Field evaluation of Trichogramma strains collected from Myanmar for biological control of Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae) and sustainable maize production
Introduction
Globally, maize is the most important cereal crop and its production has steadily increased during the last decade (OECD-FAO 2020). Maize is the second most important crop in Myanmar after rice with a yearly production of 1596.000 MT (Myanmar Agriculture in Brief Yearbook, 2019). In Myanmar, it is used as a staple crop and a cash crop for rural farmers and as an agricultural export commodity. The global increase in production is due to several factors such as an increase in planting area, and the introduction of high-yielding varieties. However, maize production is at risk from a number of insect pests, including the Asian corn borer (ACB), Ostrinia furnacalis (Guenée) (Nafus and Schreiner, 1991; Wang et al., 2000). In Myanmar, maize is generally grown year-round, either intercropped or rotationally cultivated with rice, pulses, vegetables and other crops. Consequently, infestations of ACB have increased, leading to the disruption of kernel formation, ear dropping, and plant lodging (CABI 2014; Myint et al., 2021). Data are mostly lacking from Myanmar but yield losses of 10%–30% are observed in China (Wang et al., 2014) and damage rates and yield losses are likely similar in Myanmar (Babendreier et al., 2019). A problem contributing to these yield losses is likely the continuous cultivation of maize, practiced by many farmers’ globally, including Myanmar. In addition, ACB larvae, the only life stage that causes damage in maize, feed inside maize stalks, particularly during the late vegetative stage, which makes conventional control attempts based on chemical pesticides difficult and generally not effective (Showers et al., 1976; Hellmich et al., 1998). In addition to causing direct yield losses, larval feeding on silks and kernels further leads to entry points for ear rot, reducing grain quality and increasing mycotoxin contamination which ultimately threatens consumer health (Song et al., 2009; Razinger et al., 2016).
Ostrinia furnacalis and its sibling species the European corn borer, Ostrinia nubilalis, can be controlled by a number of management tactics such as pheromone traps with chemical pesticides, transgenic insect-resistant maize and biological control agents (Afidchao et al., 2013; Chen et al., 2013; Guo et al., 2014; Wang et al., 1999, 2014). Historically, farmers in Myanmar have rarely invested in efforts to control this pest (CABI, 2014; Babendreier et al., 2019). With the arrival of the invasive fall armyworm, Spodoptera frugiperda (J.E. Smith) in 2018 (CABI 2019), farmers in Myanmar started to apply insecticides during vegetative growth stages of maize. Pesticide utilization in maize has increased considerably, reaching 1,901,266 lb during a single maize season (Myanmar Agriculture in Brief Yearbook, 2019). Reduced natural populations of Trichogramma egg parasitoids to suppress ACB infestations have recently been reported in major maize growing regions of Myanmar (Myint et al., 2021). The crop losses by ACB larval feeding are anticipated to be higher in the future due to the availability of host plants throughout the year, intensive applications of insecticide, elimination of natural enemies and the farmers’ lack of knowledge on ACB infestation inside maize stalks, tassels, ear and cobs.
Due to the long-term effects of chemicals on the environment and human health concerns, efforts are needed to promote biological control as an alternative method to reduce infestations of O. furnacalis. A number of biological control agents are or have been applied against O. furnacalis and O. nubilalis, such as larval parasitoids (Macrocentrus cingulum Goidanich (Hymenoptera: Braconidae), Eriborus terebrans Gravenhorst (Hymenoptera: Ichneumonidae), egg parasitoids of the genus Trichogramma spp., and entomopathogenic fungi (Beauveria bassiana) (Hoffmann et al., 2006; Lu et al., 2020; Wang et al., 2014; Mason et al., 1994; Zhang et al., 2018).
