Biological Control of Mole Crickets by Crabronid Wasps

Larra (LAH-ra) is a genus of crabronid wasps, with species occurring naturally in several continents. As far as is known, all Larra species are parasitoids of mole crickets. Hawaiian entomologists seem to have been the first to recognize the potential use of Larra in classical biological control. Faced with problems caused in Hawaii by an immigrant species of mole cricket now believed to be Gryllotalpa orientalis, these entomologists tried to introduce Larra species. The first attempt, with Larra amplipennis from the Philippines in 1921, was unsuccessful. The second attempt, with Larra bicolor from Brazil in 1924, also was unsuccessful. But the third attempt, with Larra polita from the Philippines in 1925, succeeded in getting a population of this wasp established. Little has been written about mole cricket damage in Hawaii since the 1930s, and some have assumed that lack of current importance of mole crickets in Hawaii is due to beneficial effects of Larra polita.

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).

An adult Larra bicolor stinging a mole cricket to paralyze it before laying an egg.

Puerto Rican entomologists, trying to find a classical biological control agent for Neoscapteriscus didactylus (Latreille), journeyed to Brazil in the 1930s. They were successful in transporting living Larra bicolor to Puerto Rico, and getting populations of it established. Little has been written about mole cricket damage in Puerto Rico since the 1940s, and some have assumed that mole cricket damage is now less than it was, and that Larra bicolor has suppressed mole cricket populations. Nevertheless, mole crickets still damage turf in Puerto Rico.

Entomologists in the southeastern United States meanwhile had studied the native Larra analis, which was found to be a specialist natural enemy of the native mole cricket Neocurtilla hexadactyla. A short-lived and unsuccessful attempt was made to import Larra bicolor to Florida in the 1940s.

Soon after the UF/IFAS mole cricket program was begun, a new attempt was made to import Larra bicolor to Florida from Puerto Rico. This time, through much preparation, the attempt was successful in 1981. However, though stocks of the wasp were released at several sites, it only became established at one. This site was the southernmost, at Ft. Lauderdale. It was hoped that the wasp population would grow and expand to additional sites and eventually spread far beyond Broward County, but this has not happened. Wasps captured at the one existing site and moved to additional sites failed to establish additional populations. Failure to establish Larra bicolor farther north than Ft. Lauderdale was attributed to the tropical origin of initial stocks: they came from Amazonian Brazil and, although they might be expected to thrive in Puerto Rico, it was thought that winters in central and northern Florida might be too severe.

It was learned by Puerto Rican entomologists that a wildflower, Spermacoce verticillata, is a favored nectar source of adult Larra bicolor. In Puerto Rico, lack of widespread distribution of the wasp was blamed on the patchy distribution of this wildflower. The same wildflower and some of its native relatives – species of Spermacoce – grow in southern Florida, and the nectar is used by the wasp. The wildflower is not at all rare in Broward, Dade, and Monroe counties. When planted in northern Florida, the wildflower is frozen to the ground in winter but seems generally to survive and put out new growth in spring. Dieback of this nectar source and perhaps others in winter may deny the wasp nectar in central and northern Florida for months. Thus, it may not be susceptibility of the wasp directly to low temperatures, but its starvation in absence of a suitable nectar source, that limit its northward spread. However, this fails to explain its inability to spread in Broward County and southward.

Plots of Spermacoce verticillata can be planted and maintained on golf courses.
Photograph by Sally Scalera.

Life cycle of Larra bicolor

A medium-sized larva of Larra bicolor developing on a tawny mole cricket.

Adult female wasps attack adult mole crickets and large nymphs on the ground surface, and will chase them out of their galleries. The mole cricket is then paralyzed temporarily by a sting to the head from the wasp. Then, an egg is laid on the underside of the mole cricket’s thorax. After a few minutes, the mole cricket recovers from paralysis and continues its activities. After six to seven days a wasp larva hatches from the egg and begins feeding on the mole cricket, destroying the latter in about two weeks. The fully grown wasp larva spins a cocoon in the mole cricket’s gallery and pupates. The pupal stage lasts six to 10 weeks. Male and female adult wasps are short lived when in captivity. In nature their flights seem of short duration. They are most active near the middle of the day.

A fully-grown larva of Larra bicolor amid the remains of a mole cricket that it has killed.

Host-specificity of Larra wasps

Larra analis, native to the eastern United States, is a specialist natural enemy of the northern mole cricket. It has not been seen to attack Neoscapteriscus mole crickets in nature. Larra bicolor, when presented with northern mole crickets in the laboratory, generally is quickly repelled by sticky fluids with which the northern mole cricket defends itself. Larra bicolor eggs laid successfully in the laboratory on northern mole crickets die as larvae.

Larra bicolor was introduced to Puerto Rico in the 1930s to combat Neoscapteriscus didactylus (Latreille). A laboratory study published in 1984 showed, however, that this species of mole cricket is not dealt with very successfully by the wasp. The three Neoscapteriscus species now present in Florida (shortwinged, southern and tawny mole crickets) were found to be better hosts.

The behavior of Larra bicolor was studied in the field at Ft. Lauderdale in the mid-1980s. There, wasp eggs and larvae were not found on tawny and southern mole crickets, even though those mole crickets were attacked by the wasp in the laboratory. Instead, wasp eggs and larvae were only found on shortwinged mole crickets. It is curious that shortwinged mole crickets are the only species shared between Florida and Puerto Rico. There is thus a possibility that it is the shortwinged mole cricket that serves as the major host of the wasp in Puerto Rico, too.

In 1988–1989 a stock of Larra bicolor was imported from Santa Cruz, Bolivia, and was released at three sites in and near Gainesville. The sites had been prepared by planting of Spermacoce verticillata from southern Florida. In 1993, this stock was found to be established. By late 1997, its population was found to have spread at least 30 km to the west and to the northeast. By late 2002, it had spread as much as 280 km to the northwest and south. Evidently it is attacking tawny and southern mole crickets and is finding suitable nectar sources other than Spermacoce verticillata. This Bolivian strain of Larra bicolor evidently is better adapted for Florida conditions than is the “Puerto Rican” strain, and probably will continue to spread. Its effects on mole cricket populations have not yet been evaluated thoroughly. Where a preliminary evaluation has been made, the wasp was estimated to kill almost 70 percent of mole crickets at two favorable sites where chemicals were not used, and where nectar-source plants were plentiful.