Garden hospitality - Welcoming the pollinators (Part 2: It's not easy being a pollinator!)
“Only in the last moment of human history has the delusion arisen that people can flourish apart from the rest of the living world.”
It’s not easy being a pollinator
It is not easy being a pollinating insect. Finding pollen and nectar of the right type – in sufficient quantities – is the first challenge. Another problem, especially for wild bees, is finding suitable nesting sites in landscapes that bear little resemblance to the ecosystems where these insects evolved. But finding food, shelter, water and a suitable place for nesting is the easy part. An insect also has to avoid multiple hazards that include predators, pesticides, diseases, parasites, and hostile conditions created by our rapidly changing climate. It’s not easy being a pollinator. Although we have incomplete data on the status of most insects, research on groups that are well studied, like bees and butterflies, is alarming (1). In many places insect numbers are in sharp decline and numerous species are threatened. In this post I will summarize the threats to insect pollinators, especially the wild bees, in order to help home gardeners understand that our contributions to pollinator conservation are very important. This post builds on my essay, “Making peace with the garden” (Feb 7 2020), and follows the Jan 24, 2021 introduction to the pollinators.
Wild bees are often specialized to feed on particular types of blossoms. Honey bees (Apis mellifera), like the one in this photo, are managed, non-native bees. They are generalist feeders and make use of many different flower types. The flowers of kale and kohlrabi attract a variety of pollinators including native and non-native species.
Threats to pollinators
There are many threats to pollinators and their effects are cumulative. While the catalogue of hazards is a bit depressing, candid recognition of specific threats allows us to make informed gardening decisions.
Pesticides: For every fruit, vegetable or ornamental plant grown in our gardens there are numerous viruses, bacteria, fungi, insects and other small (or large) animals working to thwart our efforts. The tent caterpillar grazing on my apple tree is a case in point. My first instinct might be to reach for the chemicals. But, maybe we need to rethink this approach.
Pesticides are normally applied to kill specific targets but there is often substantial collateral damage. Insecticides are a major hazard for the many beneficial insects in our gardens, those that prey on garden “pests” and especially for those that make their livelihood visiting flowers. Most insecticides are toxic for bees and other pollinators. Sometimes the kill is quick, but even when the dose is sub-lethal, lingering toxicity weakens the insect and makes it vulnerable to other threats such as parasites or disease.
Of particular concern are insecticides know as neonicotinoids (“neonics”). This family of chemicals is used widely in a variety of forms including as a seed coating, as a soil treatment, and as a foliar spray (2). Neonics are absorbed into a plant’s tissues and are toxic to insects that feed on the plant. However, these chemicals have also been found in plant pollen and in nectar, which is bad news for bees, their larvae, and for consumers of honey.
Western tent caterpillar (Malacosoma californicum)
Even herbicides and fungicides can be a problem for pollinators. Scientists have observed that exposure to some fungicides can make bees more vulnerable to parasites and pathogens (3). Many herbicides are toxic for insects. Ground nesting bees are especially vulnerable to herbicidal sprays, and herbicides used to control unwanted plants can dramatically affect pollinators’ food supply. Federal, provincial and municipal governments have made some progress in regulating the use of pesticides, but whether regulated or not, pesticides and pollinators do not mix.
And, there are other solid reasons to reduce or eliminate pesticides in our gardens. The toxification of soil, water and food through widespread use of pesticides is not just an ecological problem; it is increasingly recognized as a significant health issue. Our focus should be on producing healthy plants rather than just reacting to pests and diseases. Healthy, non-stressed plants growing in healthy, fertile soil often have few problems that require chemical intervention. In contrast, stressed plants are more prone to disease and tend to attract pests, and they don’t have the vigor to mount an effective defense. There are many (safe) alternatives to using chemical pesticides. Barriers, traps, strategic plant selection and other cultural or mechanical controls can effectively deal with most pest and disease issues (4).
Habitat loss and habitat degradation: On southern Vancouver Island, like elsewhere, natural ecosystems have been replaced by shopping malls, houses, schools, car dealerships, coffee shops and pavement...lots of pavement. Let me share a snapshot of the effects of this “development” on pollinator habitat in my part of the world; you will likely see parallels in your community.
Prior to the colonization of Vancouver Island, Garry Oak ecosystems dominated what is now Greater Victoria. These ecosystems are diverse sets of plant and animal communities, all associated with the trees known as Garry Oak (Quercus garryana). Garry Oak ecosystems include rock outcrops, coastal bluffs, vernal pools, maritime meadows and extensive park-like grasslands, each with a unique and bio-diverse collection of wildflowers, grasses, shrubs, and an abundance of insect life, including many species of bees and butterflies. Camas flowers (Camassia quamash, C. leichtlinii) were abundant in the Garry Oak meadows and camas bulbs were an important food source for local indigenous people. But, the Garry Oak ecosystems have almost disappeared – perhaps 5% remain and these remnants are badly fragmented and degraded (5). Early in the colonization process, large tracts of Garry Oak meadows were converted to farmland. Since then urban sprawl and infilling have produced the urban landscape known as Victoria, British Columbia.
It is worth unpacking what loss of habitat actually means for an insect because our gardening practices can provide valuable alternatives for pollinators – especially if enough home gardeners get on board.
