Effects of a methanolic extract of the plant Haplophyllum tuberculatum and of teflubenzuron on female reproduction in the migratory locust, Locusta migratoria (Orthoptera: Oedipodinae)

Abstract

The effects of a methanolic extract of the plant Haplophyllum tuberculatum (ME-Ht) and of teflubenzuron (TFB) were compared on several reproductive variables and ecdysteroid titers in the females of Locusta migratoria. The test products were administered orally to newly emerged females at doses of 1500 (ME-Ht) and 10 μg/female (TFB). The methanolic extract and TFB had comparable effects on several of the variables examined. Both significantly delayed the first oviposition and reduced fecundity and fertility. ME-Ht and TFB also displayed similar effects on ovarian growth, vitellogenesis and ecdysteroid titers. Both treatments induced a drop in hemolymph protein levels as well as a reduction in vitellogenin uptake by oocytes. This delay in oogenesis was accompanied by a resorption of terminal oocytes. However, whereas TFB completely blocked egg hatch, ME-Ht only had a modest inhibitory effect on this variable. Hemolymph and ovarian ecdysteroid titers, as measured by radioimmunoassay, were similar and low in both control and treated females, except for a peak observed only in control females at the end of vitellogenesis. We discuss the functional significance of the observed effects in the context of the putative modes of action of the methanolic plant extract and TFB.

Introduction

Insect reproduction and its hormonal regulation constitute potential targets for the development of bio-rational insecticides. In this perspective, the identification of molecules that interfere with insect reproduction and development, and target the associated endocrine regulatory processes has been the focus of fruitful investigations in the past few decades (Dhadialla et al., 2005). This research has led to the development of “insect growth regulators” (IGRs), which disrupt growth, development and reproduction by either mimicking key hormones such as juvenile hormone (JH) and 20-hydroxyecdysone (20E), or by inhibiting chitin synthesis (Dhadialla et al., 1998, Dhadialla et al., 2005).

Effects similar to those of synthetic IGRs have been reported for secondary plant metabolites, which constitute a potential source of naturally occurring endocrine disruptors for insect control. Céspedes et al., 2005, Rharrabe et al., 2009 have shown that plant metabolites can have various molecular targets when interfering with metamorphosis and reproduction. Research focusing on the modes of action of many of these metabolites has shown that their insecticidal and IGR activities are largely due to their anti-feeding (Feng et al., 1995) and inhibitory effects on enzymes and metabolism as a whole (Kubo and Kloche, 1983, Céspedes et al., 2000, Céspedes et al., 2001), with the exception of precocenes, which have a direct inhibitory effect on JH biosynthesis (Pratt and Bowers, 1977).

In an effort to identify novel pest-control products that are both environmentally acceptable and effective for the management of the migratory locust, Locusta migratoria, we assessed the effects of a methanolic extract of the above-ground portion of the perennial herb Haplophyllum tuberculatum (Rutaceae) on several reproductive variables in locust females. This plant is common in the Adrar region of Algeria, where it escapes feeding damage by L. migratoria (unpublished observations). Assayed against various organisms, H. tuberculatum extracts have been observed to display insecticidal (Mohsen et al., 1989), nematicidal (Onifade et al., 2008), antifungal and antibacterial (Sheriha et al., 1985, Al-Burtamani et al., 2005) properties. The plant’s chemical composition has been shown to vary as a function of geographic location and time of collection, and has so far been found to include alkaloids, lignans, flavonoids and essential oils (Khalid and Waterman, 1981, Sheriha and Abouamer, 1984, Sheriha et al., 1985, Al-Yahya et al., 1992, Al-Rehaily et al., 2001, Al-Yousuf et al., 2005, Al-Burtamani et al., 2005, Javidnia et al., 2006).

Reproduction and its endocrine regulation in L. migratoria have received considerable attention. Depending on rearing temperature, vitellogenin (Vg) first appears in the hemolymph of adult females between the 5th and 9th days after emergence, and its titers reach a peak of 25–30 mg/mL just before the onset of the second vitellogenic cycle (Chinzei and Wyatt, 1985). Although vitellogenesis in L. migratoria is primarily under the control of JH (Dhadialla and Wyatt, 1983, Wyatt et al., 1987, Wyatt, 1988), a neurohormone, the “ovary maturing parsin” (Lom-OMP), has been shown to promote Vg synthesis and vitellogenesis in adult females through a mechanism independent of JH, possibly through an ecdysteroidogenic effect (Girardie and Girardie, 1996, Girardie et al., 1998). However, although ecdysteroids play a role in the control of reproduction of many insects (Lafont et al., 2005), 20E is not essential for Vg production in adult locusts but appears to potentiate the JH-induced transcription of the Vg gene (Girardie et al., 1998). Follicular cells are the site of ecdysteroid production in L. migratoria and a high proportion (95%) is incorporated in eggs as polar or apolar conjugates (Lagueux et al., 1977, Lagueux et al., 1981, Gande and Morgan, 1979, Hagedorn, 1985), and can be bound to proteins such as vitellin (Hoffmann, 1980, Hagedorn, 1985, Tawfik et al., 1999). These conjugates are a source of ecdysteroids during embryogenesis (Lagueux et al., 1977), and their hydrolysis, in the eggs, is responsible for the peak of free ecdysone observed during embryonic development (Lagueux et al., 1981), where they are involved in the initiation of morphogenetic movements and the induction of cuticulogenesis (Lagueux et al., 1979).

For the present study, we assessed oocyte growth, vitellogenesis, oviposition, fecundity, fertility and ecdysteroid titers in L. migratoria females treated with a H. tuberculatum extract at emergence. Given that preliminary work showed similarities between the effect of this extract, on the variables considered here, and those documented for desert locusts, Schistocerca gragaria, treated with the benzoyl phenyl urea (BPU) diflubenzuron (Tail et al., 2008, Tail et al., 2010), we compared the effects of the extract to those of teflubenzuron, another IGR of the BPU family. BPUs are a group of insecticides that disrupt insect molt through their inhibitory effect on chitin synthesis (Cohen, 1987, Graf, 1993). Studies focusing on diflubenzuron, the first BPU to be commercialized, have shown that this molecule can also affect reproduction in adult females, with reports pointing to reductions in both fecundity and fertility (Lim and Lee, 1882, Soltani, 1987, Soltani and Soltani-Mazouni, 1992). Here we show that the two treatments have similar, yet distinct effects on the variables examined.