use a layer of leaves to keep warm, before finding a permanent home in the spring. So, before you drag out your rake and pruners, consider these tips!

Fall Cleanup Dos and Don’ts for beneficial bugs:

  • Leave dead or brown stalks, canes, and stems for nesting bees and butterfly chrysalises (until it warms up later in spring)
  • Leaf litter is NOT litter! Think of it as a “free” mulch that can deter weeds, and house beneficial insects
  • Keep parts of your garden undisturbed to act as habitat areas
  • Limit soil tilling, and consider the critters that might have burrowed underneath the surface