11 May 2018

Biological and environmental factors affecting wasp numbers

Environment | PPC91 May 2018

A good wasp season can give the diligent pest controller a tidy profit, and so there is the temptation to rely on these returns in subsequent years. However wasp work can be a fickle game, and you could find yourself facing a tight squeeze because wasps are mysteriously absent in the numbers that the year before promised. Alex Wade, Technical Manager at PelGar, shares some insight into what factors influence a wasp season.

Biological environmental factors affecting wasp numbers

Quick read

Wasps, what are they good for? What do they even do, other than being picnic spoiling, cider stealing, stripy balls of pure rage? I have had wasps ruin a perfectly good day out. I have had wasps ruin a perfectly good day in. I have even had a wasp announce its presence to me inside my car while I was driving along the motorway, therefore also ruining for me the parts between having a good day in and a good day out.

And what’s worse is, when they aren’t actively ruining your day with their haphazard dive bombing of your pub lunch, they can ruin an entire season of work for us… by not even being there at all.
But before we all get too down, it’s important to remember some important things about wasps.

Although we closely associate wasps with infesting eves

Biology and behaviour

Firstly, it is easy to label all wasps as that group of flying insects instantly recognisable by a black and yellow pattern, and propensity to sting first and ask questions later. But this is a largely unfair statement as there are numerous species of wasp in the UK which come in a vast array of stunning colours (like the mesmerising cuckoo wasp, Chrysis ignita) and certainly behave nothing like their more aggressive cousins. It is also often overlooked that, even within the group of social wasps which interest us as pest controllers, we are not looking at one but multiple species of wasp, of which the most commonly encountered are Vespula vulgaris and Vespula germanica.

Therefore I will attempt to explain some of the nuances of wasp behaviour and biology, and consequently how this myriad of seemingly unrelated differences can make wasps some of the most unpredictable insects in our profession.

The wasp life cycle is a fascinating process and is considerably more complicated than one might first assume. To describe it fully we must rather paradoxically start at the end, with the death of last year’s nest. Young mated wasp queens leave the dying nest of the year before and prepare to overwinter in cracks and crevices (such as tree bark, rock formations and quiet areas inside and around people’s homes), protecting themselves from scavenging predators while they hibernate through the depths of winter.

Come the Spring, with the rise in ambient temperature, these queens will emerge and start looking for suitable nesting locations. Although we closely associate wasps with infesting the eves and attics of houses, it is essential to keep in mind that these are not the preferred locations for their nests. In studies (Spradbery 1973) it was shown that almost 77% of Vespula vulgaris and 91% of Vespula germanica nest sites recorded in an area of Hertfordshire were situated in subterranean locations such as in banks, hedgerows and the soil borders of open fields. This preference towards living in the open countryside, despite happily overwintering inside our dwellings, generates a misleading bias when trying to ascertain the size of a wasp season based on the observations in the emergence of the spring queens from in and around homes.

Abandoned wasps nest

During this first couple of weeks after emergence, it is interesting to note that the behaviour of the social wasps will be very similar to that of their solitary cousins. All queens will set out to look for food to fuel the development of their internal organs (a process which is put on pause over Winter to store sufficient energy to make it through to Spring) ready to start producing eggs. At this point, they are then driven to find suitable harbourage within which to begin building their nests and deposit the first of this year’s workers.

With the establishment of the nest, the first generation of eggs are deposited into unique comb structures within the heart of the fledgeling hive. The subsequent larvae are cared for exclusively by the new queen through all five instars of their development from egg to pupa to newly emerged adult wasp. It is with the emergence of this first generation that the social and solitary wasps truly deviate in behaviour, with the offspring of the social wasps remaining as unreproductive workers, contributing to the future growth of the nest. With a fresh brood of workers to take the workload off nest building and larval nursing there is a rapid expansion of the nest, both in size and complexity of the nest’s physical structure but also in the numbers of individual wasps reared and housed within. With this sudden drive for expansion, the demand for sources of protein to feed the growing number of larva increases dramatically, with the primary sources of protein most often coming from insects which the wasps will actively hunt down and predate. Pest insects such as caterpillars and aphids form the bulk of the predatory wasps’ protein sources, which now puts the wasp (rather confusingly) into the beneficial insect category despite us being very certain since childhood of their position anywhere outside the category of ‘beneficial’.

