Dr Peter Taylor
Bat colonies living in the roofs of farm buildings are at best a barely tolerable nuisance to many farmers. What they overlook is the fact that bats are the major predators of night-flying insects, including many that are important agricultural pests.
Some 70% of the world's almost 1000 species of bats eat insects. Bats display a fascinating variety of adaptations for feeding on different groups of insects. Species of slit-faced bats have long ears to detect the sounds made by crickets and other sound-producing insects and their wing-shape enables slow flight and hovering while snatching stationary insects off surfaces. They also use "stealth-bomber"-type sonar (echolocation) calls to locate these insects without being heard. Other bats catch insects in the air, some in open spaces, others in dense vegetation, often using the wing or tail membrane as a scoop. Horseshoe bats have a special kind of echolocation (termed "high duty cycle"), which allows them to use the Doppler shift to detect the flutter of moth wings. Many moths have "ears" (termed tympani) which can detect the sonar calls of hunting bats, allowing them to take avoidance action. Most bats emit sonar at ultrasonic frequencies around 20 - 60 kilohertz (kHz) (humans hear up to 20 kHz), and this is the range at which hearing moths are most sensitive!
In what has been termed an "evolutionary arms race", bats that are moth specialist-feeders have evolved sonar calls with frequencies either above or below this peak hearing range of moths, so "outwitting" the hearing moths. This is exemplified by two rare bat species occurring in KwaZulu-Natal, both specialist moth-feeders. The large-eared free-tailed bat, Otomops martiensseni, has a low-frequency (audible to humans) echolocation call concentrated at 10 kHz, while the short-eared trident bat, Cloeotis percivali, has the highest recorded echolocation frequency of any bat - 210 kHz! Bats with strong jaws, such as the local, yellow house-bat, Scotophilus dinganii, tend to eat hard-shelled insects like beetles, while bats feeding on soft insects, like moths, tend to have weaker jaws.
To return to insects which cause agricultural losses, the Eldana saccharina moth's caterpillar causes many millions of rands damage per year to the sugar industry in coastal Kwazulu Natal (South Africa). Instead of simply wanting to eradicate "nuisance bats" in roofs, sugar growers are beginning to ask whether or not bats may be a useful asset to have around! Recent research in the United States and elsewhere offers tempting evidence that the answer may be a resounding YES! Co-operative Extension Farm Advisor from the University of California, Rachel Long, and colleagues found that agricultural pests featured prominently in the diet of bats occupying the fertile Sacramento Valley. In another study by Long and co-workers, reported in the February 1999 in the New Scientist magazine, California pear farmers suffered crop losses of less than 5% due to the corn-ear moth when a bat colony was situated within 2 km; when the bat colony was situated over 4 km away, crop losses of 60% were reported.
In other words, the presence of sufficient numbers of bats reduced crop damage by 55%! Every year, billions of moths of corn-ear worms, fall armyworms and other insects migrate in swarms from northern Mexico into Texas at altitudes of up to 3 km. These insect swarms cause massive crop losses across the southern and central United States, costing billions of dollars annually. Recent research by Gary McCracken of the University of Tennessee, Knoxville, and co-workers, using radar, weather balloons, bat detectors and analysis of insect remains in bat droppings, has estimated that the 100 million Mexican free-tailed bats occupying Bracken Cave and other major caves in central Texas can eat approximately one thousand tons of insects each night. Even if only 10% of the bat's diet was corn-ear worm moths (at 250 mg per moth), these bats would eat 340 million corn-ear worms each night, saving farmers millions of dollars.
These statistics are encouraging but, of course, do not answer the immediate question as to whether local bat populations have a significant impact on Eldana saccharina numbers. Members of the Museum's Durban Bat Interest Group (DBIG) recently visited the Sugar Association's Experiment Station (SASEX) at Mt Edgecombe to observe bats emerging at dusk from the roof of a photoperiod shed overlooking expanses of sugar cane. Bat detectors were used to detect "feeding buzzes" as bats appeared to swoop (and feed!) over the sugar cane. The visit was timed to co-incide with the period when moths of Eldana would be emerging in number. Bats were caught as they returned to their roost after feeding, stomachs full. Unfortunately, bats mince their food to such a degree that it is virtually impossible to identify the species of insect remains in their stomachs or droppings, but it is hoped that a pilot project will look at identifying a DNA marker that will allow us to determine with accuracy whether or not the bats are feeding in large numbers on Eldana. A further long-term collaborative study involving SASEX and the University of Pretoria will look at the diet of Angola free-tailed bats, Mops condylurus, and little free-tailed bats, Chaerephon pumilus, occupying sugar cane farms on the North Coast of KwaZulu-Natal.
At the same time, a number of local sugar growers are showing an interest in erecting "bat houses" which may accommodate hundreds of free-tailed bats, the species most likely to be colonising farm buildings anyway. Since it is the number of roosting opportunities, and not the food source, which limits bat populations, bat houses are a means of artificially restoring bat roosting habitat that was lost when natural habitats were replaced with sugar cane. More bats must mean fewer moths, which has to be good news for sugar cane growers!
Article by Dr Peter John Taylor, Curator: Mammology.