Know Your Friends

Nealiolus curculionis, A Parasitoid of the Sunflower Stem Weevil

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.

One important pest of sunflower is the sunflower stem weevil, Cylindrocopturus adspersus. Larvae cause damage to the crop by feeding, developing, and overwintering in the stem, which weakens the stem and can result in lodging of the plant prior to harvest. Both the egg and larval stages of the weevil are attacked by parasitoids. Weevil eggs are parasitized by the mymarid wasp Anaphes pallipes, with an average parasitism rate of 8% for the season. Larvae are parasitized by several different wasps.

The braconid wasp Nealiolus curculionis is the most abundant parasitoid attacking weevil larvae. In North Dakota, 96% of parasitized weevil larvae from 1980 to 1991 were parasitized by this species. It is found in both wild and cultivated sunflower.

Other parasites of sunflower stem weevil larvae include the eulophid wasp Tetrastichus ainsliei and the pteromalid wasp Mesopolobus sp. Nealiolus curculionis adults are active from late June to late August. Females deposit eggs in early larval stages of the weevil while they are feeding within the sunflower stalk. The immature parasitoids overwinter within diapausing weevil larvae in the sunflower stalk. Parasitism by N. curculionis has consistently averaged 27% since 1983 (an increase from levels reported in the late 1970's and early 1980's), while weevil densities have varied considerably. The ability of the female N. curculionis to effectively locate and attack hosts under varying host population densities and in crops planted on different dates is one reason for the success of a this parasitoid.

Since parasitoids are a consistent mortality factor in the population dynamics of the sunflower stem weevil in cultivated sunflower, it is important to utilize cropping practices that conserve and protect these natural enemies of the sunflower stem weevil. For example, delayed planting has been an effective cultural control tactic for reducing weevil numbers in sunflower stalks to decrease damage without harming the parasitoids. Crop rotation also helps reduce weevil numbers and damage. Parasitoids can be conserved by utilizing chemical treatments only when weevil densities exceed the economic threshold. The successful combination of these pest management strategies enables sunflower producers to reduce potential damage from this insect pest in the Northern Plains production areas.

- Larry Charlet, USDA, ARS, Northern Crop Science Lab, Fargo, North Dakota

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