Plant parks, gardens and road verges with appropriate native species

Supporting evidence from individual studies

1. A replicated, site comparison study in 1992–1996 in urban road verges in Baden-Württemberg, Germany (Schwenninger & Wolf-Schwenninger 1998) reported that road verges sown with native wildflowers had a greater species richness of butterflies and day-flying moths than verges with non-native vegetation. Results were not tested for statistical significance. Over four years, eight butterfly and moth species were recorded on two verges sown with wildflowers, compared to none on verges with non-native plants. Only one species, small white Pieris rapae, occurred every year in the sown verges. Two road verges (1,100–1,500 m², up to 5–35 m wide) on busy roads in the centre of Stuttgart were sown with annual and biennial native wildflowers including white stonecrop Sedum album, common self-heal Prunella vulgaris, greater knapweed Centaurea scabiosa and wild carrot Daucus carota. For comparison an unspecified number of vegetated road verges that contained non-native bearberry cotoneaster Cotoneaster dammeri, scarlet firethorn Pyracantha ccoccinea and cultivated roses were also surveyed. From April–August 1992–1994 and 1996, butterflies and day-flying moths were surveyed 6–10 times/year on each verge, and plants were occasionally searched for caterpillars.

   Study and other actions tested

   Referenced paper

   Schwenninger H.R. & Wolf-Schwenninger K. (1998) Modifying landscaped verges for conservation: New urban habitats for bees and butterflies? Natur und Landschaft: Zeitschrift fur Naturschutz und Landschaftspflege, 73, 386-392.

2. A replicated, site comparison study in 2001–2002 in 32 gardens and 20 mixed woodlands in California, USA (Levy & Connor 2004) found that California pipevine Aristolochia californica planted in gardens was used less by pipevine swallowtails Battus philenor and had lower egg and caterpillar survival compared to that in natural sites. Adult swallowtails visited fewer gardens (9/32) than natural sites (19/20) where pipevine occurred, and eggs were laid in fewer gardens (7/32) than natural sites (19/20). Egg survival was lower in gardens (42–70%) than in natural sites (57–91%). Adult and egg presence were higher where pipevines were at least 7- (adults) and 17-years-old (eggs), and egg and caterpillar survival were higher at sites with older (>40 years) and larger pipevines (>185 m² of foliage) than in recently planted sites (data presented as model results). Egg densities were higher on pipevines grown in the sun (2–5.5 eggs/m²/week) than in the shade (0–2 eggs/m²/week). In 2001, nine gardens where pipevine had been planted and nine riparian oak woodland and redwood forests with naturally occurring pipevine were selected. In 2002, twenty-three gardens and 11 natural sites were studied. From March–July 2001–2002, pipevine foliage at each site was inspected for >15 minutes/week, and all swallowtail eggs, caterpillars and adults recorded. In 2002, the number of eggs/m² of foliage growing in the sun and shade at four garden and four natural sites was counted weekly for 12 weeks, and the survival of marked egg masses was recorded.

   Study and other actions tested

   Referenced paper

   Levy J.M. & Connor E.F. (2004) Are gardens effective in butterfly conservation? A case study with the pipevine swallowtail, Battus philenor. Journal of Insect Conservation, 8, 323-330.

3. A replicated, paired, site comparison study in 2006 in 12 suburban gardens in Pennsylvania, USA (Burghardt et al. 2008) found that gardens planted exclusively with native plants had four times more butterfly and moth caterpillars and three times more caterpillar species than gardens with a conventional mixture of native and non-native plants. Both the abundance (12.7 individuals/site) and species richness (6.8 species/site) of caterpillars were higher in gardens with native plants than in conventional gardens (abundance: 3.0 individuals/site; richness: 1.8 species/site). Six pairs of gardens (0.13–5.26 ha) within 1.6 km of each other, and with similar area, vegetation structure and surrounding landscape, were selected. One garden in each pair was planted exclusively with native plants (canopy, understorey, shrubs and grasses), while the other contained a conventional mix of cool-season Eurasian grasses, Asian shrubs and understorey trees, and native canopy. From August–September 2006, each garden was surveyed once. All butterfly and moth caterpillars on all twigs and vegetation within reach and within a 0.5-m radius of 12 evenly-spaced points/garden were identified.

