Engineering and ecological approaches reduce Pacific tree frog intrusion into leafy green agriculture

It’s no secret that consumers expect perfection. In the case of produce, consumers expect a product that is safe for consumption and appealing to the eye. As the interface between wildlife and agriculture becomes increasingly intertwined, the challenge of providing high quality product to consumers becomes more difficult with lapses in quality carrying massive financial repercussions to the producer. Our study will use an integrated approach to improve exclusion methods used to keep frogs from entering production environments. Engineers will improve on the traditional drift fence by testing new materials, designs, and deterrents to determine an optimal design for field sites. We will also test new thermal imaging technology to detect frogs in leafy green environments. From the biological perspective, we will test the efficacy of noninvasive acoustics to attract frogs away from ag-adjacent bodies of water and conduct targeted surveys to better understand local frog populations in leafy green production areas. Our study will provide novel data on fence design, frog detection, animal responses to deterrents and acoustic signals, as well as greatly increase our understanding of frogs and the leafy green environments they use. All components of our study integrate to provide a multifaceted approach to improving frog management.

A widely recognized challenge in wildlife-agriculture conflict lies at the interface of leafy greens production and frogs. Leafy greens in the Salinas Valley requires irrigation which provides attractive habitat to local frog species. Specifically, the Pacific tree frog has become a major concern in production environments with frogs dispersing out to leafy green fields, increasing the potential that frogs will be harvested with produce. Several gaps in knowledge have made it difficult for growers to control frogs. Our project will address several gaps, including fence design, deterrent tests, noninvasive interruption of breeding cycles, and onsite frog risk assessments to gain a more complete understanding of Pacific tree frog biology and production risk. Growers have individually attempted to mitigate frog intrusion but unfortunately, results have not been as successful as desired. Therefore, a critical need exists to develop novel methods to improve frog exclusion from leafy green environments. Traditional drift fences have not been effective but few, if any modifications to the traditional form and materials have been tested. As part of our study, engineers will develop new fence designs that incorporate uniquely textured materials, angles, traps and deterrents, all of which are expected to reduce the climbing ability of the Pacific tree frog. We will use live frogs in controlled environments to test whether they are adversely affected by a given material or design. Optimal test designs will later be implemented in production environments to assess their efficacy in real-world situations. Mating and the annual proliferation of new frogs is a central feature of the conflict between frogs and growers. Frogs use acoustic signals to attract mates and ultimately produce young. We will determine the effectiveness of acoustic playback to detract frogs from ag-adjacent water. Specifically, we will record male mating calls and play them back from a selected location expecting a portion of females to investigate the false calls, thereby keeping their offspring away from sensitive production environments. In order to fully assess the effectiveness of a mitigation strategy, we must understand the local frog populations. We will complete targeted frog surveys at selected sites within leafy green production farms. We will assess frog abundance, density, vegetative cover, size and type of water bodies, as well as peaks in breeding and dispersal. The results of the surveys will guide the development of risk assessments for growers to use on their own fields. We will synthesize the data from all aspects of the project in order to develop a comprehensive and integrated approach to improving frog management. The impact of our study will be immediate in that growers will have access to new information and techniques that can be implemented on their own fields. With sound scientific data backing up the recommendations, growers can invest in management infrastructure that reduces their risk of frog intrusion.

1. Test novel drift fence designs and test non-toxic deterrents to determine whether placement of chemicals or physical materials coupled with improved fence design improve frog exclusion. 

2. Test optical sorting and field identification potential of thermal imager. 

3. Test the efficacy of non-invasive acoustics to redirect frogs away from water sources that are near/adjacent to agricultural fields. 

4. Determine the comparative risk of frog intrusion in different production environments.

Wildlife frequently incorporate agricultural fields into their home ranges, which presents a serious threat to food safety from pathogen contamination of crops. Frogs are one such wildlife group often found in lettuce fields in the Salinas Valley of California. The intense irrigation required by leafy greens provides attractive habitat for the Pacific treefrog (Hyliola regilla), which is the most oft-cited species found in these sensitive production environments. Several gaps in knowledge have made it difficult for growers to sustainably control intrusions of Pacific treefrogs and our project has set out to address these gaps. The purpose of this study was to assess habitatintrusion risks and development of a holistic framework to reduce interferences in crop fields and production environments by gaining a more complete understanding the Pacific treefrog’s biology in the Salinas Valley, Monterey County, California. 

• Fences made of aluminum siding with a 10-cm overhanging lip at the top prevented all frogs from climbing over. This novel fence design was realized at the whole-field scale and showed great promise for deterring frogs from entering a reservoir and for holding up to severe field conditions. 

• Thermal imaging cameras are not effective for detecting frogs. 

• Audio interventions hold great promise as a means to attract conspecific frogs to a wetland site. Our results, however, were based on a small sample conducted during a suboptimal time of year. Thus, even though some frogs appeared to respond positively, we have no way of knowing if the frogs would actually use these sites as breeding habitats. This noninvasive technique should be explored more thoroughly and used in combination with other methods, such as rigorous multiyear acoustic attraction protocols at newly created wetland habitats. 

• Frog calling, and presumed breeding, began in late January and was closely tied with winter rainfall. Peak calling occurred shortly after the rains ceased at the end of March, but breeding likely persisted after this time. Variation in rainfall and frog activity suggests that some regions may exhibit a delay to this general breeding pattern and that the breeding season may extend longer into the Summer at some sites. 

• Based on frog body sizes and sexes, we inferred that adult breeding began in late January and extended into May and perhaps June, with the potential for additional breeding bouts in late Summer if stimulated by rainfall. 

• Dispersing juveniles that were born this year from eggs that were laid during February to April underwent extensive egress from wetlands for starting in May, with major exodus events in June, July, and August. 

• The presence of a fence almost doubled the total number of pitfall-trap captures for small mammals, but standardizing captures revealed species-specific differences. 

• The presence of a fence significantly increased the total number of captures and captures per unit effort compared to a portion without a fence for amphibians. 

• Small mammal population dynamics was variable across sites, with a general increase in activity for all species shortly after winter rains. 

• Reptiles were active from April to September, with two species of lizards the most frequently encountered across all sites. 

• Our study provides empirical information on the vertebrate communities in the Salinas Valley and may help growers to minimize their risks of wildlife intrusion, while promoting environmental stewardship.