These fascinating winged mammals could be your secret weapon against mosquitoes
Summer in South Africa usually means having to ward off swarms of mosquitoes. Forget about chemical pesticides and get the senior management of Mother Nature’s bug patrol on your side to restore peace and quiet, says Jonathan Haw of EcoSolutions.
How many mosquitoes can a bat eat in one night? The answers from my colleagues in response to this FAQ from our clients seems to range from “1 000 mosquitoes in the first hour after dark” to “10 000 per night” to “a lot … heaps!”. Once, only once, I even heard someone proudly declare “All of them!” I then started contemplating how one could work it out…
For the sake of a simple answer, let’s use the Cape serotine bat (Neoromicia capensis) as the predator and the mosquito (Culex pipiens) as the prey – both species are common visitors to gardens throughout South Africa. Again, to keep it simple, let’s suggest a catholic mosquito diet for the duration of this dusk-till-dawn equation.
The number of insects consumed by a bat varies considerably according to species, season and reproductive cycle. It is estimated that a little brown bat (Myotis lucifugus) in captivity consumes roughly 25% of its body weight in insects each night. Obviously under natural conditions this intake would increase as lots of energy is required in the predation process. It is estimated that on a night during the peak of lactation, a little brown bat weighing 7,9g needs to consume 9,9g of insects (well over 100% of its body mass).
In the same vein, the Cape serotine bat weighs 6g to 10g, while a household mosquito on average weighs 2,5mg. If we therefore have an 8g bat requiring a conservative 65% of its body weight to sustain it, and it exclusively eats mosquitoes that weigh 2,5mg each, that equates to an estimated 2 080 mosquitoes per night. (This rough calculation also disproves the notion that bats have a natural tendency to fly into our hair… When you need to reach a nightly quota of 2 000 mosquitoes just to stay alive, who has time for that?)
Furthermore, Cape serotine bats roost in families of 15 to 20 individuals, and if each one eats approximately 2 000 mosquitoes per night, one family’s consumption comes to a staggering 30 000 to 40 000 mosquitoes each night. Of course, in reality, bats are generalists and will consume moths and other nocturnal arthropods too, and once a couple of juicy moths are included, the number of mosquitoes consumed will drop accordingly.
THE RISE OF BAT HOTELS
It is believed that a lack of suitable bat roosting sites is a serious limiting factor within both urban and agricultural landscapes. Bats tend to roost in caves and tree cavities, or behind the bark of dead trees.
The De Hoop guano cave near Bredasdorp is home to the largest aggregation of bats in South Africa. The colony of about 300 000 bats roosting there consume an estimated 100 tonnes of insects annually.
Many of the insectivorous species hibernate during the winter months and reappear in spring, when insect numbers increase.
Another fascinating aspect of Cape serotine bats is that, along with yellow house bats (Scotophilus dinganii), they will readily utilise man-made bat houses and are actually the most common occupants of the bat boxes put up by our company, EcoSolutions. We currently manage and monitor more than 200 bat hotels on citrus farms in the Western and Eastern Cape, and nearly 100 on macadamia farms in the Limpopo province. These projects are part of a master’s dissertation on the efficiency of bats in controlling the false codling moth (Thaumatotibia leucotreta), an agricultural pest in South Africa that has been estimated to cost local fruit growers between R300 million and R500 million annually.
The idea of using insectivorous bats for pest control is not new and can be traced back to the now famous Campbell Bat Towers of the 1920s. These towers, of which a few still remain, were built in accordance with blueprints supplied by Dr Charles Campbell, a physician from Texas in the US, in an attempt to reduce the incidence of malaria. Although ideologically sound, Campbell’s towers were not very successful, but fortunately we’ve come a long way since then.
A TWO-PRONGED APPROACH
Primarily, bats reduce insect numbers in two ways. The first is by direct predation, namely the actual consumption of insects by a bat or a bat colony. Secondly, bats exert significant pressure on insect populations through behaviour trait mediation.
Here’s how the latter works: tympanate insects (those with a specialised hearing system based on sound waves, that is) can detect the use of echolocation used by bats and, consequently, will attempt to evade and avoid predation. Tympanate moths exhibit different responses to bat echolocation, relating to both the distance from the bat and the frequency of the echolocation pulses. >
Moths at a certain distance from the bat will often flee, while moths closer to the source may attempt to evade more imminent predation through irregular flight, power dives or passive falls.
In addition to the response of various moths to the distance from the source of the echolocation pulse, scientists have also recorded an evasive response to the repetition rate of the pulse. The conclusion is that moths will respond to both the audibility of the pulse and its frequency, thus enabling them to determine bat feeding “buzz” and respond accordingly. Predatoravoidance behaviour is so acute in some types of moths that they will abort sexually orientated flight response to pheromone-broadcasting females. In other words, it could even affect their reproduction and thus their population negatively.
The predator avoidance behaviour recorded in moths is thought to be the major cause of the 50% reduction in damage by corn borer (Ostrinia nubilalis) when bat-like ultrasound pulses were broadcast over maize farms in the US. This means that insects not specifically targeted by the feeding bats will also leave or avoid the area where bats are hunting.
Besides the increased insect consumption and the insect behaviour modification associated with higher bat populations in agriculture, the implementation of successful bathouse programmes may lead to a reduction in the number of insecticide applications required annually. Pesticide resistance is a real problem in the chemical control of insects. As a result of the frequency of the application, insects become resistant to the pesticide very quickly. By reducing the number of pesticide applications each year, one effectively prolongs its efficacy. Finally, the incorporation of insectivorous bats into the management and control of herbivorous arthropods and the potential reduction of insecticide applications associated with this, will lower the risk of insecticide residues reaching consumers and reduce the risk to human health.
PRACTICE MAKES PERFECT
Bat boxes are a valuable contribution to any garden or agricultural undertaking. They’re fun to make and when they’re placed correctly, you’ll be rewarded with frequent occupation. Unlike rodent droppings, bat guano is dry and makes for a fantastic fertiliser, so placing a plant pot beneath your bat box is a good way to collect guano with which to nourish plants.
The use of insectivorous bats in a biological control programme is a science, of course, and should be viewed as such. Although the study of artificial roost site selection is ongoing, there are some things of which we are now reasonably sure: location, orientation, the type of box, the environment and proximity to water are all important components in turning a bat house into an occupied bat house.