For centuries, bats have been killed due to superstition, ignorance and vandalism. More recently, bat populations have been severely threatened by the destruction of their roosting sites, habitat loss and pesticides. In North America, bats have suffered mortalities from wind turbine collisions and a fungal disease called white-nose syndrome. In Africa and Asia, bats are threatened by the demand for bush meat.
Bats are the only mammals capable of true flight. They belong to the order Chiroptera, a word roughly translated as ‘hand-wing’ – an apt description of the animal’s jointed, flexible wing. The bat’s wing gives it more flight options than a bird, with more lift, less drag and greater manoeuvrability, says Prof Kenny Breuer of the school of engineering at Brown University in the US.
The bat’s body shape and flight style, adapted to the specific foraging strategy used in a particular habitat, varies widely. Insectivorous open-air feeders that hunt in open spaces tend to have a clean profile like that of a fighter aircraft. Clutter feeders are designed more like aerobatic aircraft: they are highly manoeuvrable, enabling them to hunt in the cluttered habitats of forests and orchards. Aerial, clutter-edge feeders are between these two extremes, foraging in transition zones between open and forested areas, and their body shapes tend to reflect this.
Insect-eating bats hunt using a type of sonar called echolocation. They emit a high-frequency sound that, after striking an object, returns to the bat in a bounce-back effect. This system makes it possible for the animal to accurately place and catch its prey.
Some 70% of bats are insect-eaters, with the rest being mainly fruit-eaters. There are only two species that blood-parasitise on other mammals. All species are nocturnal and roost during the day in caves, under bridges, house eaves, on trees, in hollowed-out dead trees and in abandoned buildings.
Bats and bugs
Insect-eating bats are voracious feeders. They consume a wide variety of insects, many of which are troublesome to humans. Green stinkbugs are the primary problem pest for South African macadamia growers, with pest numbers peaking from late summer through autumn, says Allan Sutton of Sutton Crest farms in the Hazyview area.
The insects feed on the nuts and cause spotting and pitting of kernels and early nut drop. Uncontrolled populations can cause crop losses of up to 80%. Another pest, the macadamia nut borer, appears from November to January.
According to Prof Peter Taylor, Chair of Biodiversity Value at the University of Venda, “the activity of bats [in macadamia orchards] is highest in summer and autumn, coinciding with annual peaks in stinkbugs, and macadamia nut borer moths, and lower in winter and spring”.
To control the stinkbugs, farmers spray pesticide. “Pyrethroids are sprayed about once a month for seven or eight months. Some farmers spray less, but they invariably have quality issues because the stinkbugs get in,” says Sutton.
Taylor, who studies bats in the Levubu Valley in Limpopo, says that there is an urgent need to demonstrate the economic value of bats to agriculture and to promote the need to conserve them. The Levubu Valley is bordered on the north by Soutpansberg mountains and on the south by macadamia, pecan, avocado, guava and banana production areas.
In Taylor’s initial study, microscopic analyses of faecal material collected from seven bat species showed a 20% to 50% component of the insect order to which the green stinkbug belongs (Hemiptera), but could not identify the families or species of bugs in faecal pellets.
A second study that used a molecular method to sequence fragments showed DNA of the southern green stinkbug (Nezara viridula) in faecal pellets of Egyptian slit-faced bats (Nycteris thebaica), Angolan free-tailed bats (Mops condylurus), African pipistrelles (Pipistrellus hesperidus) and African yellow house bats (Scotophilus dinganii).
Analyses showed 32% of the pellets contained DNA from green stinkbugs and 35% contained DNA from unidentified insects of the same order. “These data provide unequivocal evidence for predation by bats on green vegetable stinkbugs,” says Taylor. “Stinkbugs have been estimated to cause losses of some R50 million annually to macadamia orchards [nationally], and economic damage occurs at very low population thresholds of about 0,4 insects per tree.”
In older orchards where trees are higher than six metres, conventional spray equipment cannot reach the top branches. If the bugs are exposed to chemicals but in an insufficient quantity to kill them, they build up resistance. When the sprayer has passed, they simply climb down the tree to feed again. Bats can be of great benefit to those growers with older orchards and taller trees.
“Increasing local bat populations may prove to be an effective and less environmentally destructive means of keeping insect pest numbers down than simply increasing chemical pesticide use, which may ultimately prove counterproductive if populations of beneficial insect predators such as insectivorous spiders, bats and birds decline through poisoning,” says Taylor.
While there is little doubt about the role that bats play in integrated pest management, Taylor and his team are working towards quantifying this value. This would encourage farmers to take care of bats by putting up bat houses, among other things.
Because previous research was conducted in spring and early summer, the off-peak season for stinkbugs, another study is underway to more accurately estimate the biomass of stinkbugs eaten by bats. Research on an exclusion project, which proposes to exclude bats and insectivorous birds from a macadamia orchard, has been initiated.
Taylor explains the programme should provide an estimate of the savings when bats are present. A third project, involving a bat house monitoring programme, focuses on 72 bat houses in the macadamia-growing regions of Levubu and Nelspruit.
In the US, the value of bats as insect predators is well-established.
An analysis published in the journal Science in April 2011, estimated that the loss of bats in North America could lead to an agricultural loss of more than US$3,7 billion (R45 billion) a year. Mexican free-tailed bats, for example, eat billions of corn earworm moths – one of the worst pests in that country – in Texas alone.
With more research being conducted in SA, the benefits of bats to agriculture should become easier to measure, and the animal’s conservation will be seen to benefit bat populations and farmers.