The battle commenced above the cotton field one hot February night. Cotton bollworm moths, whose larvae ravage cotton bolls, cut erratic paths across the dark sky. Bats of up to half a dozen species raced in hot pursuit, their echolocation calls beeping wildly on our bat detectors. And that aerial combat is exactly what I had come to Narrabri, in the cotton country of southeast Australia, to study.
Narrabri, at the edge of the outback in New South Wales, reminded me of home in California’s Sacramento Valley. Both areas are studded with vast cotton fields interspersed with rolling hills of golden grass and trees.
A single river, the Namoi, flows through Narrabri, providing water for farmers and others. Most of the cotton in the Namoi River Valley is produced on family farms. Irrigation water is pumped from the river after rainstorms and stored in lakes scattered around the fields. These lakes, which generally cover 3 to 5 acres (1.2 to 2 hectares), are full of old-growth eucalyptus trees that had been left standing when the land was cleared for farming. The areas around the lakes were teeming with birds – and, we soon discovered, with bats.
Soon after arriving, I joined colleagues Greg Richards, an Australian bat biologist; Monika Rhodes, a Ph.D. student from Australia’s Griffith University; John Westbrook, Research Leader at the U.S. Department of Agriculture’s Areawide Pest Management Research Unit in College Station, Texas; and our host, Martin Dillon of the Australian Cotton Research Institute in Narrabri. Our project, with obvious implications for bat conservation, was to study the role of bats in controlling cotton pests. We were funded in part by Bat Conservation International.
The major pest of cotton in Australia is the cotton bollworm. The eggs of this night-flying moth hatch into larvae that feed on developing cotton bolls, severely reducing yield and fiber quality. Growers now use various methods to control these pests, including insecticide sprays that are costly to the farmer and the environment. Our goal was to document the impact of bats in helping to control this very expensive pest.
The Australian villain is the Old World bollworm (Helicoverpa armigera). It is very similar in size, appearance and agricultural devastation to its western hemisphere relative, the New World bollworm (Helicoverpa zea). Both moths lay their eggs – from about 500 to as many as 3,000 per female – on plants. The caterpillars (larvae) that emerge grow to an inch (25 millimeters) or more long and eat just about anything that grows, including each other. Besides cotton, corn and tomatoes, they feed on food crops from beans to watermelons and a variety of ornamental plants. Their commercial damage in the United States alone exceeds $1 billion a year.
In North America, insectivorous bats – especially Mexican free-tailed bats (Tadarida brasiliensis) – are prodigious predators of cotton bollworm moths. Their presence in the cotton fields can reduce the need for pesticides. We hoped to document a similar value of bats against the Old World bollworm in Australia.
Armed with Anabat echolocation detectors, night-vision goggles and infrared video cameras, we headed for the fields and spent many a night scouring the sky for the sight and sound of bats on the hunt. The nights were warm and humid, for February is midsummer in Australia, and we were overdressed in hats, gloves and long-sleeved shirts to deter mosquitoes, some of which carried the Ross River virus, a local disease that causes severe flu-like symptoms.
I also always wore boots for fear of snakes. I had spotted a couple in the area, including highly poisonous red- and blue-bellied black snakes – which my host insisted I was “lucky” to see. During our midnight breaks for tea and biscuits, I was extremely careful where I rested.
We quickly found that night skies above the fields were aswarm with flying bats and insects. In cotton fields adjacent to irrigation ponds with old-growth trees, we recorded six species of insectivorous bats. These included the white-striped freetail bat (Tadarida australis), southern freetail bat (Mormopterus planiceps), little broadnosed bat (Scotorepens greyii), little forest bat (Vespadelus vulturnus) and the less common chocolate wattled bat (Chalinolobus morio) and yellow-bellied sheathtail bat (Saccolaimus flaviventris).
All six are known to roost in old tree hollows and are listed as vulnerable to the clearing of land for agriculture. These bats were continuously active over the cotton fields from dusk until we left the fields well after midnight.
In cotton fields that had been completely cleared of native vegetation for miles around, we spotted only two species of insectivorous bats foraging over the cotton: the white-striped freetail and the southern freetail bat. In these large-scale, monoculture cotton fields, both species generally showed up after dark and activity was sporadic. Since these bats are fast flying and capable of long-distance flight, we surmised that they roosted in remnant old eucalyptus trees in the foothills of the local mountains and along streambeds and roads.
After many nights of observations in different fields, we clearly documented the importance of local roosting sites in old-growth eucalyptus trees for enhancing the diversity and abundance of bats that prey on cotton pests.
There is no doubt that the bats were hunting the insects in the cotton fields. When foraging bats appeared over a field, insects promptly took evasive action or fled. We observed moths spiraling into the cotton or suddenly increasing their flight speed. We believe bats have an impact on these moth pests by reducing numbers and disrupting their mating and egg-laying activity. Bollworms are among insects that are biologically equipped to hear bats’ ultrasonic echolocation calls, and when they do, they obviously take evasive action, attempting to avoid areas where bats are most active.
After many hours of monitoring bat activity and counting bollworm eggs on cotton plants, however, we were unable to document clear differences in the number of eggs as a result of bat activity. A severe drought in the region kept bollworm pressure too low to measure significant differences during our visit. Further investigation is needed. We were, however, able to demonstrate the great importance of old-growth eucalyptus trees in maintaining the diversity and abundance of bats on cotton farms. This information is being disseminated to cotton growers at local and regional meetings, and farmers are beginning to realize the economic value of conserving trees on their farms to enhance bat activity for natural pest control.