What factors determine the size of wasp populations?
Pest managers and suppliers always like to try and predict whether a big pest season lies ahead. Being able to predict pest numbers can certainly help with business planning – a significant increase or drop in the numbers of a particular pest can have a big impact on business. The appearance of mouse plagues in certain seasons are an obvious example. Alas it’s not always that easy to predict pest numbers, as many factors are in play. However, if you understand the biology of the pest concerned, you have a chance.
Wasps can be a significant source of business for some, especially in areas with the European wasp, so what factors may help predict a big wasp season?
The first thing to consider are the overwintering conditions. Newly mated queens hibernate over the winter. Whilst severe cold can impact surviving queens, they are actually rather tolerant of cold, so at least in Australia the winter temperature does not impact the survival rate of these new queens. Their survival over winter is more impacted by their predation by other animals such as spiders. Their emergence in spring is triggered by the rising temperature and their survival is influenced by the presence (or absence) of flowering plants; they need nectar/sugar, or they will starve.
For established European wasp nests, winter temperature can have more of an impact. Certainly, in cold winters they can die off, much like they do in the northern hemisphere, but in Australia with the warmer winters, the nests often survive over winter and continue to grow year on year.
The biggest impact on wasp nest survival is the spring weather conditions.
Firstly, there needs to be a sufficient number of flowering plants to sustain the new queens as they try to found the nest. But more importantly there needs to be sufficient insects and spiders around for the nest to establish, as these prey items provide the food for the developing larvae. Generally speaking, cold wet springs mean lower prey numbers. Warm, dry springs are ideal to drive the growth of insect populations, especially if there has been some moisture during winter. With much of the east coast having had a (very) wet spring, insect populations will have been kept in check, so survival and growth of wasp nests will have been limited.
Optimal summer conditions for wasp nests are warm and generally dry – plenty of insects and flowers means plenty of food for larvae and adult wasps. Extended hot periods during summer can be an issue. Not only can this impact overall insect numbers, but it can also have a direct impact on wasp nests. In their natural range, summer temperatures are lower than in a typical Australian summer. When there are extended hot periods in Australia, it certainly stops nests expanding and can cause some to come under stress and collapse. This is especially the case for the 20% of nests that are above ground, often in rood voids. The more common nests that are built underground are somewhat protected.
The availability of flowers (nectar) and temperature can also influence the behaviour of wasps and the likelihood of them coming into contact with humans. In hot, dry periods the number of flowers will often decrease. With this decrease in natural food sources and their need for water, wasps are more likely to be spotted around barbecues and outdoor events where they can access soft drinks and water, and pick up food. This increase in human contact will often result in an increase in calls from customers.
So, what’s the expectation for 2022-23? The mild winter would normally have been good for overwintering nests, but with the excessive rain some of the in-ground nests would have been eliminated. The continued heavy rain in spring and the lower temperatures to date will have kept insect prey numbers in check, so wasp nest growth will have been limited.
However, if the La Niña pattern is ending as predicted, the arrival of warmer, drier temperatures will see ideal conditions for most prey insects. So there may be a jump in wasp activity as we move into 2023.