Wasps
Photo: Alamy http://www.theguardian.com/environment/blog/2013/sep/05/
Wasps usually do not entice warm feelings on people - quite the opposite, these insects are often
feared, maligned and loathed. Indeed, a simple internet search will produce a great deal more
information on how to kill wasps than on understanding and appreciating them. This is
unfortunate, because wasps do have important ecological roles, and the perception of their risk is
exaggerated.
Wasp life cycle
Paper wasps and hornets (family Vespidae) are social insects that live in colonies constructed with
wood fibres (Box 1) and initiated each spring by a fertilized female (queen) that has survived the
winter. After building 20 to 30 nest cells, she lays one egg in each one. Until the first brood has
matured, 4-6 weeks later, the queen is responsible for expanding the nest, hunting for food and
caring for the young. After the first workers emerge, the queen only function is to lay eggs; she no
longer leaves the nest. The workers, all sterile females, take over the task of nest construction,
foraging for food and nurturing the larvae. As the season progresses, the colony grows rapidly,
reaching several hundred, or even thousands, workers. At the end of the summer, some eggs result
in drones (male wasps) and fertile females. These sexual forms leave the nest and mate; the males
die soon afterwards while the new queens (fertilised females) feed for a while to accumulate fat
reserves, then find a tree hole, a fence crevice or any other protected site to hibernate and start the
cycle again the following year. Once the males and fertile females have dispersed from the nest, the
colony starts to deteriorate. The old queen dies, workers also abandon the nest and die in the cold
weather.
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Box 1. The wasp nest - a feat of engineering
©2006 Fletcher http://www.solaripedia.com
Wasps build their nest with a strong, lightweight and waterproof paper-like material that
they produce by mixing saliva with scrapped wood chewed up to a pulp. The queen begins by
building a cylindrical column and coating it with a chemical that repels ants. Then she builds
cells around it, all in clusters of hexagonal units, a pattern that maximises strength and
space efficiency. The hexagonal configuration of a wasp (and honey bee) cells occurs widely
in nature, as they demand the least amount of energy and use the least amount of material.
http://waspnestremovalessex.com
After workers expand the nest by building more cells, they keep it ventilated by vibrating
their wings. Nests can be found mostly in holes in the ground, but also inside tree hollows
and any other protected sites. Wasps do not intentionally look for attics or garden sheds; it
just happens that these man-made structures are ideal nesting sites. Nests are always built
from scratch; they are never reused. Therefore it is not necessary to remove old nests from
buildings. They do not cause any damage, and will eventually crumble away.
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The ecological relevance of wasps
There are eleven wasp species in Europe, but the two most abundant in Scotland are the common
wasp (Vespula vulgaris), and the German wasp (Vespula germanica) (for more information and
photographs visit the http://www.bwars.com/ website). The adults' diet for both species consists
of high-energy sugars and carbohydrates from food such as nectar and fruit; females also
scavenge on carrion and meat. The larvae however are fed soft-bodied invertebrates, particularly
caterpillars, flies, spiders and beetle larvae, which are hunted by the workers. The impact of wasps
on their prey hasn't been quantified in Scotland, but it likely to be significant (Box
2). The large number of prey items needed to feed the wasps' brood is likely to help keeping
populations of insect pests under control.
Box 2. Prey consumption of the common wasp and the German wasp in a New
Zealand forest
Wasps are so efficient as predators that they may cause severe impact on ecosystems if they
are introduced in habitats outside their natural range. The common wasp and the German
wasp were accidentally introduced in New Zealand and rapidly spread out throughout the
country, and are having a considerable impact on the country's beech forests by consuming
honeydew and invertebrate prey. These are the figures:
- Prey load per hectare per season: ~0.8 to 4.8 million items.
- Prey biomass: 1.4 to 8.1 kg/ha.
- Carbohydrate intake (predominantly honeydew): ~ 78 to 343 litres per hectare.
The prey intake in the northern South Island was similar to that of the entire insectivorous
bird fauna.
