Everything the Bureau of Sugar Experiment Stations believed about the giant toad in 1935 rested on one paper, read to a congress in San Juan in March 1932 by a biologist from the University of Porto Rico. This piece is about that paper: what Raquel Dexter did, what she found, and which parts of it reached Queensland.

The island and the grub

The toad had been in Puerto Rico for twelve years when she began. It had been brought from Barbados in 1920 by the federal experiment station at Mayagüez, on the west coast, and set free near the station; a second stock of forty came from Jamaica in 1924; and the plantations had taken it up for themselves, one shipment to the Aguirre estate on the south coast in 1926 being 1,114 animals. By 1930 it was “found abundantly in fresh water ponds all throughout the island.”

The pest it had been brought for was the white grub, the larva of the May beetle Phyllophaga, which eats the roots of the cane. The cost of it can be read in the planters’ books. In the six months from 27 November 1913 to 14 May 1914 the Guánica Central paid $1,876.73 for the collection, by hand, of 1,662,000 adult beetles. One of the big south-coast companies spent $15,000 between 1919 and 1923 collecting ten million. Every method had been tried and, in Dexter’s words, “All other methods of control of this pest have, so far failed.”

The rule

Dexter did not begin with the field. She began with a rule, which she took from the American economic ornithologists who had spent the previous forty years settling arguments about whether birds were worth protecting. “There is only one way to determine accurately to what extent an insectivorous animal is beneficial or injurious; and that is by a careful examination of the contents of a sufficiently large number of stomachs collected at different dates and over a suitable range of territory.” Watching the animal told you what it attacked; it did not tell you what it ate, or how much. The stomach, in the phrase of F. E. L. Beal that she borrowed for her method, “constitutes the court of final appeal.”

The work started in the autumn of 1930 at the suggestion of M. D. Leonard of the Insular Experiment Station, with help from F. Seín in naming the insects. Because the toad feeds at night, she collected early in the morning, at eighteen places around the island from Rio Piedras in the north to Mayagüez in the west and Guayama in the south, five to twenty-seven toads at a time. Each was killed at once by pushing a hunting knife into the base of the skull to cut the spinal cord, opened, and the whole digestive tract cut out and preserved in 85 percent alcohol with a label giving the place, the time and anything about the ground that might help later. In the laboratory the stomach was slit along its curve and its contents washed into a porcelain dish, where the pieces were sorted by animal: heads, wing cases, legs. Each toad had an index card. Specimen 122, taken at nine in the morning on 2 April 1931 in a little garden surrounded by cane at Santa Isabel, had in it eighteen whole May beetles, two more bodies with their wing cases, eighteen right wing cases and eighteen left, fifteen thoraxes, ten heads, nine clusters of abdominal segments, two weevils and the wings of a dragonfly.

What 301 stomachs held

She opened 301 toads in all. Only six of the twenty orders of insects turned up, and one vertebrate, a small frog; the toad’s diet in the cane was narrow, not wide. Weighed by bulk, 51 percent of the food was insects injurious to agriculture, 42 percent neutral, 7 percent beneficial. Two beetles dominated: Phyllophaga, the May beetle whose grub was the pest, and Diaprepes, a root weevil. Between them they were 41 percent of everything eaten. The mole cricket, the “changa,” which was the other great pest of the island, was 2.4 percent; the toads hardly met it.

Then she sorted the toads by where they had been caught, and found the thing that governed the number. The adult May beetle flies at night to feed on foliage, and it is particular about which: bananas, casuarinas, flamboyant trees and pigeon peas are stripped, cane itself hardly touched. At dawn the beetles drop to the ground under the trees they have fed in and go into the soil. So the toads under those trees ate beetles, and the toads elsewhere did not.