For insects, the loss of habitat means a diminished food supply. Pollinators don’t survive when plants they depend on are sprayed, or ploughed under and paved over. Many pollinators are generalists, taking advantage of diverse flower types, but some native pollinators are specialist feeders, adapted to use only specific plants as food sources. They search for flowers that have the right look, shape or smell, and provide nectar or pollen with the right nutritional values. Often the insect’s mouth parts are adapted to precisely fit these blossoms, so even if they wanted to they could not switch to another food source. If these flowers are in short supply, or are widely dispersed, the specialist feeders will suffer.
On southern Vancouver Island most natural pollinator habitat has been methodically obliterated. While this wasn’t the goal of development, it is certainly the result. Victoria is called the garden city and there are many lovely parks, golf courses and sports fields. But, with a few notable exceptions, lawn grasses and ornamental plants dominate and there are few native flowers available for wild pollinators.
Loss of habitat also means many pollinators will have trouble finding suitable nesting or egg-laying sites especially if their requirements are specific. For example, some pollinators require rotting logs or wood bored by beetles or hollow plant stems of a certain diameter. Most bees, about 70% of the 4000 species in North America, are ground nesting insects. While some use almost any undisturbed patch of soil, many others have precise nest specifications. Bumble bees, for example, often nest in old rodent burrows. Other ground-nesting bees construct their own elaborate tunnels, and some bees prefer sandy soil while others prefer clay. Many line their brood chambers or tunnels, often with waxy secretions or with plant materials including plant hairs and leaf or petal fragments. In short, there are about as many different nest requirements as there are bees so diversity of habitat is essential.
The introduction of alien species is also a major contributor to habitat loss and degradation. On southern Vancouver Island the bits of land that have not been developed are often colonized by English ivy, Scotch broom, Himalayan blackberry, and an assortment of non-native grass species. These invasive aliens displace and replace the native plant species that wild pollinators depend on.
“Whether introduced by well-intentioned gardeners or imported accidentally, invasive plants are one of the biggest threats to Garry oak ecosystems, second only to land conversion. ”
Bees and other pollinators can be hosts for a variety of parasites. This Thick-legged Hover Fly (Syritta pipiens) is foraging for food and is in turn being used as a food source for parasitic mites. Two mites are attached to the leg and two more are clinging to the insect’s back. My other photographs show at least nine mites attached to this one host! The Thick-legged Hover Fly is a European introduction. It assists gardeners by feeding on lettuce aphids.
Other threats to pollinators
Parasites and disease: We know that honey bees are susceptible to parasites, such as the varroa mite, and to a variety of viral, bacterial and fungal infections. We don’t know the extent to which these parasites and diseases are affecting our native bees or other pollinators, but there is evidence that this is the case.
Alien species: Where pollen and nectar supplies are limited, competition from non-native bee species can be a problem. Another risk is the potential for parasites and diseases to spread from alien species to our native bees.
Climate change: Pollinating insects suffer from the effects of climate change in several ways. Unusually wet or cold weather as well as unusually hot or dry conditions impact insects directly and can affect the plants they depend upon. Harsh weather and storm events limit an insect’s ability to forage and can reduce the nectar and pollen supply. Native insects and plants often have unique and specific ecological relationships; changing weather patterns due to global warming can disrupt these associations. For example, abnormal spring weather can put flower blossoms and insect emergence out of sync.
Synergistic effects: The many threats to pollinators are cumulative and there is interplay between these hazards. For example, a bee that is stressed by exposure to toxins or poor nutrition is more vulnerable to diseases or parasites. Similarly, an insect that is weakened by disease or parasites is more likely to succumb to harsh weather or toxins.
So… given all these threats, what can a gardener do to help pollinators?
It is not easy being a pollinator. But it is important for home gardeners to realize that, through our gardening practices and choices, we do have an impact on the insect life in our spaces. Even in small gardens, informed pollinator-friendly actions can make a significant difference to the well-being of wild pollinators… an expression of garden hospitality. Obviously reducing or eliminating pesticides is the first step. With thoughtful planning we can provide pollinators with a safe, abundant and healthy food supply. We may also be able to offer nesting sites to some of our wild bees. All of this will be the subject of my next post. ‘Till then.
Burl
NOTES AND RESOURCES:
(1) There is a real need for “more detailed, more geographically encompassing, more species-inclusive, and longer-term studies.” Rhodes, p. 156
Rhodes, C. J. (June 01, 2018). Pollinator Decline – An Ecological Calamity in the Making? Science Progress, 101, 2, 121-160.
https://journals.sagepub.com/doi/pdf/10.3184/003685018X15202512854527
(2) Weidenhammer, L. (2016). Victory gardens for bees: A DIY guide to saving the bees: Douglas & McIntyre. p. 12.
Some jurisdictions have banned or are considering a ban on neonicotinoides. It is worth checking the status of neonics in your region.
(3) Rhodes, p. 137.
(4) Gilkeson, L. A. (2013) West coast gardening: Natural insect, weed & disease control. www.lindagilkeson.ca
(5) Garry Oak Ecosystem Recovery Team (GOERT), The Garry Oak Gardener’s Handbook, 2011
https://goert.ca/documents/GOERT_Gardeners_Handbook.pdf
See other GOERT publications at www.goert.ca