The figures for prey consumption in the British Isles are not likely to be near to the ones
observed in New Zealand, but they do nonetheless exemplify wasps' predatory potential
Reference: Harris, R.J. 1991. Diet of the wasps Vespula vulgaris and V. germanica in
honeydew beech forest of the South Island, New Zealand, New Zealand Journal of Zoology
18: 159-169.
Also, hornets and paper wasps carry yeast cells in their guts, with important ecological end
evolutionary consequences (Box 3).
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Box 2. Wasps may spoil your picnic, but without them, your party could have
no bread, beer or wine
Photo John Vielvoye http://www.agf.gov.bc.ca/cropprot/grapeipm/wasps.htm
Yeast (specifically, Saccharomyces cerevisiae) has been fundamental to baking, brewing and
winemaking throughout human history. In nature, yeast cells are found primarily on ripe
fruits during the summer months. Since yeast is not airborne, it must rely on a vector to
move from plant to plant. For years it has been assumed that birds or bees were responsible
for transporting yeast cells, but nobody could explain how they survived the winter. A
research team from the University of Florence, Italy, suspected that wasps were involved in
preserving and disseminating yeast cells; wasps and hornets feed on yeast-harbouring
grapes, their nests are hibernation havens and adults in the next generation could be perfect
dispersing vectors.
To test this hypothesis, the team collected samples from seventeen vineyards areas around
Italy and found several species and hundreds of strains of yeast in the wasps’ guts. Some
were related to wine strains of S. cerevisiae, others were similar to bread strains, and many
were yeast living in the wild. The researchers also fed new queens with a marked strain of S.
cerevisiae before they went into hibernation. The yeast survived the winter in the insects'
guts and was transferred to the larvae via the food regurgitated by the queen. The research
team believed that other organisms such as birds are involved in the life cycle of yeasts, but
wasps seem to be especially important.
Reference: Stefanini et al. 2012. Role of social wasps in Saccharomyces cerevisiae ecology
and evolution. PNAS 109: 13398–13403.
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When the wasp colony reaches its peak towards the end of September, the queen reduces the
production of larvae and the number of adults is at its highest. At this stage, adults no
longer need to hunt prey to feed the young, and are free to roam in search of sweet food,
which is often found at picnic tables and kitchens. This is the period when they become a
nuisance. However, a wasp hovering persistently over your bottle of soda or jam sandwich
is only interested in the food and will not deliberately attack you. Wasps are not aggressive
outside the nest; stings happen when a wasp is accidentally squeezed.
The best strategy to avoid a close encounter with a wasp is by minimizing the chances of
attracting them. Do not leave food exposed for too long; keep it in sealed containers and put
away the rubbish into lidded bins. Examine glasses and other liquid containers before
drinking; wasps may have flown inside. If a wasp flies towards your food, wait for it to fly
away; flaying your arms increases the chances of entrapping the wasp, which could retaliate
by stinging you.
However, wasps will defend their nest aggressively if it is disturbed or threatened. If you find
yourself inadvertently near a nest, retreat without producing much vibration or noise.
Take care with lawnmowers and other motorized equipment because they may trigger a
defensive reaction. If a nest has to be removed, leave it to a pest controller, who has the
adequate protective clothing, experience and access to appropriate insecticides.
Wasp
stings
Wasps sting to defend themselves or their colony. Like bees and ants, wasps have evolved
modified ovipositors (the egg-laying apparatus) that enable them to sting and inject a
dose of a protein venom that causes considerable pain. As males don’t have ovipositors,
they cannot sting. Wasps can sting more than once because they are able to pull out
their stinger without injury to themselves (different from honey bees, which leave the
stinger embedded in one's skin). For most people, a wasp sting will provoke pain and some
swelling that soon subsides, but a few may experience more serious allergic reactions ranging
from burning sensation, itching, redness, tenderness, severe swelling and itching. These may
last up to a week. Swelling is more serious if it occurs around the airways, so stings to the
throat or mouth should be referred to a medical professional. Very rarely, an allergic reaction
to the venom can lead to anaphylactic shock requiring urgent medical attention. Wasp stings
that cause more than a local reaction should immediately be cared for a medical professional.
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