   Study and other actions tested

   Referenced paper

   Burghardt K.T., Tallamy D.W. & Shriver W.G. (2008) Impact of native plants on bird and butterfly biodiversity in suburban landscapes. Conservation Biology, 23, 219-224.

4. A replicated, paired, controlled study in 2004–2006 in 18 urban gardens in New York, USA (Matteson & Langellotto 2011) found that planting native plants did not increase the number of butterflies in gardens. In gardens where native wildflowers were planted, the number of species of butterflies was similar to gardens where no additional flowers were planted (data not presented). In addition, 88% of butterflies seen on flowers were using non-native species (statistical significance not assessed). In August 2004, in each of nine gardens (224–2,188 m², 0–33 years old), 70 plants of seven native wildflower species were planted in a sunny, composted 10 m² plot (or additional 24-inch diameter pots where limited soil was available). Any lost plants were replaced in May 2005. In a further nine gardens, similar in size and floral area, no wildflowers were planted. Prior to any planting, the majority of plants in the gardens (69%) were non-native. From June–September 2005–2006, butterflies were visually counted in each garden every two weeks, spending 5 minutes/600 m², and their use of native or non-native flowers was recorded.

   Study and other actions tested

   Referenced paper

   Matteson K.C. & Langellotto G.A. (2011) Small scale additions of native plants fail to increase beneficial insect richness in urban gardens. Insect Conservation and Diversity, 4, 89-98.

5. A replicated, site comparison study in 2009–2010 in 20 residential gardens and five meadows in Pennsylvania, USA (Cutting & Tallamy 2015) found that common milkweed Asclepias syriaca planted in gardens was used by monarch butterflies Danaus plexippus more than milkweed planted in meadows, but caterpillar survival was similar across the sites. Milkweed patches in gardens contained more monarch eggs (47–109 eggs/plot) than milkweed patches in meadows (7–45 eggs/plot). Egg and caterpillar survival was similar in gardens (6.9–8.7%) and meadows (3.9–11.4%). In May–June 2009, twenty milkweed plants were planted in each of twenty 2-m² plots (130–1,500 m apart, >500 m from the nearest known milkweed) in heavily managed lawns and gardens and forty 2-m² plots across five minimally managed native meadows. Plants were grown from seed in greenhouses, surrounding vegetation was cut prior to planting, and sites were watered periodically. Plants were searched for eggs and caterpillars nine times from July–September 2009, and six times from 19–29 August 2010. Eggs and caterpillars were removed or marked to avoid double-counting. Monitoring ended if fewer than four healthy plants remained. On half of the plants at each site, survival of marked eggs and caterpillars was monitored over 11–14 days from the third week of August each year.

   Study and other actions tested

   Referenced paper

   Cutting B.T. & Tallamy D.W. (2015) An evaluation of butterfly ardens for restoring habitat for the monarch butterfly (Lepidoptera: Danaidae). Environmental Entomology, 44, 1328-1335.

6. A replicated, randomized, controlled study in 2016–2017 in an arboretum in Kentucky, USA (Baker & Potter 2018) found that following planting of eight milkweed species Asclepias spp. the number of monarch butterfly Danaus plexippus eggs and caterpillars was higher on taller species than on shorter species, but that caterpillar growth and survival were similar across all species. Taller milkweed species (Asclepias syriaca, A. speciosa, A. incarnata, A. fascicularis) had a higher number of monarch butterfly eggs and caterpillars (3.0–16.8 individuals/plot) than shorter species (0.0–5.4 individuals/plot). However, caterpillar growth (final weight: 169–437 mg) and survival (56–100%) were similar on all milkweed species in two of three trials. In the third trial, survival (40–65%) was similar but caterpillars were larger on A. verticillata, A. tuberosa and A. speciosa (868–1,032 mg) than on the other species (300–706 mg). Eight species of milkweed native to Kentucky or the central or western USA (common A. syriaca, swamp A. incarnata, butterfly A. tuberosa, green antelopehorn A. viridis, whorled A. verticillata, showy A. speciosa, Mexican whorled A. fascicularis, broadleaf milkweed A. latifolia) were grown from seed in a greenhouse. In May 2016, seedlings were transplanted into five (1.22 × 9.75 m) garden plots, each with eight subplots (1.22 × 1.22 m). Four individuals of a single species were sown into each subplot. From May–October 2016 and April–September 2017, monarch eggs and caterpillars were counted on all plants once every two weeks. The following three experiments measured monarch caterpillar growth and survival. In August 2016, two first or second instar monarch caterpillars were caged in white fine mesh bags (25 × 40 cm) on each of two plants/plot for nine days. In September 2016, three caterpillars were caged on each of nine plants for seven days. In August 2017, a single 1-day-old caterpillar was caged on each of 10 plants of each species in a greenhouse for five days.

