letting nature back in

I noticed a decoratively marked black-and-white pollinator, studiously visiting flowers on our lavender bushes last week. I had not seen one of these unusually marked insects before, but guessed that it might be a solitary bee.

When checking up on it I was surprised to learn that some solitary bees are kleptoparasitic (or cleptoparasitic), that is they lay their eggs in the nests of other bees, hence their common name: cuckoo bee. After looking at photos of South African solitary bees, I concluded that the lovely looking bee that visited our lavender plants is likely to be one of the cuckoo bees.

There are estimated to be 20 000 to 25 000 bee species worldwide, with over 1000 bee species in South Africa. Only a very few of these are social bees, including the honey bee, and most are solitary bees. Although there are some species of solitary bees that nest gregariously, that is in colonies, even then each female solitary bee independently constructs her own nest.

Solitary bee species have varying nest-building behaviours. Nests may be made in pre-existing cavities such as in hollow reeds, or in tunnels in wood or stalks that have been made and abandoned by other insects. Some species, such as carpenter bees, make their own tunnels in wood. Carder bees make external nests using plant material or partition their nests with plant fibres. Daubers make and partition their nests out of mud or resin. Others, known as mining bees, make or use tunnels in the ground.

Solitary bee nests consist of a series of chambers that are partitioned off from each other. The female fills each chamber with a mixture of nectar and pollen and lays an egg in each chamber. She does this sequentially sealing and partitioning each chamber as she goes. Some solitary bees, known as leaf-cutters, line and partition their nests with small fragments of leaf. Some gardeners, ignoring the fact that the pollinating services of bees are invaluable, eradicate leaf-cutter bees, even though the tidy notches the bees cut from leaves do not harm the plants. Other species of solitary bees partition their nests with resin, wood fragments, fine plant hairs/fibres, or mud.

When the eggs hatch, the bee larvae feed on their supply of nectar and pollen, they then pupate before emerging, usually in the spring as adult bees, whereupon they mate, the females nest, and the cycle continues.

Of course, cuckoo bees do not make nests at all. Instead, after mating, a female cuckoo bee looks for newly completed nests made by other species of bees. She waits for the host female to go away and then opens the nest and lays her eggs inside. When her faster-developing eggs hatch, the larvae are equipped with jaws to eat or kill the host eggs and larvae, and then they remain in the nest and consume the stash of nectar and pollen the host female left there for her own young

Bees (Anthophila) are divided into ten families, with six of these occurring in sub-Saharan Africa. I realise that a little bit of knowledge is dangerous thing, but as far as I could figure out, the cuckoo bee visiting our lavender flowers is a member of the Megachilidae family of long-tongued solitary bees. Megachilidae bee species can be anything from 3 mm to 22 mm in size. Megachilidae collect pollen on hairs on their abdomens and do not have pollen baskets on their legs, as do honey bees. Although Megachilidae are often referred to as leaf-cutters, in fact species in this family include also carpenter bees, daubers, carder bees and soil nesters.

Adult cuckoo bees feed on nectar from flowers, but they lack pollen collecting (and nest building) abilities

Cuckoo bees are not numerous enough to threaten the species survival of their hosts, but unfortunately there are many real threats to the survival of pollinators, including bees. The decline of honey bees gets a lot of media attention, but numbers of solitary bees too are on the decline. Loss of bee diversity is not only a threat to natural ecosystems, but also to agriculture.

Most solitary bees collect pollen on specially adapted brushes of hairs on either their abdomens or legs.  As this pollen is more easily shed when visiting other flowers than the pollen collected by honey bees that is packed into sacs on their legs, solitary bees are more effective pollinators of the plants they visit than honey bees.

