Population Density and Parasitism Evaluation of Helicoverpa armigera(Hubner) and Spodoptera exigua (Hubner) on Twelve Varieties of Tomato in the Field

Introduction Tomato, Lycopersicon esculentum Miller has a special place among agricultural products. A wide range of pests and diseases are causing damage to tomato. Tomato fruit worm, Helicoverpa armigera (Hubner) 1809 and beet armyworm, Spodoptera exigua (Hubner) 1808 are two important pests of the Lepidoptera order, which aggress to the tomato. Biological control by predators and other natural enemies is an important components of a comprehensive pest management program (IPM). Among the natural enemies, parasitoids has special significance and can be used in biological control of pest plants. Habrobracon hebetor Say (Hymenoptera braconidae) is a cosmopolitan, gregarious, ectoparasitoid that attacks the larval stage of several species of Lepidoptera and is considered one of the best potential biological control agent. Study of pest density on different cultivars has an important role in choosing appropriate method and time control, and is basis of plant restance. Therefore in this study mentioned pest density and efficiency of h. hebetor was assessed.
Materials and Methods Population density and larval parasitism by H. hebetor was examined on twelve tomato varieties including, Rio fujio, AB2, Mariana, Gem pride, King ston, Super queen, 9704, 9706, 9553 and PS 6515 in field conditions during 2014. All experiments were conducted in a randomized complete block design. Five weeks after transplanting, sampling performed randomly. At three-days intervals, egg, larvae and parasitized larvae of each moth were collected on different cultivars of tomato. The whole of each tomato plant were checked for the presence of pests and the number of eggs. 400 and 570 wasps were released in two steps in june 24th and july 23th during 2014, respectively. Number of healthy and parasitized larvae of each pest were counted and recorded to calculate the parasitism percentage. The trichomes density of defferent varieties of tomato was assessed by a stereo microscope with a magnification of 70. The density of trichomes in the leaves after the terminal bud per plant (per square millimeter) were counted and recorded at three points around the midrib. Statistical analysis of data was carried out using Exel 2013 and SPSS 18.0 software.
Results and Discussion The results of the study showed that the pest density had no significant differences among different cultivars of tomatoes, while there were significant differences in different dates of sampling. The highest density of tomato fruit worm and beet armyworm larvae was observed on AB2 and Kingstone, respectively, while the lowest density of these larvae was obtained on the Kingstone and 6515 respectively. Also, the highest density of h. armigera was in 17 and 22 july 2014 that coincided with the ninth and tenth weeks after transplanting in the field. Parasitism percentage of tomato fruit worm and beet armyworm was significant among different dates of sampling but no significant differences were observed among different cultivars. The highest parasitism percentage of tomato fruit worm larvae was obtained in 22 and 27 july 2014. The highest density of pest was occured in 24 and 27 June and 3 July 2014 coincides with the sixth and seventh tomato growth. The highest larval parasitism of beet armyworm was observed in 17 July 2014, three weeks after bracon release. Study on tomato leaf trichomes density in different varieties with highest and lowest population of tomato fruit worm larvae indicated that, the highest density was observed in Kingston variety. Biological and behavioral differences of tomato fruit worm and beet armyworm are could be effective on host preference by these pests, in addition chemical compounds produced by different varieties of tomato (repellents and attractants) are effective on host selection for oviposition and influence pest population.
Conclusions Release of parasitoid wasp, H. hebetor to control the larvae of the tomato fruit worm and beet armyworm, plays an effective role as a biological agent and can be used in a management program of theses pest. According to the text, due to the compatibility of resistant cultivars with other control methods and environment, more study on the role of trichomes in plant resistance is recommended to introduce a resistant cultivars of tomato to mentioned pests.