Densovirus offers hope for biological control of Egypt’s major cotton pest

Cotton plantations are highly important in Egypt, covering between 400 000 and 500 000 ha, 1/6 of all cultivated land. These crops are a vital source of foreign currency revenue through exports, and their state of health is therefore permanently under close surveillance. Cotton plants are indeed the target of a leaf-eating insect, the noctuid Spodoptera littoralis (Lepidoptera), or Egyptian Cotton Leafworm.

Known to be the main pest of cotton, it also attacks the leaves of cereal crops and lucerne which is the principal fodder crop used in Egypt. This noctuid is one of the most intensively studied insects in that country. The problem that arises concerns how to eradicate populations of this pest without massive use of chemical pesticides and thus minimize damage to the environment, and in such a way that crop yields are not jeopardized.

The solution put forward by researchers from the IRD, the University of Cairo and their French and Canadian partners (1) lies in the domestication of an insect-infecting virus isolated in Egypt in 1995, Densovirus MlDNV (2). This organism could be used locally as an element of biocontrol programmes.

Caterpillars of this moth become infected when they eat leaves contaminated with the virus. The microorganism passes through the intestinal wall, then propagates through most of the caterpillar’s body, causing considerable tissue damage. Caterpillars then secrete substances carrying viral particles, which initiate a new cycle of infection, and die a few afterwards. This virus of local origin, used as a biological control agent, or biopesticide, to reduce noctuid populations, could therefore provide protection of cotton fields without disturbing ecosystem balance.

Densovirus biology was studied in terms of both its natural habitat by observation and tests and its genomic organization and expression by using high-performance molecular diagnostic tools, in order to pave the way for its authorization in Egypt. The research team was therefore able to characterize the biology, virulence and the propagation of this virus in relation to population fluctuations of insect pests.

Information on its distribution in cropped areas in Egypt was sought by studying its presence in seven species of noctuid, including Spodoptera littoralis. Densovirus was isolated in these seven species and throughout Egypt (from cotton plantations of the Nile Delta in Lower Egypt to fields of lucerne and clover in Upper Egypt and the oases in the West), whatever the time of year. The virus appears therefore not to be strictly and solely associated to S. littoralis but is polyspecific. This gives it the ability to sustain itself in crops throughout the year. Transfection of the virus would occur by passage from winter and spring noctuid species to summer and autumn ones (3). Large infestations would thus develop in each of these pest species.

Characterization, partial cloning and sequencing of the Densovirus MlDNV genome were performed. Results gave new information on the mechanisms that govern the virus’s multiplication. The different samples of the virus taken in Egypt all belong to the same species of Densovirus, MlDNV, although some correspond to genetically distinct strains. This genetic biodiversity could explain the wide spread of the virus and its polyspecificity -ability to infect several species of noctuid- without altering its high virulence.

Other studies involving in vitro cell-infection experiments indicated potential infection of mammals by this insect virus to be improbable. This was the case also for most other animals in the agricultural areas investigated (earthworms and snails, in particular). Research projects are continuing in Egypt to verify the harmlessness of the virus for insect species not targeted by pest control, with a view to its authorization. Such ratification would be the first step towards the development of a densovirus-based biocontrol agent.

Marie Guillaume – IRD

(1) This research work is the fruit of a joint project between the IRD and the University of Cairo at Gizeh (Egypt), in partnership with the University of Quebec at Laval (Canada) and the University of Montpellier II.

(2) This Densovirus was isolated for the first time in 1995, in Egypt, from the leaf-eating noctuid maize worm Mythimna loreyi (Lepodoptera Noctuidae). Reference: G. Fédière, M.A. K. El-Sheikh, S. Abol-Ela, M. Salah, M. Massri and J. C. Veyrunes, 1995 – Isolation of a new densonucleosis virus from Mythimna loreyi Dup. (Lep. Noctuidae) in Egypt, Bull. Fac. Agric., Cairo Univ., vol.46, pp. 693-702.

(3) Agrotis ipsilon, A. segetum, A. spinifera and Spodoptera exigua (winter and spring species), Heliothis armigera and Autographa gamma (summer and autumn species, like S. littoralis).