'Grubby' Research Promises Environmental, Economic Benefits

Tomberlin, a Texas A&M entomologist, is looking into the possibility that black soldier fly larvae – "grubs" to the uninitiated – could be used to turn livestock manure into high-protein feed.

The concept itself has been proven practical for reducing poultry litter: The flies lay their eggs in the animal manure without much encouragement. The eggs hatch into larvae that eat the manure as if it's caviar, growing into fat little creatures that are 40 percent or more protein. The chickens do what chickens do naturally, eat the worm-like larvae with relish, said Tomberlin, who has a joint appointment with Texas Cooperative Extension and the Texas Agricultural Experiment Station.

"No special harvest equipment is needed," said Tomberlin, who is based at the Texas A&M University System Agricultural Research and Extension Center at Stephenville.

Tomberlin expects to find the same manure-reduction techniques that work with poultry can be adapted to Central Texas dairy farms and other livestock operations, including feedyards. Whether the larvae can be recycled as livestock feed is another question, but he has plans to investigate this as well with feeding trials.

Tomberlin does know that when large numbers of soldier fly eggs are introduced to a manure pile, the resulting larvae can reduce dry weight of the manure by 30 percent to 50 percent in two weeks. Preliminary work with calf hutches shows this to be true. (Calf hutches are small, individual, roofed pens for young calves.)

"We also showed the residual manure nitrogen and phosphorus was reduced by half. Reducing phosphorus levels are particularly important, as excess phosphorus can be a primary pollutant," he said. "We're just learning to use what Mother Nature has already provided to reduce pollution."

The soldier fly occurs naturally in Texas, but unlike other fly species, it does not invade houses or become a pest to domestic animals. "The adults are short-lived and prefer to live in a wild environment," he said.

Even better, Tomberlin said, introducing black solder flies to manure can actually reduce the numbers of house flies because the two species compete for larval habitat. Female house flies will not lay eggs where soldier fly larvae are abundant.

In his preliminary study, Tomberlin compared four treatments to calf hutch manure at a cooperating Comanche dairy. Sixteen calves were used in the study. Catch trays, 3 x 4 feet in size, were placed under each calf. After allowing manure to accumulate for two weeks, he inoculated four of the manure buildups with 2,000 immature black soldier fly larvae.

"That's about a 1/4 pint of larvae," Tomberlin said.

He repeated treatment four times over the course of the study, which lasted three months, from late June to late August, the time during which conditions are most favorable for flies to breed and lay eggs.

Eight of the other trays were sprayed with a commonly used fly control pyrethroid pesticide. Four were treated at the pyrethroid's lowest labeled rate; four at the highest labeled rate.

A fourth of the catch-trays were left untreated.

Throughout the three months, Tomberlin and Bob Whitney, Extension agricultural and natural resources agent in Comanche County, weighed the accumulated manure and took samples to estimate house fly larval populations. Tomberlin also measured moisture levels of the accumulated manure so he could determine if any manure reduction was due to its drying out or from actual loss of solids.

Soldier fly larvae did reduce manure. The untreated – either without soldier fly larvae or pyrethroids – averaged 60 pounds of accumulated manure. Those inoculated with soldier fly larvae averaged 43 pounds.

The average weight of the piles treated with the high- and low-levels of pyretheroid was heavier, 65 and 57 pounds respectively. This is because naturally occurring house fly larvae also reduce manure, though not to the degree as do black soldier fly larvae, Tomberlin said. And house fly larvae – were their counts reduced in the manure piles inoculated with soldier fly larvae?

Averaged, the counts show a reduction of numbers, but from a scientific standpoint, there was such a wide range of variation between the various treatments, that he could only honestly label the results as "inconclusive," Tomberlin said.

"The weekly counts were just all over the place, probably because of measuring problems," he said.

Because he was more interested in manure reduction over time, Tomberlin couldn't screen the entire contents of any given tray for house fly larvae. Instead, he resorted to taking small samples and extrapolating estimates for the entire pile.

One interesting note: The house fly larvae counts were actually higher in the piles treated with the pyrethroid insecticide. Tomberlin believes this is not due to a measurement error, but that "some of these insecticides for fly control aren't very effective."

"They also kill biological beneficials that feed on house fly larvae," he said."for instance, parasitic wasps, mites and predatory beetles." Tomberlin believes these measurement problems could be eliminated with an expanded study.

"This was first-year preliminary data. A larger study would (show) what's actually occurring in nature."

He also thinks inoculating piles with larger numbers of soldier fly larvae would show better results. The larvae are easy to rear in trays, he noted.

Tomberlin also said the use of black soldier fly larvae should be well adapted for small- and medium-size livestock producers. His work in Central Texas has to date been only with dairies, but the biological controls show promise to work in other confined livestock operations, such as poultry and swine.

"I'd be eager to collaborate with any swine or poultry producers," Tomberlin said.