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Sexual selection is one of the most active areas in studying behavioural ecology because it directly influences the reproductive fitness of insects. Individuals select their partners because potential mates vary in quality, quantity and availability. Sexual selection may occur through intrasexual selection where males or females compete for mates or through intersexual selection where females or males choose

their mates with certain characteristics (Jennions and Petrie 1997; Panhuis et al. 2001).

For aphidiide species, females are monandrous and males are polygynous. Therefore, there is no issue about sperm competition. For females, premating mate choice is extremely important especially in species with very female-biased sex ratio; they should be very choosy theoretically and mate with ‘best’ possible mates. Age and body size may be the major characteristics that affect the sexual selection. It is expected that females should choose large and young males to obtain more sperm, and males should select large and young females to fertilize more and better eggs.

CHAPTER 3

GENERAL BIOLOGY OF APHIDIUS ERVI

3.1 Introduction

This chapter described the general methodology applied throughout this

research, investigated the general biology of A. ervi in emergence, sex maturation and

oviposition patterns of A. evi and then determined the effect of parasitism on host

reproductive potential.

3.2 General Methodology

The materials, procedures, environmental conditions and definitions detailed in this section were used throughout the thesis.

3.2.1 Materials

Breeding colony: A breeding colony started from A. ervi emerged from blue- green lucerne aphid, collected on lucerne in Palmerston North, New Zealand in

December 2002. Both blue-green lucerne aphid and pea aphid are hosts of A. ervi, and

lucerne is the host plant of both species; however, because of the difficulty of growing

lucerne in greenhouse or laboratory, pea aphid was used as host of A. ervi and broad

bean, Vicia faba L. cv. Pride, was used as host plant of pea aphid. The A. ervi colony

was reared on pea aphid for five generations before used in experiments.

Breeding cage: A. ervi and pea aphid were maintained separately in aluminium

framed cages (64 cm in length × 45 cm in width × 40 cm in height) with fine metal

screen (0.2 mm in length × 0.2 in mm in width) on the back and both sides and

Figure 3.1 Breeding cage used for rearing A. ervi and pea aphid.

Figure 3.2 Experimental cylinder.

Experimental cylinders: All experiments were carried out in transparent plastic

cylinders (8.5 cm in diameter × 10.5 cm in height) with gauze-covered holes, one in

the top (5 cm in diameter) and two (2 cm in diameter) in opposite sides for ventilation

(Figure 3.2). A plastic container (6.5 cm in diameter × 8.5 cm in height) was firstly

inserted into a cardboard (10.5 cm in length × 10.5 cm in width) via hole (6.5 cm in

diameter) cut in the centre before the container was filled with tap water and covered. A broad bean cutting was placed in the plastic container via a hole (0.6 cm in diameter) cut in the centre of a cover. The plastic cylinder was then placed on and sustained by

Plastic cylinder

Cover of plastic container

Plastic container Cardboard

the cardboard (Figure 3.2). The broad bean cutting was replaced when necessary. Honey solution (10%) was supplied for parasitoids as food daily in a cotton wool wick (1 cm in length), inserted through a hole (0.6 cm in diameter) in the top of the cylinder (Figure 3.2).

Microscope: A stereomicroscope (Leica MZ12, German) equipped with a micrometer eyepiece was used for dissecting parasitised aphids and measuring body size of newly emerged adults. A compound microscopy (Olympus, GH, Japan) equipped with transmitted light and a micrometer eyepiece was employed for

counting the number of eggs in newly emerged A. ervi females.

3.2.2 Procedures

Eggs laid: To determine the number of eggs laid by A. ervi in a parasitised aphid, aphids were dissected in a droplet 70% alcohol 4 d after parasitisation under the stereomicroscope and the number of larvae in each aphid was counted. The number of parasitoid larvae recorded from dissecting was assumed equal to the number of eggs laid (Bueno et al. 1993).

Egg load: To determine the egg load of A. ervi females in ovaries at

emergence, females were killed by freezing at -20°C and dissected in a droplet of

70% alcohol on a slide under the stereomicroscope. One droplet of acid fuchsin was added to the alcohol for staining of the ovaries. After 3 ~ 5 min, the ovaries were covered with a slide cover and spread by gently pressing the slide cover. The number of eggs in the ovaries was counted and recorded under a compound microscope.

Emergence: A. ervi adults used for experiments emerged from mummies that were individually maintained in glass vials (1.5 cm in diameter, 5.0 cm in height). Mummies from each experiment were removed from bean plants and maintained in

Petri dishes (5.5 cm in diameter × 1.3 cm in height) for emerging, and emerged adults

Sex identification: The females could be distinguished from males by the sharp abdomen tip.

3.2.3 Environmental Conditions

The colonies were maintained and experiments were carried out in bioassay rooms at 20 ± 1°C and RH 60-70% with a photoperiod of 16:8 h L:D. Lighting was provided by high frequency broad-spectrum biolux tubes (Osram, Germany).

3.2.4 Definitions

Number of parasitism: number of aphids parasitised by female A. ervi.

Parasitism rate: percentage of aphids parasitised by female A. ervi. Superparasitism: when two or more eggs were laid in an aphid.

Fecundity: the total number of eggs laid detected by dissecting.

Fertility rate: the proportion of female progeny.

Reproductive fitness: female parasitoids - fecundity, fertility, longevity, number and body size of progeny, and egg load of newly emerged female progeny; male parasitoids – longevity, number of mates inseminated and progeny fathered, and body size of progeny.

3.2.5 Statistical Analysis and Reported Values

All statistical analysis were set at P < 0.05 and carried out using SAS STAT (SAS Institute 2006). Unless stated otherwise, all reported values are means ± SE.