I have been prompted by several private e-mails to respond to the findings in the paper written by Harris (Diet of the wasps Vespula vulgaris and V.germanica in honeydew beech forest of the South Island, New Zealand) as cited by Beefriendly. Sadly the forum archives where I previously addressed the paper by Harris a few years ago on this very same subject no longer appear to be available as the archives only extend back to 2015/2014.
There are two critical factors which undermine the relevance of the findings of Harris compared to the UK.
The first and most important is that the work conducted by Harris singularly failed to take into account the different dietary behaviour of wasps depending on where they are in their life cycle and the amount of brood in the nest. Sampling of wasps in the Harris study in both years occurred during the sweet feeding phase, i.e. the equivalent of August through to October in the UK. Typically the hunting phase in the UK comes to an end in July (January in New Zealand) when the queen stops brood production which peaks with the production of sexual castes. It is no surprise therefore in the Harris paper that the prey loads retrieved were miniscule because the nests that were sampled contained little if any brood.
The observation is further corroborated in the Harris paper by the disproportionately high number of foragers returning with nectar filled crops. Moreover, during the sweet feeding phase, foraging behaviour changes dramatically in terms of nest aggregation. During the hunting phase adult wasps are fed regurgitated carbohydrates digested from insect skeletons by the grubs which encourages a significantly higher traffic rate. During the sweet feeding phase, when there is rapidly declining or no brood to feed the adult population, individual wasps spend most of their time away from the nest sweet feeding so the traffic rate falls away drastically and the nests almost appear deserted.
The second major factor which undermines the relevance of the Harris study to the UK is the difference in the availability of honey dew which skews both dietary behaviour (away from hunting) and population densities. The UK simply doesn't have the carbohydrate profile that exists in New Zealand. The Harris paper quotes a wasp biomass in the Nelson region as 5,200 g/ha, i.e. 1,346,800g/sqm. The average weight of a wasp is between 0.07 and 0.1g which calculates through to 13,468,000 wasps per square mile or 2,694 nests (based on an average nest size of 5000 adult wasps). This far exceeds the population dynamics of wasps in the UK and understandably so.
What is interesting though is that the Harris paper quotes the amount of honeydew taken per hectare as between 78 litres and 343 litres, i.e. 78,000g to 343,000g (assuming roughly the weight per ml to be 1) per hectare or 20.2 tonnes to 88.8 tonnes per square mile.
Honeydew contains between 16% (at 9.00am) and 40% (around noon) and up to 70% (on standing) glucose. Insects typically contain between 3 and 6% glucose. Assuming that wasps need to consume the same amount of carbohydrates irrespective of where they are in their life cycle, this extrapolates the tonnage to 202 to 888 tonnes per square mile of glucose equivalent insect mass in free (as opposed to fixed) carbohydrate poor environments.
Nominally the pay load carried by wasps doesn't tend to exceed 30mg. Interestingly wasps manipulate their prey into convenient pay loads by dismembering and dissecting their prey so much of the prey doesn't make it back to the nest. Insect weights vary tremendously from a few mg to tens of grams. The Harris paper quotes wasps feeding on wetas which are large crickets typically between 3 to 6g in weight, i.e. between 100 to 200 times the weight of the average wasp pay load. Just extrapolating by a conservative 10 fold gives 2020 to 8880 tonnes per square mile of insects required to sustain wasps (which should not be confused with the pay load weight).
Taking into account the 2694 nests this gives a figure of circa 3.3 tonnes per nest and these inferences drawn from the Harris paper put the tonnage into the right ball park.
Incidentally, the figure of 4 to 5 metric tonnes is one that I read in a reference book which at the time I thought was a 1000 fold mistake. (For the life of me I can't dig out the reference but it's not too dissimilar to what Nige.Coll posted) and I spent quite a bit of time trying to disprove it but finally came to the conclusion that it is a realistic figure.
Anyway, perhaps the most interesting conclusion from the Harris paper is that wasps represent the most important predatory group preying on insect pests and we eradicate them injudiciously at our peril.
Please please don't set spring traps for Velutina unless its presence has been confirmed. Doing so will IMHO only serve to open the door to allow it to become established.