It is at this point that the number of wasp nests, the abundance of wasps residing within them and the comparative sizes of the wasps start to alter from what may have been observed in the previous year(s). Foremost, wasp populations undergo an endogenous mechanism which brings early wasp populations into an equilibrium. This phenomenon is what is mostly responsible for the two-year cycle of wasp abundance and wasp scarcity that we usually observe. Historically, this two-year cycle was so pronounced it was possible to almost set a watch by this cycle of abundance and scarcity. So, what has changed?

Populations of wasps in and around the buildings tend to be closely

Blame it on the weatherman?

Common wisdom would point the finger squarely at a changing climate, increases in rainfall and temperature upsetting some delicate natural balance, but studies (ME Archer 2001) showed that this was not entirely the case. These studies showed that the weather had no significant effect on the number of new nests established in the Spring (and therefore upsetting the two-year cycle), but it did have a significant impact on the sizes of those nests towards the end of Summer. This relationship between the weather at various stages of the nest’s developments ties in nicely with another limiting factor of nest size: food abundance.

Favourable weather conditions see an increase in plant growth and, with that, an increase in the insects that reside on and feed on those plants. So far this all seems somewhat predictable, the two-year pattern of behaviour is largely uninterrupted. So, again, what has changed?

It is now that we must look towards ourselves and point the finger at a different cause. The effect the humans have on the environment is profound, but specifically how we grow and raise plants for our own use is probably one of the most crucial factors in the world’s ecology. When one considers the fact that 80% of wasp populations will nest, feed and overwinter near areas of agriculture, it is not then surprising to imagine that even slight changes in the way that agriculture is undertaken will have a significant impact. Removal or encouragement of hedgerows will alter the amount of available harbourage to one species of wasp or another. The type and density of crop being grown will change the species of prey insect available for foraging wasps as well as their abundance and size. It has been clearly documented (Spradbery 1973) that, between Vespula vulgaris and Vespula germanica, there is a noticeable difference in the manageable size of prey item which can be flown back to the nest. This means some crops and their associated insects will be favourable for one species and not the other. Finally, and possibly the most devastating of all impacts to the growing wasps, is the broadcast use of pesticides over these crops, removing all but a fraction of the available food for the nests as well as coincidently removing those nests close to the boundaries of the crop itself.

All these factors, and more, have an unnatural dampening effect on ‘wild’ populations of wasps. This begins to be of real relevance when one considers that populations of wasps in and around buildings tend to be closely monitored by their human neighbours and will be removed with great prejudice at the slightest provocation. We, as a species have become exceedingly adept at this task and it would be fair to assume, therefore, that most wasp nests found in urban areas do not originate from nests which have managed to survive this annual purging. Instead, they have probably migrated in from more rural locations with queens having been documented travelling up to a kilometre from their overwintering locations to find suitable nesting locations (Crossland 1991). This was clearly shown in a case study undertaken at the Royal Horticultural Society Gardens at Wisley. The study showed that, despite the successful 100% year-on-year eradication of wasps from the grounds, there were always new nests occurring each year without fail. Mainly this can be explained by large reservoirs of wild wasps found out in the greater environment, and that favourable nesting sites would almost always recruit fresh queens each Spring. Therefore the abundance of nuisance nests around peri-domestic locations will fluctuate in direct relation to the success of rural, and not domestic, wasp populations in the previous year.

The bottom line

You can see that wasps are far more than they appear. The historical wisdom of a two-year cycle has been skewed by numerous external factors with many of those factors taking place concurrently with the growth of the population. With this in mind, the final thoughts on wasps would be to prepare for the worst and hope for the best. While you may now understand better some of the underlying mechanics of what builds a healthy population of wasps, you certainly cannot rely on being able to forecast the scale of a wasp season with any reasonable accuracy.

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Pelgar iconAlex Wade
Technical Manager, PelGar

1 May 2018  |  PPC91

Source: PPC91

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