Even the smallest areas of foliage can make a buzzing contribution to biodiversity. Whether insect biodiversity on balconies and terraces is not only theoretically, but also practically possible depends upon the size of the given plant container and the choice of plant species. Plants like these can then provide food (nectar, pollen, fruits), building material, or shelter to wild bees and other insects. However, many highly cultivated plant species are no longer capable of doing this. Plants living in window boxes or flower pots have a highly limited root space; for this reason, they must be resistant to overheating and fluctuations in the availability of water and nutrients. This investigation presents a comparison of the attractivity of various plants for people and insects, their resistance to disease, and flowering duration.
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Biodiversity on the balcony and terrace:
Vegetables, herbs, and window box flowers as sources of forage for bees and other insects
Plant species under investigation
A total of 250 different plant species were planted in 45 balcony boxes having a width of 60 cm and in 14 larger containers (Fig. 1); two individuals of each species were planted. There were combinations for shadow, for half-shadow, and for full sunlight; all plants were watered automatically. Two peat-free products were used as substrates. Annual plants, but also perennials and woody plants, were used. All combinations were evaluated weekly until the end of September with respect to health and development, the presence of insects, esthetics, and usefulness for people.
In order to ensure that wild bee queens immediately found sufficient food sources when they became active after winter in early spring, the planting of early-flowering geophytes (wild tulips, botanical daffodils, dwarf irises) proved valuable. It was also important that they had a flowering season from late May into autumn since forage then becomes less available in the wild. Some plant species developed well and attracted a noticeably high number of insects (Fig. 2); these species included the Greater Knapweed (Centaurea scabiosa), species of basil (Ocimum basilicum ‚African Blue‘), Blueweed (Echium vulgare), the Lesser Calamint (Calamintha nepeta), Dwarf Catnip (Nepeta racemosa ‚Snowflake‘), Hyssop (Hyssopus officinalis), Wild Carrot (Daucus carota), Anise Hyssop (Agastache foeniculum), species of thyme (Thymus sp.), Garden Heliotrope (Heliotropium arborescens ‚Laguna®Blue‘), Blue Sage (Salvia ‚Rockin‘®Deep Purple‘), Knautia (Knautia dipsacifolia), as well as all spring-blooming geophytes.
Observed insects
Over the course of the investigation, the following kinds of insects were sighted: honey bees, bumblebees (the Garden Bumblebee, the Buff-Tailed or Large Earth Bumblebee, the Red-Tailed Bumblebee, Cuckoo Bumblebee, and the Common Carder Bee), wild bees (the Carpenter Bee, the Pantaloon Bee, the Masked Bee, the Heriades Bee, European Wool Carder Bee, the Nomada Bee, the Leaf-Cutting Bee, the Mason Bee, bees of the genera Anthophora / Amegilla), wasps (e.g., Scolia hirta), various species of Lady Bugs (Coccinellidae), Hover Flies, Green Lace Wings, Heteropterans, spiders, and butterflies (Monarch Butterflies, Hummingbird Hawk Moths, various Gossamer-Winged Butterflies and members of Pieridae, Checkered Spot Butterflies, and Smoky Moths).
It is possible to grow balcony box or terrace plants which attract a wide range of wild bees and other insects. Of the approx. 250 species of plants used in this investigation, only twelve failed to thrive; all the others showed good to very good development. The use of peat-free substrates changed the availability of water and nutrients for the plants but is to be recommended in order to reduce the worldwide exhaustion of peat and thus make an additional contribution to climate protection and the preservation of ecosystems. However, further investigations in this area are needed.