By Khuong B. Nguyen, 1988
DISSERTATION SUMMARY
A search for natural enemies of mole crickets in South America revealed a steinernematid nematode parasite of mole crickets which may limit increase of the population. The nematode was brought to the University of Florida to determine its potential as a biological control agent of mole crickets imported accidentally from South America about the turn of the century. Studies have shown that the nematode does not fit currently described species, and it is described herein as Steinernema scapterisci n. sp. This nematode can be distinguished from other species of the genus as follows: female with thick cheilorhabdions, an excretory duct that forms a large doughnut-shaped structure near the base of the esophagus, and an epiptygma. The spicules of the male taper gradually to narrow point, the shaft of the spicules bears a sheath, and the gubernaculum has a long neck, which is bent upward anteriorly. The body length of the juvenile is in the range of that of S. carpocapsae, but it differs by having a ratio of the measurement from the head to the excretory pore divided by the tail length, which is 0.73 for the new species and 0.60 for S. carpocapsae. It does not reproduce well, if at all, in Galleria mellonella, which is a universal host for all other species of the genus. Its protein patterns are different than those of S. carpocapsae, and it does not cross hybridize with it.
Biology and Ecology
Steinernema scapterisci enters the host through the mouth and spiracles, then either penetrate the digestive system or break through the tracheal system to enter the body cavity of the insect. The life cycle and sex ratio of the nematod is influenced by temperature. At 10 to 15 C the life cycle is not completed, at 20 C the cycle takes 12 days to complete, at 24 C 10 days and at 30 C 8 days. At 15-24 C the number of females in the population is greater than the number of males, but at 30 C the reverse occurs.
When released on the soil surface, the nematode moves down to seek out, enter and kill mole crickets at least 10 cm, in the soil. When placed in the center of a column of soil, some of the nematodes move upward and some downward with more moving downward.
S. scapterisci can survive in soil without a host for at least 10 weeks and retain its ability to kill mole crickets.
Some factors which may influence the effectiveness of the nematode to kill mole crickets are the fact that mole cricket hemolymph can encapsulate the nematode, and the presence of a mite, Rhizoglyphus sp., which consumes nematodes, on mole cricket body. There are many other factors, however, that increase the killing ability of the nematode. Infected mole crickets die in the soil, which increases the survival chances of the nematode; mole crickets can fly a long distance which increases the dissemination of the nematode; and since mole crickets in soil, usually use tunnels made by other mole crickets increases the chance of a mole cricket being infected by nematodes which were produced in the cadaver of a mole cricket which died in its tunnel.
The nematode can be produced in vivo in mole crickets and house crickets, and in vitro in liver medium.
Steinernema scapterisci as a Biological Control Agent of Mole Crickets
This new species appears to have a relatively narrow host range being quite specific to mole crickets and a few other Orthoptera. In a host range experiment with mole crickets, house crickets, field crickets, granulate cut worms, wax moth larvae, fall army worms, cock roaches, honey bee adults, velvet bean caterpillars, earth worms and two different beetles which prey on mole crickets, only mole crickets and house crickets were killed at a rate of 100%. Field crickets were killed at 22 %, and all others at 10% or less. None of the earth worms or the two predatory beetles were killed.
When released in the field 3 1/2 years ago in North Florida, the nematode became established, continues to kill mole crickets, and has become disseminated at least 24,000 meters from the original release sites.
Results of these investigations indicate that S. scapterisci is a good candidate for biological control of mole crickets.
Nematode Parasites and Associates of Mole Cricket
A survey of nematodes on mole crickets was carried out and the following nematodes were found:
Parasite
Steinernema spp.
Associates
Internal: Cameronia sp., Cruznema sp., Binema sp.,
Pulchrocephala sp., Talpicola sp. were found in the rectum
and hind gut of the mole crickets.
External: Diplogaster sp., Mesorhabditis sp., Pelodera sp., and Rhabditis sp. were found on the bodies of living mole crickets or on their cadavers in the soil.