Laboratory screening supports the selection of sesame (Sesamum indicum) to enhance Anagrus spp. parasitoids (Hymenoptera: Mymaridae) of rice planthoppers

Abstract

Planthopper (Delphacidae) pests have broken out frequently in Asia over the last decade leading to interest in enhancing the impact of natural enemies by growing nectar plants on the bunds that border rice fields. Such targeted use of plant diversity is popular in other crop systems but there is a marked lack of information on the scope for its use in rice, particularly the important aspect of which plant species to use. This study used Y-tube olfactometer assays to measure the response of two important parasitoids of delphacid pests to candidate nectar plants. Anagrus optabilis exhibited significant attraction to the air from six of the seven plant species whilst Anagrus nilaparvatae appeared more selective, exhibiting attraction to only seven of the 23 plants screened and repulsion to one. Sesamum indicum, Emilia sonchifolia, and Impatiens balsamena were the only three plants attractive to both parasitoids. Laboratory longevity of adult female A. nilaparvatae and A. optabilis with access to sesame flowers was significantly greater than with access to sesame from which the flowers were removed plus water. Similarly, both parasitoids parasitized significantly more brown planthopper (Nilaparvatae lugens) eggs in the presence of sesame flowers. Handling time of A. nilaparvatae was reduced from 31.29 to 18.36 min by access to sesame nectar. Findings show that sesame has a marked beneficial effect on key parameters of Anagrus spp. and justifies further evaluation of its utility as a nectar plant to improve biological control in Asian rice systems.

Introduction

Rice accounts for more than 65% of caloric intake in the low-income countries in tropical Asia and is the most important staple food worldwide (Peng and Hardy, 2001, Zeigler and Barclay, 2008). Rice planthoppers (Delphacidae), the brown planthopper (Nilaparvata lugens (Stål)), white-backed planthopper (Sogatella furcifera (Horváth)), and small brown planthopper Laodelphax striatellus (Fallén)), are amongst the most destructive insect pests of rice in Asia and outbreaks have occurred frequently in recent years (Cheng, 2009, Savary et al., 2012). In China alone an average of 26.7 million hectares were affected by these species in the seasons between 2005 and 2007 (Xia, 2008). Their impact is now so severe they are considered to be substantial threats to the world food security (Lou and Cheng, 2011).

Anagrus spp. parasitoids are important egg parasitoids of planthoppers in Asia (Gurr et al., 2011). Their parasitism incidence normally reaches about 10–70% in rice fields in China where Anagrus nilaparvatae (Pang et Wang) dominates (Yu et al., 2001). The same species is important also in Cambodia, India and Philippines (Chandra, 1979, Kalode, 1983, Preap et al., 2001). The congeneric, Anagrus optabilis (Perkins) is the dominant egg parasitoid of the green slender planthopper, Saccharosydne procerus Matsumura, which is the key insect pest of a perennial vegetable crop Zizania caduciflora L. often grown by rice farmers in adjacent fields in eastern China (Lu, 2003). This is relevant to rice because A. optabilis also attacks eggs of rice planthoppers so, whilst able to overwinter using S. procerus on its perennial host plant, it can move to nearby rice fields after spring to attack rice planthoppers (Zheng et al., 2003b). This phenomenon is important in non-tropical Asian rice systems where neither brown planthopper nor white-backed planthopper are able to overwinter so are unavailable as hosts for parasitoid survival.

Rice is an annual crop so subject to high levels of disturbance from cultivation, seasonal inundation, transplanting, and harvesting as well as the abuse of pesticides and fertilizers, all of which can adversely affect natural enemies leading to low levels of biological control (Heong, 2009). Enhancement of biological control by habitat management (Landis et al., 2000) has been widely explored in many crop systems. The main goal of habitat management is to conserve natural enemies and enhance their performance by providing resources such as non-host foods which can be particularly important when hosts or prey are unavailable to natural enemies (Gurr, 2009), because foods such as nectar often have a great influence on the female parasitoid longevity, searching efficiency and parasitism (Mitsunaga et al., 2004, Mitsunaga et al., 2006, Rivero and Casas, 1999, Shearer and Atanassov, 2004, Jervis et al., 2004). In ecological engineering for pest management, habitat management approaches are used in a targeted manner whereby the benefits of candidate plants are evaluated to identify the right kinds of diversity to introduce into a farming system (Gurr et al., 2004). Despite the importance of rice in world agriculture and the fact that insect pests cause serious losses, surprisingly little information is available on the scope for ecological engineering to contribute to more sustainable pest management in this crop (Gurr et al., 2011). The only available study (published in Chinese) showed that longevity and parasitic ability of A. nilaparvatae was strongly improved by feeding from soybean flowers (Zheng et al., 2003a). Accordingly, the aim of this study was to study the behavioral and physiological response of A. nilaparvatae and A. optabilis to a range of potential nectar plant species that could be grown on the bunds surrounding rice fields. The rationale for plant selection was that each species is either a food, medicinal herb or ornamental crop of potential value as a secondary income source or a weed species that farmers could be advised to conserve when present on rice bunds.