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School of Life and Environmental Sciences, Kyoto Prefectural University, Kyoto, Japan

Email: ^*nakao@kpu.ac.jp

Copyright © 2014 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Received 4 March 2014; revised 3 May 2014; accepted 17 May 2014

ABSTRACT

Tobacco thrips, Frankliniella fusca, which is a vector of Tospovirus, is a serious pest of tobacco, peanuts, and cotton in the United States. It was said that immigration of the adult thrips from weed hosts into crop fields is a cause of TSWV epidemy in the field. Recently this pest thrips was introduced into Japan. We investigated survival rate, developmental time, and fecundity of a Japanese strain of F. fusca on leaves of tobacco, peanut, and green bean at 18˚C. Thrips could not complete larval development on 3 Nicotiana species, N. tabacum, N. gossei, and N. rustica. On the other hand, survival rate from hatching to adult emergence on peanut and green bean is 78% and 86%, respectively. A female adult thrips lived during about 25 days and deposited 101 eggs on green bean, but died within 10 days without ovipositing on Nicotiana spp. The tobacco is not a reproductive host plant that permits tobacco thrips to propagate, but an adult food source that is susceptible of TSWV.

Keywords:Tobacco, Thrips, Groundnut, Green Bean, Fecundity, Survival Rate

1. Introduction

Tomato spotted wilt virus (family Bunyauirdae, genus Tospovirus) causes economic losses to many cultivated crops. Tomato spotted wilt virus (TSWV) has a wide host range that includes several hundreds of plant species over 20 families [1] , and it is a serious economic threat to tobacco, Nicotiana tabacum L. and peanut, Arachis hypogaea L. [2] -[6] .

TSWV is vectored exclusively by several thrips species, including the tobacco thrips, Frankliniella fusca, which are commonly observed on tobacco plants in USA [2] [3] . The virus must be acquired by thrips larva in which it multiplies and survives through the larval developmental stages. Mature adults became viruliferous and remain so throughout their lives [7] . Recently, the tobacco thrips was introduced into Japan [8] .

The tobacco thrips is highly polyphagous and has been reported as the predominating thrips species on tobacco foliage in USA [2] [9] , but few immature thrips have been collected on tobacco plants [2] [9] . Thus, it was suggested that there is little potential for secondary spread of the virus for populations on tobacco plants. They suppose that adults of tobacco thrips immigrate into tobacco fields from weed host in early season [1] [9] [10] . However, development and oviposition of the tobacco thrips on tobacco foliage have not been confirmed. Because F. fusca is a thrips species most likely to vector TSWV to a lot of susceptible crops, the development and the reproduction on not only adequate host plant species but also unsuitable host plant should be evaluated. Especially in crops fields where resources are provided in large areas, some may become source of viruliferous thrips, even if the crop plant is not good host for tobacco thrips. Biotic performance of the tobacco thrips has been investigated on a few crops, including cotton, Gossypium spp. [11] [12] , peanut [13] [14] , and tomato, Lycopersicon esculentum Mill. [14] and it is known that green bean is adequate to maintain population in laboratories [14] . Although pods of green bean were typically used to maintain culture of F. fusca [1] [15] [16] , foliage is more available in time and space than pods in fields. Thus, we investigated suitability of tobacco plants as host plants of F. fusca in comparison with peanut and green bean, using leaf discs.

2. Materials and Methods

A thrips strain was originated from population on narcissus bulbs (Narcissus pseudonarcissus L.) produced in Fukushima Prefecture, Japan, in 2002 [17] . The strain was maintained on germinated broad bean seeds Vicia faba L. (provided by International Fodder Industry as food for pigeons) which were placed on a moist Kimtowel® (produced by Cresia), in polystyrene containers (97 × 137 × 50 mm), as described in previous papers [17] [18] . The strain was kept at 22.5˚C and 15L9D.

Transparent plastic tubes (18 mm in diameter and 49 mm in length) with leaf discs were used as rearing cages for all experiments [17] [19] . Both ends of the tubes were covered with M-film®. Leaf disc (18 mm in diameter) was fixed between one end of the tube and the film. Each thrips in the cage was able to feed on surface of leaf discs (ca. 2.3 cm²). Several female pupae were transferred from the stock culture to rearing cages with the surface of green bean Phaseolus vulgaris L. (cv. Tsurunashi-purasu-namame) leaf discs and reared under 18˚C and 15L9D. Within 3 days after adult emergence, the females were allowed to mate with 2 - 5 day-old males and reared at the same photothermal condition. Number of mated females per cage was two, and they were transferred into fresh cages every day. Cages with leaf discs containing eggs laid by the mated females were kept at the same condition as the parent. Newly hatched larvae were placed in fresh cages with leaf discs of one of 5 plant species: Nicotiana tabacum L. (cv. BY-1), N. gossei Domin, N. rustica L., A. hypogaea L. (cv. Chibahandachi), and P. vulgaris (cv. Tsurunashi-purasu-namame), and reared under the same conditions. The larvae were transferred into fresh cages every 4 days, and number of individuals per cage was 2 - 6 until emergence. Leaves were taken from young plant before flowering. Survival and developmental stages were recorded daily.

Some newly emerged female adults on P. vulgaris leaf discs were allowed to mate with newly emerged males. Each of the mated females was transferred to fresh cages with leaf discs of one of the 3 plants: N. tabacum, N. gossei, and P. vulgaris, every 2 days and reared individually throughout their lives under 18˚C and 15L9D. Numbers of eggs deposited by them were recorded every day until death.

Since leaf discs of food plant dried up and deteriorate only a few days after supplying at higher temperature conditions, we surveyed rearing trials at 18˚C. It is known that F. fusca can complete immature development above 13˚C [13] and deposit eggs above 18˚C [17] . Tobacco plants in the fields are usually planted in April in the mainland of Japan, and mean daily air temperature in the early growing season is between 14˚C - 18˚C.

Statistical analyses were performed by PASW® Statistics Base ver. 18.0 [20] and the R statistical package version 2.15.3 [21] .

3. Results

On three species of Nicotiana, F. fusca died in larval stage (Table 1). On two species of Leguminosae, the toTable1 Mortality and developmental period from hatching to adult emergence, and fecundity of F. fusca reared on leaf discs of five plant species under 18˚C and 15L9D. Numbers in parentheses indicate sample size.