Larra wasps were traditionally classified in the family Sphecidae, but phylogenetic analysis suggested to W. Pulawski and colleagues that they should be placed in the family Crabronidae (O'Neill 2008).
Larra bicolor is native to South America, where it is widespread. The story of its introduction to Florida is given below. The population that now exists in the Ft. Lauderdale area originated from Belém, Brazil (by way of Puerto Rico). The population that exists in some counties of northern Florida originated from Bolivia. Adults of these two populations can be distinguished by microsculpture of the head. The microsculpture patterns are illustrated in scanning electron micrographs provided by Menke (1992). Its hosts are the pest Scapteriscus mole crickets whose origin, likewise, is South America. Three species of Scapteriscus - S. abbreviatus Scudder, S. borellii Giglio-Tos, and S. vicinus Scudder occur in Florida, and all are attacked by L. bicolor.
Apart from attraction to flowers, wasps are attracted to the chemical phenylacetaldehyde (Meagher and Frank 1998).
When they are not feeding at flowers, wasps are seldom seen. A lucky or patient observer may see a female wasp running on the ground stopping and running again, during the brightest hours of the day. With luck, the observer will see the female wasp enter a gallery (= horizontal burrow) made by a mole cricket. Shortly thereafter, a mole cricket may be seen emerging from the ground, with the wasp in pursuit. She will pounce on it, wrestle with it, and sting it on its soft underside, thus immobilizing it for a few minutes. As the mole cricket lies inert on the ground, the wasp lays a single egg on its underside. The oviposition may easily be staged in a laboratory, by inserting a mole cricket into a glass or plastic vial containing a female wasp. She wastes little time in attacking it. Female L. analis oviposit behind a hind leg of the host (Smith 1935) whereas female L. bicolor oviposit between the first and second pairs of legs of the host (Castner 1988b).
wasp and mole cricket emerging from ground
After the mole cricket is paralyzed, the female wasp may imbibe a little hemolymph from it before she oviposits, in a process called "host-feeding." The purpose of "host-feeding" is obscure.
Once an egg has been laid, the wasp leaves the mole cricket. The latter soon recovers from its temporary paralysis and burrows back into the ground.
The venom of female Larra wasps seems designed to cause only temporary paralysis of the host, in contrast to the sting of closely related wasps in the genus Liris, which attack crickets (but not mole crickets) and whose host does not resume its full repertoire of behavior (Steiner 1984). Yet other relatives paralyze their prey permanently, drag the prey items into to cells that they have dug in the ground, and there lay eggs on them. Larra is thus exceptional among these wasps because females of the other genera dig cells in which to deposit their paralyzed hosts.
Mole cricket nymphs that are less than about a third to half grown are not attacked by the wasps. Otherwise, the wasps seem to show no preference for size of host.
Incubation time of the eggs is temperature-dependent and may be as rapid as four days in summer. Total development time from oviposition to pupation is also temperature-dependent, and ranges from a minimum of about 12 days to a maximum of about 30 days (Smith 1935). Wasp larvae develop as ectoparasitoids, with mandibles inserted into the host. The larvae have five instars (Cushman 1935).
The fully-grown wasp larva pupates among the remains of the dead host, or close to it, in the ground. A cocoon is formed by cementing sand grains or other particles together. The duration of the pupal stage is highly variable, ranging from a little less than 50 days in summer to many months during the winter, in diapause. It is not yet known what initiates or ends this diapause. In the vicinity of Baton Rouge, Louisiana, L. analis undergoes at least three generations during the summer and autumn, but winter causes above-ground activity to cease. The situation is the same with L. bicolor in northern Florida. In effect, adult wasps are present continuously during the autumn months, and generations are not discrete. Adult wasps have been found dead on the ground after the first night of the winter that has below-freezing temperatures.
In 1938, Larra bicolor was introduced successfully into Puerto Rico from Amazonian Brazil as a biological control agent of Scapteriscus didactylus (Latreille). This mole cricket, whose common name in Puerto Rico is la changa (sometimes called West Indian mole cricket) was an important pest (Wolcott 1938). Despite confused accounts in the entomological literature, this mole cricket species does not occur in the mainland U.S.
The importance of Scapteriscus mole crickets as pests in Florida, and news of the successful introduction of Larra bicolor into Puerto Rico as a biological control agent, led to attempts to introduce L. bicolor into Florida. The first attempts were made in the 1940s. The attempts were unsuccessful, and were abandoned when the efficacy of chlordane was discovered as a pesticide for controlling pest mole crickets (Frank 1990). Thirty years of use of chlordane followed, with mole crickets as targets in plantings of vegetables, turf grass, and pasture grasses.
When chlordane was banned by the US Environmental Protection Agency, the Florida Legislature ordered in 1978 that a mole cricket research program be initiated by the University of Florida's Institute of Food and Agricultural Sciences (UF/IFAS). This led to renewed attempts to import Larra bicolor and other biological control agents for pest mole crickets, using earmarked State funds. Because of administrative rules that barred use of such funds for foreign travel, for example to search for biological control agents in the South American homelands of Scapteriscus mole crickets, one option was to import Larra bicolor from Puerto Rico (a U.S. commonwealth, not a foreign country).
