Mermis nigrescens Dujardin, 1842
by John L. Capinera
On rainy mornings in May or June, large nematodes sometimes can be found up on foliage, where they are depositing their eggs. Because they are so large (up to 160 mm or about 6 inches in length), they attract attention. Under some circumstances they also function as important biological control agents, especially of grasshoppers.
Mermis nigrescens is normally associated with grasshoppers (Orthoptera: Acrididae, Romaleidae, Tettigoniidae) but also is reported to occur naturally in earwigs (Dermaptera), beetles (Coleoptera), caterpillars (Lepidoptera), and even honeybees (Hymenoptera). Because it is similar in appearance to other species of Mermis, and to Amphimermis, Longimermis, Agamermis, and Hexamermis, some host records may be inaccurate.
Mermis nigrescens occurs widely in North and South America, Europe and Asia. Distribution in Africa seems to be quite limited, but uncertain. It is found in Tasmania, but not yet in Australia, though other Mermis spp. and related genera occur there.
Mermis spp. display the curious habit of depositing their eggs on vegetation, where they are accidentally ingested by herbivorous insects; other mermithids lay their eggs in the soil and when they hatch the larvae move to the surface and penetrate the hosts. After ingestion, the M. nigrescens eggs quickly hatch and the immature nematodes break through the gut wall and enter the body cavity of the host insect. Juvenile M. nigrescens nematodes initially are about 0.75 mm long. Infection by M. nigrescens inhibits development of the ovaries in grasshoppers, and the host may perish before or after the nematodes emerge from the insect. After emerging, the nematodes enter the soil where they reach the adult stage after 2-4 months. They are not sexually mature for another 6 months, and may remain in the soil for up to 3 years, though a two-year life cycle is normal. M. nigrescens may either mate, or reproduce parthenogenetically. Males are not as long-lived as are the females. Males are about 40-60 mm long, females 60 to 160 mm. Females with fully developed eggs come to the soil surface and deposit eggs on vegetation only following protracted rainfall, often in the morning hours before the sun dries the foliage. Females produce large quantities of eggs, estimated to be up to 14,000 per female. When laden with eggs, the tan-colored females appear to have a dark stripe running the length of the body, but this dark coloration disappears as the eggs are deposited. The eggs are about 0.05 mm in diameter. They are brown or reddish brown in color. The eggs bear two clusters of branched filaments, one at each pole. As rainfall and dew evaporate, the filaments adhere to the foliage, causing the egg to remain fixed to the vegetation, where it can be consumed along with the plant. Irrigation can increase parasitism of grasshoppers by these nematodes by simulating rainfall conditions; rainfall is a prerequisite for adult female nematodes to leave the soil to deposit eggs.
These nematodes are most abundant where suitable insect hosts have been plentiful, usually grasshoppers. Various studies have reported high levels of parasitism in grasshoppers ranging from 20-75%. Although often considered to be absent from arid areas such as the western Great Plains of USA, they do occur where water is available. In Colorado, up to 50% of Melanoplus femurrubrum from irrigated cropland was found to be infected whereas the same species was unparasitized in nearby arid rangeland. Although impressive claims sometimes are made about the suppressive effects on grasshopper populations by M. nigrescens, their long life cycle and need for high moisture conditions reduces their reliability as a biological control agent.
Mermis and related nematodes are often confused with horsehair worms (Nematomorpha: Gordioidea). Horsehair worms superficially resemble Mermis nigrescens nematodes, but may be twice as long, and usually are found near water. Horsehair worms typically are completely dark in color, with a uniform thickness throughout almost the entire length of the body. Mermis nigrescens nematodes are not uniformly dark and at least one end of the body tapers to a blunt point. Identification aids can be found in Nickle (1971, 1973) and Baker and Capinera (1997).
Mermis nigrescens is not available commercially. However, their egg stage can be stored and applied as a suspension in water, so if economic artificial rearing techniques could be developed M. nigrescens might make a useful augmentative biological control tool.
Baker, G.L. and J.L. Capinera. 1997. Nematodes and nematomorphs as control agents of grasshoppers and locusts. 171: 157-211.
Christie, J.R. 1937. Mermis subnigrescens, a nematode parasite of grasshoppers. Journal of Agricultural Research 55: 353-364.
Nickle, W.R. 1971. A contribution to our knowledge of the Mermithidae (Nematoda). Journal of Nematology 4: 113-146.
Nickle, W.R. 1973. Identification of insect parasitic nematodes – a review. Experimental Parasitology 33: 303-317.
Poinar Jr. G.O1979. Nematodes for biological control of insects. CRC Press, Boca Raton, FL.