Nealiolus curculionis
Hymenoptera: Braconidae

Sunflower, an important oilseed crop in the Northern Plains states, is one of only a few cultivated crops that are native to North America. Insects associated with the sunflower genus Helianthus - plant feeders, pollinators, and natural enemies - have evolved with the plant for centuries and many have moved from wild species to the cultivated crop. Only a small number of the hundreds of insects recorded from sunflower have become economic pests; indigenous natural enemies have been a significant factor in preventing many insects from becoming pests.

Appearance

N. curculionis is a small (~3 mm long), dark wasp that is seen on all parts of the sunflower plant, but most often probing the stems. Parasitized sunflower stem weevils are not visibly distinguishable from healthy larvae.

Habitat (Crops)

Sunflowers, both wild and cultivated, and other plants where its several hosts are found.

Pests Attacked

Sunflower stem weevils (Cylindrocopturus adspersus) and at least 10 other species in the Curculionidae, including the red sunflower seed weevil (Smicronyx fulvus), the boll weevil (Anthonomus grandis grandis), and the plum curculio (Conotrachelus nenuphar).

Life Cycle

N. curculionis is well-synchronized with its sunflower stem weevil host, suggesting that it has a high degree of adaptation with its host's physiology. N. curculionis overwinters as a late first or early second instar within the mature diapausing sunflower stem weevil larva in a chamber constructed in the lower stem or root crown of the sunflower plant. N. curculionis does not complete development in one season, and its host continues to function after parasitization. After diapause, the parasitoid larva develops for approximately 21 days, then exits its host, continues growing and feeding externally on the stem weevil (sometimes consuming the entire weevil larva), and pupates. From the end of diapause to adult emergence is usually about 31 days. Only a single adult normally emerges from a host larva, the males approximately 4 days earlier than females.

N. curculionis adults were observed from late June to late August during 1980 to 1985 in the northern plains. Early instar sunflower stem weevils feed and tunnel in the conductive stem tissue which is located under the epidermal surface of the plant stem and it is there, within reach of N. curculionis' ovipositor, that parasitoid oviposition takes place. Adults in the study were provided honey and moisture and survived about 9 days at 26°C. (In the field, nectar is plentiful on sunflower, so nutrients are not likely a limiting resource.)

In two of the observation years (1981 and 1984), there were two peaks in the number of N. curculionis adults in field plots. This could be due to late season emergence of adults of this generalist parasitoid species from other overwintering curculionid species.

Relative Effectiveness

In a North Dakota study of sunflower stem weevils from 1980 to 1991, parasitism rates by N. curculionis ranged from about 5% to almost 32% (average = 27%), and comprised about 96% of the parasitoids attacking the weevils. N. curculionisis also the predominant parasitoid of sunflower stem weevils in Minnesota and South Dakota.

Study results did not show a positive correlation between parasitism rates and host density, indicating that the female parasitoid is able to locate and attack hosts under varying host population densities. The degree of mortality from the host's immune response also varies from year to year due to variation in the intensity of the host's defensive reaction over time.

The parasitoid appears to be a consistent mortality factor in the population dynamics of the sunflower stem weevil in cultivated sunflower, though adult population levels are often low.

Conservation

For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, or the Volume II, No. 1 Feature Article on conservation in the Midwest Biological Control News Online.

Pesticide Susceptibility

Materials that were the most effective in controlling stem weevil larvae in one study were also most harmful to the parasitoids. The majority of these compounds were carbamates, which tend to be more persistent that organophosphates. This may be one of the reasons for their destructive effect on the natural enemies. Carbofuran was the most toxic insecticide tested to both the parasitoid and its host.

The nontarget effect of these materials on natural enemies must be weighed against the benefits of their activity on stem weevils in the development of any pest management program.

Commercial Availability

Not commercially available.

Taken from:

Charlet, Laurence D. 1994. Seasonal abundance and impact of the sunflower stem weevil parasitoid, Nealiolus curculionis (Hymenoptera: Braconidae), in the northern Great Plains, Biol. Control, 4: 26-31.

Additional Reference

Charlet, L.D. and Oseto, C.Y. 1983 Toxicity of insecticides on a stem weevil, Cylindrocopturus adspersus (Coleoptera: Curculionidae), and its parasitoids in sunflower, Env. Ent. 12: 959-960.

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