Where Are All The Pollinators?

Yard in the fall with leaves on the ground

Leaving fallen leaves, plant twigs, and wood in your landscape can help pollinators survive the winter. Photo: Anahí Espíndola

Even though I love the heat of summer, I have to admit that fall makes me happy in a different way. I like the trees that turn into beautiful colors, the crisp air, quiet nights, and the days that slowly become shorter and make me want to drink tea and eat cookies. There is however one thing that makes me a little sad about fall, and it’s that all those beautiful pollinators I love so much are now gone! But are they? Actually, have you ever asked yourself where pollinators go in the fall? Well, thanks for asking — today is your lucky day! In today’s blog post we will talk about what happens to pollinators in the fall, and what we can all do to continue helping them during this quiet time.

When the season reaches an end, pollinators find themselves in a hard spot. They could hopefully collect food (nectar, pollen, etc.) during the spring and summer, but now all the flowers are gone and decisions need to be made if they are to survive until next year. When the season reaches an end, pollinators have basically two options to make sure they or their progeny survive until the next season: migrate for the winter, or stay and protect themselves against the cold.

How to Help Pollinators in the Fall

#1 – Let them go!

Some famous pollinators migrate. You may be familiar with the impressive and beautiful Monarch butterfly migration, which happens every year, and which allows Monarch populations that are far North reach latitudes where the climatic conditions are more benevolent to their survival. Other less famous pollinator migrations are those of hummingbirds, which also migrate to less harsh conditions at the beginning of the fall.

The best thing to do to help migratory pollinators is to help them migrate! This may sound a bit counterintuitive but providing plant resources for migrant pollinators for too long can be a bad idea, because that may make them stay in the region for longer, expose them to parasites for longer, and not reach their final destinations in time.

Let’s take the Monarchs as an example. Monarch caterpillars feed on milkweed, and there are both native and non-native species of milkweed they can develop on (Figure 1).

Tropical Milkweed

Figure 1 – Monarchs can develop on many species of milkweed. Non-native milkweed such as the tropical milkweed (Asclepias curassavica; left) can hinder their migration. Favoring native milkweed species (such as the common milkweed; right) is a way to help Monarchs stay safe in the winter.

The native milkweeds naturally start drying out by the end of summer. This changes the chemistry of the plant and “tells” the caterpillars that the end of the season is coming. This is one of the triggers for the caterpillars to start transforming into adult butterflies. Without that trigger from the host plant, the caterpillars continue feeding. This is what happens when they develop on non-native milkweeds that stay green for longer. In fact, caterpillars that develop on those non-native species become adults later in the season, and when they finally emerge, it is too late in the season and they are unable to migrate and make it to the next spring. One of the best ways to protect these migrants is to let them finish their natural cycle and leave in time (I know it’s hard to see them leave!), which in the case of Monarchs requires favoring planting native milkweeds over their non-native counterparts.

#2 – Let them diapause!

Most of the pollinators in our region, however, are adapted to spend the winter right here. Where are they? As I mentioned in another blog post, the vast majority of pollinators in our area are insects. Insects can’t move, fly, or feed if the temperatures are too low. To deal with very low temperatures, insects in temperate regions like ours enter a physiological stage called diapause.

During this stage, the insect physiological rate is reduced and all development is put in a pause until conditions are more favorable. Even though we usually think about these stages when the season comes to an end for us (the fall), it is interesting to note that many of our pollinators reach this stage at the beginning of the summer and they maintain it until the following spring. The practical consequence of this is that if we want to protect pollinators, we don’t just need to provide food for them; we also need to make sure that wherever they decide to spend their diapause is safe from disturbances.

So, how to do this? First, it’s important to realize that each pollinator species enters diapause at different times and places and at different developmental stages (e.g., larva, pupa, adults). Our native bees diapause in nests (solitary or communal), which can be built in different places, depending on the species. The majority of our native bees are ground-nesting bees and they can enter diapause as early as the beginning of the summer and as late as the fall. For nesting, these bees usually prefer loose soils such as those that are sandy or rocky (Figure 2). Making sure that we are not disturbing the ground in places where we see nests will be key for them to survive until the following year. Practically speaking, this means that if you see bees digging holes in the ground of your garden, you may not want to till that part of it.

Ground nesting bees

Figure 2 – The Rufous-Chested Cellophane Bee (Colletes thoracicus) establishes nests in the ground. They can be identified by the mounds left around their entrances. These bees are solitary and non-aggressive, so if you see them nest, don’t disturb that area and rejoice in the bees’ return next spring! Photo by CsabaVadasz.

The second most common place for bee nesting is in cavities. These cavities can be plant twigs and branches, cracks in rocks or walls, or even, in some regions of the world, empty snail shells! If you would like to help these bees in your garden or yards, just leave the remains of all your dry plants through the winter. Chances are that some bees have chosen your dry plants as a place to set their nest (Figure 3). These bees also are the ones that like nesting in homemade bee hotels, and it is really fun to see them emerge early in the spring from the little tubes.

Bees nesting in twigs

Figure 3 – Some bees nest in twigs and branches, like these nimble Ceratina Bees (Ceratina strenua) which have chosen a raspberry branch as a perfect place to raise their progeny. Waiting until the spring to clean your dry raspberry branches is a good way to keep these bees protected throughout the winter. Photo by Terry Miesle.

Other bees prefer to build their nest completely above ground. You may have seen little mud “amphoras” or other structures made of little rocks that hang from walls. If you see these nests close to your house, try to not disturb them and keep an eye on them next spring!

Other pollinators, like moths and butterflies, diapause in leaf litter, on wood, or in the ground. They usually do so by enveloping themselves in dry leaves, by digging themselves in the ground, or by attaching their chrysalis onto sticks and branches (Figure 4). To protect these pollinators, you can leave parts of your yard or garden soil undisturbed, keeping at least some of your dry leaves on the soil.

Black swallowtail butterfly chrysalis and polyphemus moth cocoon

Figure 4 – Some moths and caterpillars diapause in cocoons, such as this Black Swallowtail (left; photo: Megan McCarty) or this Polyphemus moth (right; photo: Leckie Seabrooke). Note how the moth has enveloped itself in dry leaves.

Finally, some moths and butterflies diapause as adults, hiding in wall cracks or small orifices. In these cases, it can be hard to spot them before it’s too late. To avoid that, you can observe around your garden or yard (and around the house) to try to find them. This way, you will know what places you should not disturb when you are doing yard or garden work.

For more information on this topic, check out Where Do Maryland’s Butterflies Overwinter? and Bewitching Butterflies and Moths with Fall and Winter Habitat.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park

One Comment on “Where Are All The Pollinators?

  1. Thanks. I did not know butterflies could diapause in their cocoons. I had thought one of my Black Swallowtails that had turned brown and failed to emerge had died. Now I know to leave it alone and wait for spring.

    Like

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