Beginning in 1979, UF/IFAS researchers visited Puerto Rico in an attempt to collect living Larra bicolor and bring them to Florida. Researchers involved were R.I. Sailer, J.A. Reinert, W.G. Hudson, J.L. Castner, and H.G. Fowler. Adult wasps were collected in Puerto Rico and were released in Florida at sites where Spermacoce verticillata had been planted as a nectar source to support their existence. Releases were made in Gainesville, Tampa, Lakeland, Bradenton, and Ft. Lauderdale (Frank 1990, Sailer 1985). This was done with the required USDA-APHIS federal permits. A special dispensation was allowed by USDA-APHIS: most such permits require the biological control agent to be reared for at least a generation in a quarantine facility to obviate contamination of biocontrol agents by hyperparasitoids. Because of the difficulty of rearing L. bicolor in the laboratory, it was permitted that the adult wasps be imported and released directly, without a generation in quarantine.
Only wasps released at Ft. Lauderdale established a population. At the four other sites, releases were unsuccessful. The Ft. Lauderdale population dates to 1981 importations. By 1987 it seemed to exist only at the UF/IFAS Ft. Lauderdale Research and Education Center, having failed to survive at a nearby golf course (the golf course management inadvertently destroyed the Spermacoce plants and applied methyl bromide). Subsequent research by J.L. Castner (a) revealed details of its behavior, (b) failed to distribute it to additional localities, (c) documented a parasitism level of < 1% on the mole cricket Scapteriscus abbreviatus (and none on S. borellii or S. vicinus), and (d) showed experimentally that overwintering survival of wasp pupae at Gainesville was much less than at Ft. Lauderdale (Castner 1988a). These findings did not show the "Puerto Rican" strain (originally from Belém, PA, Brazil) of the wasp to be of much value in controlling pest mole crickets in southern Florida, and suggested that winters in northern Florida were disastrous for it.
UF/IFAS researcher F.D. Bennett saw J.L. Castner's findings as a suggestion that Larra bicolor wasps from cooler climates in South America might be better adapted to survival in Florida. With funds (from the UF/IFAS mole cricket research program) and support from Bolivian resident C.J. Pruett, he investigated behavior of L. bicolor at Santa Cruz, Bolivia (Bennett and Pruett 1991, Pruett and Bennett 1991). Then, in 1988 and 1989 he imported living adult L. bicolor wasps from Bolivia for direct release into Florida. Again, USDA-APHIS permitted this direct importation. Female wasps were exposed in quarantine to Florida-grown Scapteriscus mole crickets; these mole crickets, parasitized by Larra eggs, were released in the field by F.D. Bennett, J.H. Frank, and J.P. Parkman. The female wasps, after they had parasitized many mole crickets in quarantine, were likewise released in the field. All imported adult wasps, before release in Florida, were examined microscopically for presence of rhipiphorid larvae: such beetle larvae are reported as parasitoids of Larra, and would be expected to harm and reduce the effectiveness of the wasps; no rhipiphorid larvae were detected. Three release sites were employed, all in the Gainesville area.
In the autumn of 1993, the Bolivian stock of L. bicolor was found to have established a population in the Gainesville area (Frank et al. 1995). Subsequent observations through 2007 showed that the population had spread to at least 40 counties (not yet published). Evaluation by two graduate students (L. Treadwell, H. Cabrera, not yet published) showed the high level of effectiveness of L. bicolor as a biological control agent of Scapteriscus mole crickets at two sites in the Gainesville area.
It is expected that L. bicolor will eventually spread its populations through most of Florida. However, such is its measured effectiveness as a biological control agent that deliberate attempts to hasten its colonization of the rest of Florida are worthwhile. Toward this objective, plans are now being developed to distribute stock to other regions of Florida that yet have no L. bicolor population.
In answer to the question "What if L. bicolor evolves to overcome the defenses of N. hexadactyla?" we have two responses. First, nobody can be sure what direction future evolution will take in any wild organism. Second, a 100-year inadvertent experiment already has taken place. The native wasp Larra analis has not evolved to attack Scapteriscus mole crickets in the 100 years during which it has been exposed to them even though they must surely represent a substantial food source because they are abundant. So, what would be the evolutionary advantage for the introduced wasp to adapt to attack the native mole cricket, which is a relatively rare food resource?
close up of S. verticillata (southern Larraflower)
Larra bicolor adults have been seen abundantly at flowers of a crop of pea plants (garden peas). This offers to growers the opportunity to intercrop peas with other plants needing protection from pest mole crickets.
Authors: J.H. Frank, Department of Entomology and Nematology, University of Florida; and Andrei Sourakov, Florida Museum of Natural History
Photographs: Paul Choate and Andrei Sourakov
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-268
Publication Date: March 2002. Latest revision: April 2009.
Copyright 2002-2009 University of Florida
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adult L. analis
adult L. bicolor
wasp stinging a mole cricket
ovipositor
egg
host-feeding
developing larva
fully grown larva
fully grown larva
S. verticillata