Fact or Fiction?
Is this statement fact or fiction?: "Wait to clean up your garden, until temperatures consistently reach 50ºF or higher, in order to help conserve insects."
Many butterflies, bees, and other pollinators are currently overwintering in the dead leaves and hollowed out stems of last year’s plants. If you clean your garden now, you will literally be throwing away this year’s butterflies, bees, and other beneficial pollinators.
The verdict: Mixed
Terrestrial insects overwinter in a variety of habitat types that can be generally classified as:
- Protected: beneath the soil
- Partially exposed: within vegetation or underneath leaf litter
- Exposed: above the ground, on vegetation or other surfaces
It is true, that if you destroy an insect’s habitat while they are still overwintering, that they are highly unlikely to survive into the next season. However, the typical maintenance and clean up tasks for spring gardens (i.e. pruning berries in March or cutting back ornamental grasses in April) are unlikely to broadly harm insect pollinators, with the possible exception of cavity-nesting bees.
Where do garden pollinators nest?
Soil nesting bees
Across the globe, most bee species nest in protected sites beneath the soil (Danforth et al. 2019, page 104). For gardens located within ultra-urban landscapes, such as New York City, less than half of all bees are soil nesters (Matteson et al. 2008). In more suburban gardens, a majority of bees are soil nesters (Fetridge et al. 2008).
For most gardens, unless you are tilling the soil as a spring garden chore (which, we don’t advise you do, due to the negative effects of soil tilling on soil structure), most bee species should not be harmed by spring garden chores.
Cavity nesting bees
In urban gardens, bees that nest above ground in stems and other cavities are more common than they are in non-urban sites (Langellotto 2017).
One of the most common bees (~10%) of the 2,100 specimens that we found in Portland-area gardens is the small carpenter bee, Ceratina acantha. This bee nests in pithy stems, including native and cultivated Rubus (caneberry) and Sambucus (elderberry) species, and many introduced species (e.g. Brassica, Daucus, Foeniculum, Rumex). Ceratina acantha is thus able to nest in the pithy stems of across many plant families (e.g. carrots, mustards, buckwheats, roses, and elderberries) although they seem to prefer stems that are 5-8mm in diameter (McIntosh 1996).
Research from Portland State University shows that these bees emerge in April in the Portland Metro Region (Steele et al. 2019). Thus, for cavity nesting bees in urban gardens, waiting until May to clean up garden debris that includes pithy stems would allow any overwintering small carpenter bees to successfully emerge and start their own nests, in the spring.
Although it is possible that urban and suburban gardens offer suitable habitat for butterflies, few studies have looked at butterfly overwintering in different types of garden habitat (Lang et al. 2019). Studies that have censused butterflies in gardens have found that the vast majority are the ubiquitous, non-native cabbage white butterfly, Pieris rapae (Matteson and Langellotto 2010).
This butterfly is reported to overwinter in bark and crevices above the soil. Thus, typical gardening cleaning tasks are unlikely to harm the most dominant butterfly in urban and suburban gardens.
Temperature thresholds for insect emergence?
I tried to find the source of the suggestion that gardeners wait until it is 50oF or higher, before cleaning up garden debris. The closest suggestion that I could find is a study of bat foraging in Wisconsin (Meyer et al. 2016). This study found that bat foraging strongly coincided with insect spring emergence, which sharply increased at 50oF.
However, it should be noted that 78% of the insects captured in this study were flies, and almost all of the flies captured were aquatic. These, obviously, would not be disrupted by spring garden maintenance, unless you are cleaning and disrupting a garden pond or stream.
What you can do to conserve garden insects
One of the best things you can do, if you want to conserve insects in the garden, is to set aside a portion of your garden that is left unmanaged, as perennial habitat. The larger the space you are able to set aside as unmanaged habitat, the more insects you will be able to host in your garden.
Keep in mind, however, that habitat set asides will provide space for the insects that are generally desirable in a garden (such as pollinators), but may also provide space for insects that are less desirable (such as herbivores that can become garden pests).
To learn more about how you can protect pollinators in your garden, via pollinator-friendly garden designs and maintenance practices, check out the ‘Enhancing Urban and Suburban Landscapes to Protect Pollinators’ publication from OSU Extension (Melathopolous et al. 2020). Experiment, and do what works best for you. The mere fact that you’re thinking about and are aware of the importance of insect conservation in the garden is a huge step in the right direction.
Danforth, B. N., R. L. Minckley, J. N. Neff. 2019. The Solitary Bees: biology, evolution, conservation. Princeton University Press, Princeton and Oxford.
Fetridge, E., J. S. Ascher and G. A. Langellotto. 2008. The bee fauna of residential gardens in a suburb of New York City (Hymenoptera: Apoidea). Annals of the Entomological Society of America101: 1067-1077.
Lang, B.J., Dixon, P.M., Klaver, R.W., Widrlechner. 2019. Characterizing urban butterfly populations: the case for purposive point-count surveys. Urban Ecosyst 22: 1083–1096.
Langellotto, G.A. (2017). An analysis of bee communities in home and community gardens. Acta Hortic 1189: 491-496.
Matteson, K. C., J. S. Ascher and G. A. Langellotto. 2008. Richness and composition of the bee fauna of urban gardens in New York City (Hymenoptera: Apoidea). Annals of the Entomological Society of America 101: 140-150.
Matteson, K.C., Langellotto, G.A. 2010. Determinates of inner city butterfly and bee species richness. Urban Ecosyst 13: 333–347.
Melathopoulos , A. Bell, N. Danler, S., Detweiler, A. J. Kormann, I., Langellotto-Rhodaback, G. A., Sanchez, N. Stoven, H. and Smitley, D. 2020. Enhancing Urban and Suburban Landscapes to Protect Pollinators. Oregon State University Extension and Experiment Station Communications. EM 9289.