Abstract
Baculoviruses are known biocontrol agents of several crop pests, however, their activity is deteriorated rapidly when exposed to the sunlight. Many UV protectants were used to improve the efficacy of these viruses under field conditions. The objective of the current study was to evaluate the effectiveness of plant extracts to improve the persistence of Spodoptera littoralis multiple nucleopolyhedrovirus (SpliMNPV) against beet army worm, Spodoptera exigua (Hubner) under harsh sunny field conditions in Saudi Arabia. A preliminary test of SpliMNPV was performed to determine the lethal concentrations of this virus against the first and second larval instar of S. exigua. The potency of ten plant extracts as UV absorbers were done by measuring the absorption spectra of the 0.5% extracts using UV spectrophotometer. Based on preliminary data, clove, green tea and henna extracts at 10% (v/v) concentration were added to the virus and tested under semi-field conditions on the cabbage plantations for 7 days. The preliminary experiment showed that the LC50 and LC95 for the first instar larvae were 1.59 x 10(3) PIB and 4.91 x 10(7) PIB, respectively. Whereas, these values for the second instar larvae were 4.99 x 10(6) PIB and 1.06 x 10(8) PIB, correspondingly. The field experiment data indicated that the mortality of the first instar larvae was highest at day 0 of the sunlight exposure. In virus alone treatments, after 7 days of sunlight exposure, the efficacy recorded was 88%. However, in clove-virus treatment, the mortality was higher (96%) than other treatments. These data also revealed that the second instar larvae were more resistance than the neonates. The efficacy of SpliMNPV at 0 and 7-day against the second larval instar was 88% and 5%, respectively. In clove-treated applications, the mortality was was 96% and 32%, at 0 and 7 d post treatment, respectively. Findings of this study demonstrated that the addition clove extract enhanced the effectiveness and persistence of SpliMNPV under arid conditions against the beet armyworm.