About 60% of the frogs were able to scale the window screening, and 55% made it over the silt fencing. But only 8% were able to climb over the sandpaper barrier, and none made it over the aluminum siding.
Davidson said the two top barriers work in different ways. Pacific tree frogs use suction-cup-like toe pads and secrete a liquid to aid suction as they climb. The sandpaper's rough texture reduces the efficacy of the frog's suction system.
The aluminum barrier - and particularly the lip - creates an uncomfortable environment for frogs, which don't like to be in the open, Green said. The frogs that did climb the aluminum barrier hid under the lip and never made it over the top.
The aluminum barrier also was effective on small mammals, lizards and other small trespassers, she said.
Part of the project looked at whether pre-recorded frog calls could be used to prompt frog movement. Although the results were promising, Green cautioned they were only from three sites.
"There was one male that would position itself right by the speakers," she said. "They do respond to the vocals. There would be no calls but when the recorded calls started, it induced activity and that could potentially get the frogs to move. Then the question becomes, where do you want them to move to?"
The researchers also sought to learn more about the Pacific frog biology to determine when they were the most active and potentially could move into fields.
"The take-away is we saw activity at different times of the year than expected," Green said. In February and March, for example, full-sized adults were ramping up to breed. This differs from a guide book that said Pacific tree frogs are most active in November and December.
"It helps to know when frogs are likely to be active and near their fields so growers can decide what type of fencing to use," Green said. "They can put in fencing that will be permanent or they can do something that is temporary."
The methods also may reduce frogs in the growing environment without negative impacts to wildlife in the area.
The researchers credit their Salinas Valley cooperators and the California Leafy Greens Research Program advisors for their project's success.
"I don't know if we could have done the study without the cooperators," Davidson said. "They helped us find sites for the study. They gave Danny and our whole team access and provided people to help install the fences. They did more than we could have asked."