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Comparison and contrast of cotton and Desmodium tortuosum as hosts of Heliothis virescens

Published online by Cambridge University Press:  19 September 2011

Guy J. Hallman
Guy J. Hallman
Affiliation:
Instituto Colombiano Agropecuario, Espinal, Tolima, Colombia
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Abstract

Desmodium tortuosum (Sw) DC (Papilionaceae) is a preferred wild host of Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae) which is an important pest of cotton in Tolima, Colombia. From November 1976 until March 1979, cotton and D. tortuosum were compared and contrasted in the field as hosts of the pest. A very high infestation level of H. virescens in cotton in 1977 was preceded by an extremely low infestation level of the insect in D. tortuosum previous to the cotton-growing season. In 1978 and 1979 moderate infestation levels in cotton were preceded by moderately low infestation levels of H. virescens in D. tortuosum previous to the cotton growing season in both years.

Progressively less eggs were laid on cotton versus D. tortuosum as the cotton plants aged. Peak numbers of H. virescens occurred during the latter part of the cropping season and immediately after. A mean of 13% of first-instar larvae of H. virescens on D. tortuosum were found stuck by the glandular trichomes of the plant. On cotton significantly more eggs were found during the week of full moon than new moon or first quarter. Apparent sterility and pre-hatch mortality of eggs of H. virescens varied between 1–5 and 0 and 7%, respectively, for both hosts combined.

Egg parasitism by Trichogramma sp. (Hymenoptera: Trichogrammatidae) in cotton was 1.5% in 1977, presumably due to excess use of pesticides, and 23.5% in 1978 when pesticides were not used much. Parasitism of eggs on D. tortuosum by the same parasite averaged 1%; the glandular trichomes of D. tortuosum trapped and killed adult Trichogramma.

The fact that infestation levels of H. virescens in cotton were very high in the year that they were very low in D. tortuosum previous to the cotton-growing season, and moderate in cotton during 2 years that they were not as low previously in D. tortuosum suggests that the continued presence of the pest many aid its natural regulation, perhaps by providing a source of hosts to H. virescens specific parasites.

If relative levels of oviposition in cotton could be predicted by lunar cycles this would aid in planning control strategies, such as massive liberations of egg parasites and more intensive monitoring of infestation levels during predicted peak periods.

Résumé

Desmodium tortuosum (Sw) DC (Papilioniceae) est un hôte sauvage préféré par Heliothis virescens (Fabricius), (Lepidoptera: Noctuidae), un organisme nuisible et important pour le coton au Tolima, Colombie. De Novembre de 1976 à Mars de 1979, sur le champ, on a comparé et établit les diffèrences entre le coton et D. tortuosum comme hôtes de H. virescens. En 1977, avant le cycle du coton, un niveau d'infestation extrêmement bas de l'insect sur D. tortuosum a précédé un niveau d'infestation très haut de H. virescens sur le coton. En 1978 et 1979, avant le cycle du coton, des niveaux d'infestation modérément bas de H. virescens sur D. tortuosum ont precédé des niveaux d'infestation modérés de H. virescens sur le coton.

Au fur et à mesure que les plantes du coton veillissaient un nombre plus petit d'oeufs étaient déposés en comparaison avec D. tortuosum. La population la plus nombreuse s'est presentée pendant la dernière partie et immédiatement après le cycle du coton. On a trouvé une moyenne de 13% de larves de H. virescens, au premier état, collées aux tricomes glandulaires de D. tortuosum. Sur le coton, on a trouvé un nombre d'oeufs significativement plus grand pendant la semaine du plein de la lune que pendant la lune naissante et le premier quartier. L'infertile apparente et mortalité des oeufs avant l'eclosion ont variées entre 1–5 et 0–7%, respectivement pour tous les deux hôtes.

Sur le coton, en 1977, le parasitisme des oeufs par espéces de Trichogramma (Hymenoptera: Trichogrammatidae) a eu une valeur de 1.5% probablement à cause de l'usage excessif de pesticides, et de 23.5% en 1978 quand ceux-ci n'ont pas été utilisés beaucoup. Le parasitisme des oeufs du meme parasite sur D. tortuosum a eu une valeur moyenne de 1%; les tricomes glandulaires de D. tortuosum attrappaient et tuaient les adultes des espéces de Trichogramma.

Le fait que la même année, les niveaux d'infestation de H. virescens aient été plus hauts sur le coton et plus bas sur D. tortuosum (avant le cycle du coton) et de que ils aient été modérés sur le coton pendant deux années dans lesquelles sur D. tortuosum ils n'ont pas été si bas comme avant, suggére que la presence continue de l'organisme nuisible aide àl son prope contrôle en proportionnant une source d'hôtes pour les parasites spécifiques de H. virescens.

Si on pouvait, en utilisant les cycles lunaires, prédire les niveaux relatives d'oviposition sur le coton, on pourrait projeter des stratégies de contrôle, telles que liberations en masse de parasites des oeufs et un étude plus intense des niveaux d'infestation au cours des périodes dans lesquels la population est la plus nombreuse.

Keywords

CottonDesmodium tortuosumHeliothis virescensegg parasitismnatural regulation
Type
Research Article
Information
International Journal of Tropical Insect Science , Volume 6 , Issue 5 , October 1985 , pp. 573 - 578
Copyright
Copyright © ICIPE 1985

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