Bats as a component of integrated pest management

Bats as a component of integrated pest management

Ecosystem Services provided by bats.

Through dietary analysis, many species of prominent agricultural pests have been recorded in the diet of insectivorous bats (Whitaker, 1995)  Research in the Midwestern United States calculated that a colony of 150 big brown bats (Eptesicus fuscus) consumes approximately 600 000 cucumber beetles, 158 000 leafhoppers, and 335 000 stinkbugs.  Assuming that each female cucumber beetle lays 110 eggs it is suggested that predation by the big brown bats could prevent the production of 33 000 000 cucumber beetle larvae, which are recognised as a severe agricultural pest (Whitaker, 1995). The value of the pest consumption service provided by bats to the North American agricultural industry has been estimated to roughly $22.9 billionper annum (Boyles et al., 2011).

In South Africa, a colony of 300 000 Schreiber(‘)s long-fingered bats (Miniopterus schreibersii natalensis) in the De Hoop Nature Reserve consumes an estimated 100 tonnes of insects annually, making an invaluable contribution to the pest control on farms in the Bredasdorp area (Taylor, 2000). 




Bats as an IPM tool

The predatory nature of insectivorous bats coupled with their voracious insect consumption has indicated their significant ability to suppress insects over agriculturally intensive areas (Federico et al., 2008; Cleveland et al., 2006). As a consequence of their high mobility, bats are able to cover great distance to take advantage of localised insect abundance (Lee and McCracken 2005) and have been found to respond quickly to these insect aggregations (Pocock and Jennings 2008). In addition to their quick response time to prey abundance, insectivorous bats will also revisit the area of abundance over consecutive nights (Kennard 2008). There have to date been various studies which attest to the valuable contribution made by insectivorous bats as part of integrated pest management strategies. A colony of 30000 bats (spp) residing within a mile of a pear orchard in Rockville (USA) were responsible for a 83% reduction in Codling moth (Cydia pomonella) damage (Long and Hogan, 1998), while an Oregon farmer, through the placement and occupancy of bat houses reduced insecticide applications for Corn ear-worm pests from 13 per annum to just 2 (Murphy 2003).

In addition to the studies showing the positive correlation between insectivorous bats and their impact on insect pest species, there are also studies that indicate the negative findings associated with the absence of insectivorous bats. A study undertaken where bats were excluded from coffee plants through the netting of individual plants within a plantation indicated an 84% increase in insect densities on the plants from which bats had been excluded (Williams-Guillen et al., 2008).

When evaluating the contribution made by insectivorous bats to the agricultural sector, both the direct consumption of invertebrate arthropods and the trait mediation interactions that affect prey populations should be considered (Preisser et al., 2005). Tympanate insects can detect the use of echolocation used by bats and consequently, will attempt to evade and avoid predation (Fenton & Fullard, 1979).   Tympanate 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 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).