• Eight toads taken in the cane fields proper of Central Aguirre, a mile from the mill, on 6 April 1931, had not one May beetle among them. They had seventy Campsomeris wasps, which are parasites of the white grub. “From an agricultural standpoint this is not desirable.”
• Thirteen toads from the banks of a reservoir in the same district planted with bananas and pigeon peas had eaten 104 May beetles and no wasps.
• Thirteen toads from the reservoir called El Destino at Santa Isabel, surrounded by cane but with its banks overgrown with weeds, had eaten eight May beetles and fourteen wasps.
• Five toads from the little garden half a mile away, among the cane at Santa Isabel, had eaten 120.

Adding up the toads taken where the beetles fed, she had eighteen toads with 224 beetles between them from one night, “an average of 12.5 beetles per toad per night.” Adding up the toads taken where nothing grew that a beetle wanted, she had twenty toads with eight. Both numbers are in the paper, a paragraph apart.

The arithmetic and the recommendation

From the first number she did the sum that would be repeated in every newspaper that touched the subject for the next five years. At 12.5 beetles a night, a thousand toads would have eaten the Guánica Central’s 1,662,000 beetles in 133 nights, “a little over half the time taken for the collection,” and the south-coast company’s ten million in twenty-six and a half months. Each female beetle killed before laying was, by an older estimate she cited, forty-five grubs that would not hatch.

Her recommendation followed from the mechanism and not from the number. The toads could only eat beetles that came to the ground where the toads were, so the banks of the reservoirs and the main irrigation ditches, which were damp and full of toads, should be planted with bananas and pigeon peas to bring the beetles down there. “This method will very materially reduce the ‘white grub’ injury after a number of years of persistent practice.” The toad was a tool that worked where it was given the conditions, and she said what the conditions were.

She also cleared the animal of two charges. Planters had reported that toads killed chickens by sucking out their entrails. Amphibians cannot suck; she and Seín put a two-day-old chick in a cage with a large toad and watched the toad flatten itself on the floor in fear, then swallow three mole crickets the moment the chick was taken out. The same toad, offered a small mouse, caught it on the third attempt and swallowed it whole; the casting afterwards held only hair. The toad would eat anything of a size it could take that moved in front of it. That was the point of it, and the warning in it.

What travelled

Three things in the paper reached Queensland. The 51, 42 and 7 percent went into Mungomery’s article of July 1935 unchanged. The 12.5 went into the Cairns Post as “an average of twelve cane-beetles,” and from there into the public mind as the rate at which the toad would eat the greyback. And the general proposition, that a toad in a cane field is a beetle-eating machine, went everywhere.

Three things did not. The eight beetles for twenty toads where no feeding tree stood. The seventy parasitic wasps in the stomachs of the toads from the cane field proper, which is the earliest record of the toad eating the natural enemies of the pest it was meant to control, and the thing that the analysis of 2020 would find had probably undone whatever good it did. And the sentence in which the conditions were stated: that the toad could be used “provided the proper conditions and favorite host plants are used in the banks of the water reservoirs and main irrigation ditches.” Dexter’s beetle came to the ground every morning; her recommendation was to make it come to the ground where the toads were; her number was measured there. In Queensland the beetle stayed in the trees for a fortnight, and nobody planted anything to bring it down, because nobody carried the sentence that said why they should.

Sources

• Raquel R. Dexter, “The Food Habits of the Imported Toad Bufo marinus in the Sugar Cane Sections of Porto Rico,” Proceedings of the International Society of Sugar Cane Technologists, Fourth Congress, San Juan, 1–16 March 1932, Bulletin 74, with its eleven tables. Everything here. dochub.sasri.org.za
• R. W. Mungomery, “The Giant American Toad (Bufo marinus),” Queensland Agricultural Journal 44 (2), 242–248, 1 August 1935, for what the Bureau took from her. era.dpi.qld.gov.au
• “Cane-Beetle. Reported Remedy. Introduction of Toads,” Cairns Post, 26 June 1935, page 6, for “twelve cane-beetles.” trove.nla.gov.au
• Richard Shine, Georgia Ward-Fear and Gregory P. Brown, “A famous failure: Why were cane toads an ineffective biocontrol in Australia?”, Conservation Science and Practice 2 (12), e296, 2020, for the toad’s consumption of the beetle’s natural enemies. doi:10.1111/csp2.296