An unidentified solitary bee visiting an Everlasting flower in our garden, alongside another insect (on the left) that I also have not identified. Because the pollen is adhering to the abdomen of the solitary bee, as can been seen in this photo, I would guess that this bee is a member of the Megachilidae family

 Plant diversity depends on pollinator diversity, and the conservation of pollinators is important for our food security and survival. Ongoing threats to bees include the destruction of nests, nesting material and nesting sites. For example, removing dead trees and the widespread collection of firewood can have a detrimental effect on species of solitary bees that nest in wood. Commercial bee keeping can potentially introduce pests and diseases to wild species of bees.  Agricultural activities can destroy or make unavailable sites used by ground dwelling bees, both through tilling and sites being trampled by livestock. Flowers that are sprayed with insecticides can kill adult pollinators directly or contaminate the food they collect for feeding their larvae. Polluted water is another threat to bees and where natural water sources are replaced by reservoirs and canals, mud sources for nest building are less available for bees. Agricultural monoculture and destruction of habitats and plant diversity are major threats to bees worldwide.

Farmers can take action to mitigate some of these threats. For example, farmland can leave narrow (1-metre) strips of land to provide diverse vegetation and provide year-round food resources for bees, and provide nesting, breeding, resting and refuge sites for bees. Pesticide use on flowers should be minimised or adjusted so that farmers avoid poisoning pollinators, and pesticide drift to neighbouring areas should be reduced as much as possible (BioNET-EAFRINET, 2011).

Home gardeners too can take steps to make their spaces more accommodating to bees. There is a welcome and growing interest in setting up bee houses suitable for some species of solitary bees to nest in, but planting a diversity of plants, including plants that are native to the area, should also be part of the plan to help pollinating bees survive and breed. Ensuring potential nesting material simply by not creating a sterile garden, and leaving at least some corners in the garden to grow naturally and so be suitable for nesting sites, including for soil nesting bees, are also easy habits to get into.  I was touched to read this eloquent plea to gardeners to conserve mining bees: Let mining bees be.

Small carpenter bees (Allodapula variegata)  on a flowering Paintbrush or Snake Lily (Scadoxus puniceus). The bees are collecting pollen that adheres to fine brushes of hairs on their legs

There are some species of bees that are more generalist than others in the plants they visit. Generalist bees include honey bees and bumble bees. However, studies have shown that even bees with generalist potential learn which plants to visit and that they tend to make repetitive visits to the same plants. Even generalist bees cannot pollinate all types of plants.

Some solitary bees are fairly generalist in the plants that they visit, others visit only a narrow group of plants, and a few specialist species are restricted to pollinating only a single species of plant. The other side of the coin is that some plants have evolved to be pollinated by specific species of pollinators only.

A carpenter bee (Xylocopa caffra) visiting an African Dog Rose in our garden. Large carpenter bees are often mistakenly referred to as bumblebees, but there are no South African bumblebees

Studies in the field of plant-pollinator interactions are ongoing. It stands to reason that the more diverse plant communities are, the more species of pollinators can be sustained, and that more complex networks of pollinators and plants are likely to be more sustainable and resilient should any one species be compromised in any way.

 Sources:

BioNET-EAFRINET. 2011. Keys and Fact Sheets: Megachile bees. https://keys.lucidcentral.org/keys/v3/eafrinet/bee_genera/key/african_bee_genera/Media/Html_eafrica/Megachile_bees.htm

Eardley,Connal. [n.d.]  Bees at the Pafuri River Camp. http://www.pafuri.co.za/bees.php

Eardley, Connal; Kuhlmann, Michael & Pauly, Alain. 2010. The Bee Genera and Subgenera of sub-Saharan Africa. ABC Taxa, vol.7.  http://biodiversityadvisor.sanbi.org/wp-content/uploads/2012/11/bee-genera-of-SA.pdf

Gous, Annemarie; Willows-Munro, Sandi; Eardley, Connal & Swanevelder, Zacharias H (Dirk). 2017. Pollination: Impact, role-players, interactions and study – A South African perspective. South African Journal of Science, 113 (9-10). http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-23532017000600010

Grow Wild: Flowers to the People. [n.d.] About Solitary Bees. Kew Royal Botanic Gardens. https://www.growwilduk.com/content/about-solitary-bees

Spence, Steven. 2016. Parasitic Bees: Natural-Born Robbers. GotScience Magazine.  https://www.gotscience.org/2016/08/parasitic-bees-natural-born-robbers/

Posted by Carol