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Mermithid nematodes are common parasites of black flies and play a significant role in the natural regulation of these medically important insects. Infection levels tend to he moderate and perennial, with epizootics rare and highly localized. Mermithid parasitism almost invariably results in the death of the black fly, and thus considerable attention has focused on the potential of these nematodes as biocontrol agents. Early instar black fly larvae appear most susceptible to infection, and integumental penetration hy mermithid preparasites is the only known mode of entry. Postparasitic nematodes typically emerge before host pupation. However, carryover of parasitism into adult simuliids is an important mechanism for local dispersal and recolonization of upstream areas. Following emergence, the mermithids molt to the adult stage. Copulation ensues, the females then laying eggs which eventually give rise to the next generation of infective preparasites. The number of described species is conservatively estimated at 35-40, with most species within the genera Mesomermis, Gastromermis, and Isomermis. The taxonomy of this group of mermithids is a challenging and little explored area. Host-specificity statements, therefore, must be made cautiously because of these systematic problems and others within the Simuliidae. In most instances, temporal and spatial factors limit the host range of these mermithids among simuliid species. Differential susceptibilities anmng larvae concurrently present within the same microhabitat probably reflect varying degrees of host attractiveness and behavioral-physiological resistance. Effects of parasitism on the host may include prevention of metamorphosis, sterility, intersexual development, and behavior modification. Evaluation of the technical feasibility of mermithid control of black flies has been stymied by the limitations of current inoculum-production technology. Continued advances in in vivo and in vitro culture methods are required to accelerate the research process.

The RNA and protein content of perennial ryegrass root-tip galls induced by Longidorus elongatus were measured from transverse sections and the morphology described. Galls progressed through five distinct stages and were viable for only 10-12 days at 18 C, after which they collapsed and became necrotic. In the initial stage hypertrophy occurred and cells contained enlarged nuclei and nucleoli, a greater proportion of cytoplasm, and increased concentrations of protein. This was followed by hyperplasia; cells divided to give two or four daughter cells, accompanied by a proportionate reduction in volumes of cytoplasm, nuclei, and nucleoli and reduced concentrations of RNA and protein. The third stage was secondary hypertrophy with enlarged, amoeboid nuclei and nucleoli and a significant increase in concentration of RNA and protein. In the final two stages, as feeding by L. elongatus progressively removed cell contents, most cells were devoid of inclusions and galls collapsed and were invaded by soil bacteria. This ordered development and exploitation of galls suggests that L. elongatus may have two phases in its feeding.

Histological responses to Meloidogyne incognita infection in Rhizobium nodules of clover, horsebean, lupine, and pea were investigated. The formation of giant cells in vascular bundles of nodules and roots, and the basal connection of the nodule, were usually associated with abnormal xylem and/or deformed xylem strands. However, giant cells did not disturb or prevent the development of nodular tissues. Areas in which galls formed, wall thickness of giant cells, and number of giant cells around the nematode head varied with plant species. Ranking by gall size and giant-cell wall thickness was horsebean > lupine and pea > clover. The multinucleate condition in giant cells resulted from repeated mitoses without subsequent cytokinesis. The resulting nuclei agglomerated in irregularly shaped masses in some giant cells.

Meloidogyne hapla is the dominant root-knot nematode found in Tennessee woody ornamental nurseries. In greenhouse tests, M. hapla produced galls and formed egg masses on roots of Abelia x grandiflora, Comus florida, Hydrangea paniculata grandiflora, Photinia x fraseri, Spiraea x bumalda, Spiraea x vanhouttei, and Viburnum carlesii. Galls on H. grandiflora and V. carlesii were mostly large and fusiform. Galls on C. florida were spherical and usually terminal, whereas those on the other species were minute. Lateral roots grew from galls on all susceptible plants. Two Acer spp., two Buxus spp., three llex spp., five Prunus spp., three Rhododendron spp., Euonymus alata, Ligustrum sinense, Magnolia x soulangiana, Nandina domestica, and nine conifer species were nonhosts or very poor hosts.

Pterotylenchus cecidogenus did not inhibit germination of Desmodium ovalifolium but did reduce survival of seedlings and root and stem growth. Stem gall formation was associated with nematode population densities which were positively correlated with plant age at time of inoculation. Nematode population densities increased 100 times in 52 days following inoculation.

Sugar beet (Beta vulgaris L. cv. Monogerm C.S.F. 1971) seeds sown into Vineland fine sandy loam, infested with 15,500 H. schachtii juveniles/pot, showed little growth during an 11-week test in the greenhouse. Seedlings transplanted at 2, 4, and 6 weeks of age had 32, 30, and 31% less top weight and 71, 68, and 59% less root weight, respectively, compared to controls grown in nematode-free soil. Nematode reproduction in both direct-seeded and transplanted sugar beets was limited and related to root weight. Shoot/root ratios were increased by the nematodes in all nematode-infected beets compared to those grown in soil without nematodes. In contrast to seeding or transplanting sugar beets into nematode-infested Vineland fine sandy loam, an inoculation of Beverly fine sandy loam supporting 0 (seeds), 2-, 4-, and 6-week-old sugar beet seedlings with 7,400 juveniles/pot, followed by 11 weeks of growth in the growth-room, resulted in top weight losses of only 13, 3, 18, and 15% and losses in root weight of 44, 38, 36, and 38%, respectively. Nematode reproduction was high and all shoot/root ratios were increased by the nematode compared to the noninoculated controls. These experiments have shown that sugar beets sown into nematode-infested soil are damaged much more heavily by H. schachtii juveniles than seeds inoculated with the nematode immediately following sowing. Results indicate that an increase in tolerance of sugar beets to attack by H. schachtii does not occur beyond the first 2 weeks of growth and that transplanting damage lowers the tolerance of seedlings to nematode attack.

Four populations of the lesion nematode Pratylenchus brachyurus were tested in a greenhouse on seven selected plant species to determine host suitability and usefulness in identifying physiological races of the nematode. The differential plants were 'Florida 77' alfalfa, 'Harvester' snap bean, 'Rough Lemon' citrus, 'Pioneer 304C' corn, 'Florunner' peanut, 'Braxton' soybean, and 'Rutgers' tomato. Fresh shoot or root weights of plants inoculated with all populations were similar to each other and to the uninoculated control in both experiments. The numbers recovered from Harvester snap bean were different (P = 0.05) between populations in each test and from Braxton soybean in the second test. The differences between populations on snap bean were different in the two tests.