The idea behind using insectivorous bats to control pests is not new and can trace its early origins to the now famous Campbell bat towers of the 1920’s. These towers, of which a few still remain, were built by Dr. Campbell, who installed them in an attempt to reduce the malaria cases he was seeing at his medical practice. The towers, although ideologically sound, were unfortunately not very successful. We have come a long way since then.
Let us start with a few basic questions and a few basic answers…
Q. If bats can control insects on my farm, why aren't they doing it already?
They would if they could, however, as a consequence of early farming practice, bat roost sites in the form of dead trees were removed. This was done either to clear areas for the planting of crops, or for the provision of firewood to staff and farm residents. Additionally, bats were often persecuted when they attempted to find artificial roosting sites within the ceilings and eaves of farm buildings.
Q. I have so many insects, are bats really capable of making a difference?
Primarily, bats reduce insect numbers in two ways. The first is direct predation, which roughly translates into how many insects a bat or a bat colony consume per night. The number of insects consumed by a bat varies considerably by 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 greater energy expenditure is required in the actual predation process. (Kurta et al., 2010) estimates that at a peak night of lactation, a 7,9g Little Brown Bat needs to consume 9.9g of insects (over 100% of its body mass). This increase in consumption is a result of the costly energetic expenditures required during the reproductive cycle (I’m sure most of us can relate?). At peak lactation, a female Brazilian Free-tailed Bat (Tadarida brasiliensis) can consume up to 70% of her body mass in insects each night. Like the Little Brown Bat, the Cape Serotine Bat (Neoromicia capensis) weighs between 6 - 10g. On average, a False Codling Moth (Thaumatotibia leucotreta) weighs 65mg. There are 1000 milligrams to a gram (just a reminder). If we have an 8g bat exclusively eating False Codling Moth nightly and requiring a conservative 65% of its body weight to sustain it, we come up with a moth consumption of 80 moths per night.
Secondly, bats exert significant pressure on insect populations through behaviour trait mediation. Tympanic insects can detect the use of echolocation used by bats and consequently, will attempt to evade and avoid predation (Fenton & Fullard, 1979). Tympanic moths exhibit different responses to bat echolocation relating to both the distance from the bat and the frequency of the echolocation pulses (Roeder, 1964; Acharya & Fenton, 1992). Moths at a distance from the bat will often flee, while moths closer to the source may attempt to evade more imminent predation risk through an irregular flight, power dives or passive falls (Roeder, 1964). In addition to the response of various moths to the distance from the source of the echolocation pulse, an evasive response to the repetition rate of the pulse has also been recorded. The inference is that moths will respond to both the audibility of the pulse and the frequency of the pulse, thus enabling moths to determine bat feeding buzz and respond accordingly (Roeder, 1964). Predator avoidance behaviour is so acute in some pyralid and noctuid moths that they will abort sexually orientated flight response to pheromone broadcasting females (Acharya and McNeil 1998). 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 broadcasted over corn farms in the US (Long & Hogan, 1998). The benefits of trait mediation may also extend to the suppression of herbivorous arthropods that may not be the selected dietary preference for insectivorous bats where aggregations of specific insect species are preferred (Preisser et al., 2005).
Q. I am considering using sterilised insect technique to control moths, will bats and bat houses be in conflict with this?
Not only are bats and bat houses not in conflict with SIT, they actually compliment the sterilised insect technique. The release of large numbers of sterilised insects into an area acts a bit of dinner bell for insectivorous bats. Bats have been recorded frequenting areas where insect numbers are high over consecutive nights. The high mobility afforded bats allow them to travel to insect eruptions. The use of sterile insect technique should, as a point of departure, include the placement of well situated bat houses which will afford bats that are feeding on the higher insect numbers, the opportunity to take up more permanent residence and contribute to the pest management of both targeted pests and also the potential crop pests not targeted by the application of Sterile insect technique.
Q. I have tried bat houses/my neighbour has tried bat houses, yet we have never experienced any occupancy.
Although the study of artificial roost site selection in insectivorous bats is ongoing, there are some things that we are now reasonably sure of. We know that location, orientation, type of box, environment and proximity to water are all important components in turning a bat house into an occupied bat house. A project undertaken by EcoSolutions in conjunction with the University of Venda and Subtrop, placed 100 bat houses within Macadamia nut orchards to assess the mitigation potential of bats on Stink bug populations. A year after installation, bat house occupancy has been recorded at all sites. The use of insectivorous bats in a biological control programme is a science and should be viewed as such. There is now a blueprint to successful bat house placement and bat house occupancy. EcoSolutions currently installs and manages bat house projects nationally. Through satellite imagery, we assist with the correct placement of bat banks on farms, the correct bat house designs ideally suited for potential bat occupancy and additionally, the management and service of bat houses within these projects.
In addition to the increased insect consumption and the insect behaviour modification associated with higher bat populations in agriculture, the implementation of successful bat house programs may additionally lead to a reduction in the number of insecticide applications required annually. Aside from the cost benefits associated with reduced insecticide applications (Banaszkiewicz, 2010), it is held that the reduced number of applications will prolong the evolved resistance period by the target species to the insecticides active ingredient.
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.
