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Continuous rearing on Ephestia kuehniella reshaped quality of the parasitoid wasp Trichogramma brassicae (Hymenoptera: Trichogrammatidae)

https://doi.org/10.1016/j.aspen.2021.02.013Get rights and content

Highlights

  • Quality of T. brassicae decreased over long-term mass rearing on E. kuehniella.

  • G5 and G10 had the highest GRR, R0, r and λ.

  • G5 and G45 had the highest and lowest values of c0, ψ and ω, respectively.

  • The efficacy of wasps decreased dramatically after G20.

Abstract

Continuous mass rearing of Trichogramma brassicae (Bezdenko) at commercial mass-rearing insectaries may affect both quality and performance of natural enemies. In the present study, we studied the quality and performance of a colony of T. brassicae reared for over 45 generations (G) on Ephestia kuehniella Zeller using two-sex life table parameters and parasitism capacity. Our results revealed that although different generations showed no significant difference in terms of female longevity or total life span until G35, G5 and G10 had the highest values of fecundity, gross reproductive rate (GRR), net reproductive rate (R0), intrinsic rate of natural increase (r), and finite rate of increase (λ). No significant difference in male adult longevity was found among different generations. The longest and shortest mean generation times (T) were found in G10 (13.65 ± 2.31 d) and G45 (13.25 ± 3.37 d), respectively. The finite rate of parasitism (ω) ranged from 0.355 ± 2.332 host/parasitoid/day in G5 to 0.242 ± 0.017 host/parasitoid/day in G45. However, ω did not show any significant difference until G20. These results indicate that T. brassicae wasps held under continuous laboratory rearing declined in quality after 20 generations, and therefore periodical rejuvenation of the colony by adding feral parasitoids is strongly recommended.

Introduction

Egg parasitoids, especially trichogrammatid species, are among important natural enemies because of their important role in the timely control of insect pests (Cônsoli et al., 2010). Trichogrammatids can parasitize eggs of insects in 11 different orders (Knutson, 1998). Trichogramma brassicae (Bezdenko) has a wide geographical range in Iran (Ebrahimi et al., 1998, Iranipour et al., 2009) and other countries (Smith, 1996, Trdan et al., 2020), where it is commonly mass reared in insectaries on various factitious hosts, such as the Mediterranean flour moth, Ephestia kuehniella Zeller (Iranipour et al., 2009, Wang et al., 2014). This parasitoid has been used successfully against several important insect pests (including armyworms, borers, cutworms, fruit worms, leaf worms, leaf rollers, loopers etc.) in inundative release programs (Parra, 2010). The Mediterranean flour moth is commonly used for mass rearing of Trichogramma species because of its favorable egg size, high production from rearing medium, ease of mechanization, and ease of sanitation (Moghaddassi et al., 2019, Smith, 1996).

Natural enemies used in inundative releases are generally mass reared continuously for many generations under artificial conditions in insectaries to increase productivity and lower cost (Kölliker-Ott et al., 2003). However, long-term mass rearing in laboratory is known to cause inbreeding and loss of important traits needed to control pest in field conditions (Kölliker-Ott et al., 2003). Field population of natural enemy holds high genetic diversity which gets impacted due to laboratory colonization thereby altering their status from open to a “closed population”. All subsequent genetic event will apply selection pressure on this population, and therefore fitness of natural enemies reared under laboratory conditions may differ from those collected directly from the natural field ecological niches. Furthermore, such conditions may negatively affect traits necessary for diapause, hosts/prey location, or mating and may lead to a genetic revolution and the establishment of new balanced gene systems (Van Lenteren, 2003). Inbreeding may also occur in laboratory colonies, resulting in more genetically homozygous progeny (Chambers, 1977, Sørensen et al., 2012, Van Lenteren, 2003).

A population of natural enemies selected for use in inundative release should have an accepted level of fecundity, emergence, sex ratio (percentage of female offspring), longevity, a host preference for the target species, good host-searching ability, and tolerance to local weather conditions. Such populations are defined as high quality populations (Chambers, 1977, Smith, 1996, Trdan et al., 2020). However, the quality of any natural enemy should be examined before use. Life table parameters (Chen et al., 2018, Chi and Liu, 1985), parasitism capacity (Chi et al., 2011, Karami et al., 2018) and foraging behavior (Fathipour and Maleknia, 2016) are among the most important criteria to estimate the behavioral dynamics of insects under mass-rearing conditions.

Although Trichogramma species are frequently studied, assessment of their quality during long-term mass rearing has been neglected. Although the loss of preference for the target host has been demonstrated for some Trichogrumma species due to prolonged rearing on an artificial host (Lü et al., 2015, Lü et al., 2017, Nordlund et al., 1997, Pratissoli et al., 2004), there are cases in which the efficiency of natural enemies has not decreased significantly (Kölliker-Ott et al., 2003). Pratissoli et al. (2004) studied the undesired changes in biological traits of T. pretiosum Riley reared on Anagasta kuehniella (Zeller) (Lepidoptera: Pyralidae) for 23 generations and found reduced longevity in later generations. Lü et al. (2017) conducted an investigation on the life history traits of T. dendrolimi Matsumura in vitro and in vivo for over 30 generations. Although, they revealed higher pupation and adult emergence rates were found in parasitoids reared on a modified artificial medium, they had lower adult emergence rate and fecundity compared with those reared in vivo. A number of other studies have demonstrated the importance of quality control measures in the production of biological control agents reared for a long time in laboratory or insectary conditions to ensure the subsequent sustainability of the biological control program (Bellutti, 2011, Khanamani et al., 2017b, Miyatake, 1998).

In the previous study, we investigated the efficiency of T. brassicae reared for a long-time on S. cerealella (Ghaemmaghami et al., 2021). The aim of this study was to evaluate the quality and performance of T. brassicae reared on E. kuehniella for over 45 generations using two-sex life table and parasitism parameters.

Section snippets

Insect rearing

A colony of Mediterranean flour moth was established on a medium consisting of one kg of flour (prepared from Omid wheat cultivar), 200 g of wheat bran and 5 g of bread yeast, and reared at 25 ± 1 °C, 30 ± 5% RH and a 0:24 h (L:D) photoperiod in a laboratory at the Technical and Vocational Training Center of Estahban, Fars, Iran. The emerged adults were transferred to 20 cm dia funnels covered with a fine mesh net over the mouth. After mating, the eggs were collected on papers placed under the

Parasitoid life stage duration, survival rate and fecundity

The highest female longevity (7.30 ± 0.30 d) and total life span of all individuals (18.02 ± 0.42 d) were observed in G5. T. brassicae in G45 had the shortest female longevity (5.11 ± 0.25 d) and total life span (16.26 ± 0.28 d). In contrast to females, no significant difference in male longevity was found among different generations. All adult wasps emerged after approximately 12 days and in this respect, no significant difference was observed among different generations (Table 1).

Different

Discussion

Trichogramma is one of the most well-known genera of Trichogrammatidae, with a worldwide distribution. Many of its species have been mass reared due to the ease or low cost of rearing (Ayvaz et al., 2008, Cônsoli et al., 2010). Millions of these parasitoid wasps are produced and released each year to control different insect pests on various agricultural crops (Knutson, 1998). For an augmentative release program, parasitoids need to be reared on a large scale, which can be labor-intensive.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study is a part of the Ph.D. dissertation of the first author that was funded by Tarbiat Modares University, whose support is greatly appreciated. In addition, we would like to thank the Technical and Vocational Training Organization for assistance in providing facilities for this research.

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