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).
Entomologists in the southeastern USA 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 of them. 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.
Larra analis, native to the eastern USA, is a specialist natural enemy of the northern mole cricket. It has not been seen to attack Scapteriscus 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 defend themselves. 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 Scapteriscus didactylus. 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 Scapteriscus species now present in Florida (short-winged, 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 short-winged mole crickets. It is curious that short-winged mole crickets are the only species shared between Florida and Puerto Rico. There is thus a possibility that it is the short-winged 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% of mole crickets at two favorable sites where chemicals were not used, and where nectar-source plants were plentiful.
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