   Study and other actions tested

   Referenced paper

   Baker A.M. & Potter D.A. (2018) Colonization and usage of eight milkweed (Asclepias) species by monarch butterflies and bees in urban garden settings. Journal of Insect Conservation, 22, 405-418.

7. A replicated, randomized, controlled study in 2014–2015 at a research farm in Alabama, USA (Clem & Held 2018) found that planted native trees supported a greater abundance and species richness of moth caterpillars than non-native trees. The abundance (260–281 individuals/year) and species richness (17 species) of moth caterpillars on planted native red maple Acer rubrum was higher than on non-native Norway maple Acer platanoides (abundance: 98–102 individuals/year; richness: 10 species) and crepe myrtle Lagerstroemia indica (abundance: 8 individuals/year; richness: 3 species/year). In March 2014, a native red maple was planted in the centre of each of 28 plots (5 × 5 m, 15 m apart) and surrounded by four further trees (3.5 m away) in one of four randomly assigned treatments: native red maples, non-native Norway maples, non-native crepe myrtles, or no trees. Herbicide was applied monthly, exposed ground covered with pine straw at the beginning of each season and nitrogen fertilizer applied to each tree in April and August each year. From June–September 2014, and May–September 2015, caterpillars were surveyed twice a month on the central maple and one neighbouring tree in each plot. All caterpillars on 30 leaves in each of four cardinal directions were counted.

   Study and other actions tested

   Referenced paper

   Clem C.S. & Held D.W. (2018) Associational interactions between urban trees: Are native neighbors better than non-natives? Environmental Entomology, 47, 881-889.

8. A replicated, paired, controlled study in 2013–2014 in a managed park in Georgia, USA (Majewska et al. 2018) found that areas planted with native flowers had a similar overall abundance and species richness of adult butterflies, but fewer adults and/or eggs, and more caterpillars, of specific species than areas planted with non-native flowers. The total abundance and species richness of butterflies was similar in plots planted with native and non-native plants (data presented as model results). However, in native flower plots the abundance of monarch Danaus plexippus (0.1–0.2 butterflies/plot) and gulf fritillary Agraulis vanillae adults (0.3–0.5 butterflies/plot), and of monarch and queen Danaus gilippus eggs (0.1 eggs/plant) was lower than in non-native flower plots (monarch: 1.6–3.1 butterflies/plot; gulf fritillary: 1.3–1.4 butterflies/plot; eggs: 0.7 eggs/plant). The abundance of black swallowtail Papilio polyxenes caterpillars was higher in native plots (0.7 caterpillars/plant) than in non-native plots (0.6 caterpillars/plant). Authors suggested that the difference in the number of butterflies of some species may have been caused by the fact that there were fewer flowering plants in native than non-native plots. In spring 2013, four fenced, irrigated experimental plots (7.6 × 15.2 m, 20–88 m apart) were established in each of three blocks, 250 m apart. Each plot was planted with 128 plants of 13 species, and was surrounded by mown grass. Plots were assigned to four treatments/block: planting with species native to Georgia or not native to the USA, and low (every other month) or high (every other week) weed maintenance. From May–September 2014, adult butterflies were surveyed for 7 minutes/plot, 1–4 times/month, and eggs and caterpillars of the four species were counted weekly or monthly on 2–14 host plants/butterfly species/plot.

   Study and other actions tested

   Referenced paper

   Majewska A.A., Sims S., Wenger S.J., Davis A.K. & Altizer S. (2018) Do characteristics of pollinator-friendly gardens predict the diversity, abundance, and reproduction of butterflies? Insect Conservation and Diversity, 11, 370-382.