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Effect of Lefenuron, a Chitin Synthesis Inhibitor, on Encapsulation Response of Spodoptera Littoralis Larvae to Surplus Microplitis Rufiventris Larvae

Published online by Cambridge University Press:  19 September 2011

E. M. Hegazi ,
W. E. Khafagi  and
G. M. Abd El-Aziz
E. M. Hegazi
Affiliation:
Department of Entomology, Faculty of Agriculture, Alexandria University, Alexandria, Egypt
W. E. Khafagi
Affiliation:
Plant Protection Research Institute, Sabahia, Alexandria, Egypt
G. M. Abd El-Aziz
Affiliation:
Plant Protection Research Institute, Sabahia, Alexandria, Egypt
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Abstract

The chitin synthesis inhibitor lefenuron [N-{2,5-dichloro-4-(1, 1, 2, 3, 3, 3-hexafluoropropoxy)-phenylaminocarbonyl}-2, 6-difluorobenzamide] was found to have a negative effect on the encapsulation responses of Spodoptera littoralis larvae towards supernumeraries of Microplitis rufiventris parasitoid. Superparasitised hosts fed on compound-free diet showed strong encapsulation reaction (ca. 40.8%) to surplus parasitoid larvae having scars of physical combat. Treated hosts exhibited a significant reduction in either capsule thickness or encapsulation rate to the excess parasitoid larvae. Even when a capsule was formed, it had a thinner haemocy tic layer than tha t of control hosts. Studies on the differential haemocyte counts showed that failure of treated larvae to encapsulate injured surplus parasitoid larvae could be attributed to the following factors: (1) disturbance in plasmatocytes-granular cells (GR) ratio, e.g. abnormal increase in GR levels possibly causing faint haemocytic reaction to injured parasitoid larva(e); (2) the parasitoid surface becoming less attractive to the granulocytes and plasmatocytes; (3) the haemocytes involved in capsule formation (granulocytes, plasmatocytes) losing their ability to spread out over the surface of parasitoid larvae; and (4) the blood cells of the treated host failing to recognise wounded larvae as foreign bodies. Reduced encapsulation capacity might be due to a secondary effect(s) of IGR treatment inducing a suppression of adhesive activity of the plasmatocytes. These results may be useful for improving our understanding of insect immune reaction and/or manipulating parasitoids activity against nonhabitual Parasitoida.

Résumé

Il a été observée que l'inhibiteur de synthèse de la chitine—le lefénuron [N-{2,5 dichloro-4-(1, 1,2,3, 3,3-hexafluoropropoxy)-phenylaminocarbonyl}-2,6 difluorobénzamide], comportait un effet négatif sur les réponses des larves de S. littoralis, à encapsuler les larves surnuméraires issues du superparasitisme par le parasitoïde Microplitis rufiventris. Les larves hôtes superparasitées et nourries sur un milieu sans cet inhibiteur montraient une forte réaction d'encapsuler le surplus de larves (environ 48%) du parasitoïde, qui avaient des cicatrices résultant d'un combat physique. Chez les hôtes traités on a observé une réduction importante, soit de l'épaisseur de la capsule ou du taux d'encapsulation des larves excédentaires du parasitoïde. Même si une capsule était formée, celle-ci était caractérisée par une mince couche d'hémocytes en comparaison de celle des témoins. Les comptages différentiels d'hémocytes ont montré que l'échec des larves traitées à encapsuler les larves blessées excédentaires du parasitoïde pourrait être attribué aux facteurs suivants: (1) un déséquilibre du rapport entre plasmatocytes et granulocytes c-à-d une augmentation excessive du nombre de granulocytes, qui probablement débouche sur une faible réaction des hémocytes chez les larves blessées du parasitoïde; (2) la paroi corporelle du parasitoïde devient moins attrayante pour les granulocytes et les plasmatocytes; (3) les hémocytes (granulocytes, plasmatocytes) intervenant dans la formation de la capsule perdent leur pouvoir de couvrir la paroi corporelle du parasitoïde; et enfin, (4) les cellules sanguines de l'hôte traité sont incapables de reconnaître les larves blessées comme des corps étrangers. La capacité réduite d'encapsulation peut résulter des effets secondaires de régulation de la croissance de l'insecte due à l'inhibiteur qui annule l'activité adhesive des plasmatocytes. Ces résultats pourraient nous aider à mieux comprendre les mécanismes qui régissent la réaction immunitaire de l'insecte et/ou manipuler l'activité des parasitoïdes dans le sens d'éviter un parasitisme inhabituel au sein du groupe Parasitoida.

Keywords

Spodoptera littoralisMicroplitis rufiventrissuperparasitismIGRlefenuronencapsulationhost haemocyte profileSpodoptera littoralisMicroplitis rufiventrissuperparasitismerégulation de croissance de l'insectelefénuronencapsulationprofile hémocytaire de l'hôte
Type
Research Articles
Information
International Journal of Tropical Insect Science , Volume 18 , Issue 4 , December 1998 , pp. 357 - 363
Copyright
Copyright © ICIPE 1998

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