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Taming Tropical Invasives

Raised on a central Florida orange and cattle ranch, Joe Walter has been battling invasive tropical plants since he was a kid. “Because of our climate, there is no letup,” he says. “Around here, historically, it has been a 365-day-a-year job.”

Walter, who runs an Angus-cross cow-calf operation and also holds a position with Florida Extension Service, believes that a major change to that cattle producer/weed relationship could be in the future. “As we do more research on these weeds, we are learning more about their vulnerabilities and how to turn those vulnerabilities to our advantage,” he says. “In a sense, we are working smarter, not harder.”

He notes that one area of study that has proven to be particularly productive is the practice of investigating the interspecies relationships of invasive plants in their countries of origin. 

“Originally, all these plants come from an environment that has natural enemies to control their populations,” says Walter, adding that identifying those agents is the first step to developing an effective long-term biological-control program.

Tropical Soda Apple

One South American invasive has plagued southeastern cattle operators since it first appeared in Florida’s Glades County in 1988. The tropical soda apple (TSA) is known for its thorny presence and fierce competitiveness.

“Tropical soda apple was a real problem for ranchers across the South,” says Walter. “In order to deal with it, we needed to look beyond our conventional control systems.”

In 1994, researchers began identifying insects that specifically preyed on the TSA in its native habitat. One insect identified and later introduced from Paraguay and northern Argentina was the TSA leaf-feeding beetle, Gratiana boliviana. In 2003, after it was determined the insect did not pose a threat to Florida agriculture or the environment, the beetle was released.  

“This little green bug only feeds on TSA plants,” says Walter. “Before being released, the scientists made sure that they weren’t introducing an insect that would also attack one of our crops.”

TSA Busted

As Walter goes on to explain, the objective of introducing beneficial insects and other biological-control agents isn’t so much to eradicate the host plant as it is to impose a significant negative impact on its population. Once that has occurred, equilibrium is usually reached between the insect population and the host population.

He says when biological controls have successfully established themselves, other control costs drop, while more acreage is restored to grazing and made available for shade.

In order to evaluate the TSA beetle’s effectiveness as a biological-control agent, a 2010 survey asked Florida ranchers about the financial impact the beetle had on their TSA-control programs. Preliminary results indicated a cost savings in control measures of approximately 50% statewide. If verified, it would mean a statewide savings in control costs of $3.25 to $8 million annually.

Cow-calf operator Buzz Eaves of St. Lucie County doesn’t have to wait for verification to be convinced of the value of the tiny green beetle from Paraguay. Since 2004, when UF/IFAS researchers first released a colony of  Gratiana on his 1,300-acre LL Ranch, he has seen a steady decline in his resident tropical soda apple population. “I used to spend up to $20,000 a year on TSA control,” he says. “Now it’s down to almost zero.”

Ken Gioli, one of the University of Florida Extension agents involved in introducing the beetles to the LL Ranch, refers to a follow-up survey of release sites throughout Florida that documented a 70% to 90% reduction in TSA plant material after the insects established themselves. 

Smut Grass Breakthrough

If tropical soda apple has a grass counterpart, it is giant smut grass. An invasive bunch grass perennial from tropical Asia, the smut grass is a serious competitor in improved perennial grass pastures throughout the southeastern U.S. It is estimated that over 75% of Florida’s pastures contain smut grass.

“It will get into a Bahiagrass pasture and just take over,” says Walter. “Cattle won’t eat the mature smut grass, so the carrying capacity of the pasture decreases as the once-dominant Bahiagrass is replaced by the smut grass.”

He says conventional weed-control agents are often too expensive to justify their use. Economic analysis shows that in order for herbicide treatments to be cost effective on smut grass, ground cover infestations need to be 30% or higher.

In 2009, a working group of interested ranchers, members of the University of Florida Extension Service, and the NRCS participated in a unique study that explored the use of rotational mob grazing at a high stocking rate to control smut grass infestation in Bahiagrass limpograss pastures.

Initially, plots were segregated into three separate treatments. The first was the control, which involved intensive grazing only. The second required mowing off the mature smut grass and then exposing the newly emerging grasses to intensive cattle grazing. The third used burning to remove the mature smut grass and then, like the others, it was followed by intensive grazing. Plant counts were done prior to launching the study and then done one year later.

While the number of smut grass plants rose in the control group and fell slightly in the mowed group, the number of smut grass plants in the burn group dropped significantly – from 3,100 plants per acre to 2,000 plants per acre. There was also a reduction in the circumference of the surviving plants from an average of 321 inches to 29 inches. 

“Unlike the mature inedible smut grass that dominated the pastures before the burn, the newly emerging smut grass shoots were highly palatable to the cattle,” notes Walter. He says the heavy grazing on the smut grass regrowth offered additional space for the Bahiagrass to reestablish itself.

In spite of the promising results with burning and grazing, Walter admits that the practical implications of congregating hundreds of cattle for short-term mob grazing events could place serious limitations on it becoming a widespread system for controlling smut grass.

“It is obvious that this system isn’t for everyone,” says Walter. “For those having to deal with smut grass on a daily basis, it is another tool in their weed-control toolbox.”

If you want to learn more, contact Joe Walter at 321/633-1702, ext. 234. 

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