Releasing Trichogramma egg parasitoids has been reported as the most successful biological control of O. furnacalis and O. nubilalis worldwide (Smith, 1996; Hassan, 1981; Wang et al., 1984, 2014). At least five Trichogramma species are commonly used for field application against both species, i.e., T. evanescens, T. brassicae, T. ostriniae, T. nubilale and T. dendrolimi (Bigler, 1986; Hassan, 1993; Felkl et al., 1990; Wang et al., 2014). The parasitoids T. ostriniae, T. dendrolimi, T. chilonis and T. evanescens have been successfully released in large quantities to control O. furnacalis in China and the Philippines (Tran and Hassan, 1986; Wang et al., 2014; Zhang et al., 2018). Mass releases of T. ostriniae (5 releases of 150,000 wasps/ha) were also successfully applied in DPR Korea with a significant reduction of ACB larval damage and an increase of 28% fresh yield compared to non-release plots (Zhang et al., 2010). The effectiveness of biological control using Trichogramma depends primarily on the biological traits of the species and strains used (Smith, 1996; Wang and Shipp, 2004; Tabone et al., 2010). Furthermore, it is highly important to conduct the release in a timely manner and at appropriate release rates to achieve successful biological control (Hoffmann et al., 2006). In addition, the performance of Trichogramma depends on factors including temperature, rain, availability of host eggs, and intraguild predation of natural enemies under field conditions (Fournier and Boivin, 2000; Zhang et al., 1995; Zhou et al., 2019). Recently, Huang et al. (2020) and Zang et al. (2021) reviewed the use of Trichogramma in China and indicated that sophisticated techniques have helped to mass produce egg parasitoids, leading to e.g., 5,500,000 ha treated with Trichogramma in 2015 in maize alone. Such inundative releases of T. dendrolimi achieved 70% ACB egg mass parasitism in the first generation (Zhang et al., 2015). Biological control agents have also been evaluated under field conditions for control of O. nubilalis in Iran (Movahedi et al., 2014). Despite these studies in neighboring countries, biological control of ACB has not attained a lot of attention in Myanmar until recently when Babendreier et al. (2020) reported on the establishment of Trichogramma production facilities. Myint et al. (2021) also found that three indigenous Trichogramma strains, i.e., two strains of T. ostriniae from Taunggyi and Yatsawk, and one strain of T. dendrolimi collected in Yatsawk of Shan State, Myanmar, were best performing in terms of host searching capacity and egg parasitism under laboratory and semi-field conditions. Based on such laboratory selection tests, the most promising strains should be further studied to assess the efficacy under field conditions before mass production (Hassan, 1994; Smith, 1996; Tang et al., 2017; Cherif et al., 2021; Zang et al., 2021). To date, there are no previous reports on inundative releases of any Trichogramma strains from Myanmar for the control of O. furnacalis control under field conditions. The main objectives of this study were to assess the performance of three Trichogramma strains collected in Shan State, Myanmar under field conditions, to evaluate the optimum release density and the number of releases, as well as to estimate economic benefits of O. furnacalis biocontrol by using Trichogramma.
Section snippets
Trichogramma rearing
The Trichogramma strains tested here were identified by a combination of molecular and morphological methods (Myint et al., 2021). The two strains of T. ostriniae (collected from Southern Shan State, T. o YS collected from Yatsawk, and T. o TG, collected from Taunggyi), and one T. dendrolimi strain collected in Yatsawk (T. d YS), were the best performing both in host searching capacity and egg parasitism under laboratory and semi-field conditions (Myint et al., 2022), and were therefore used in
Evaluation of egg mass and egg parasitism
In 2020, release density significantly affected egg mass and egg parasitism rate in the second and third release, while in the first release only egg parasitism rate was significantly increased with increased number of released wasps (Table 2, Table 3). Egg mass and egg parasitism rates did not vary among strains except for egg parasitism rates in the third release (Table 3). Overall, the T. ostriniae, T. o YS strain showed highest parasitism rates compared to the other two tested strains (
Discussion
This is the first report of Trichogramma strains collected from Myanmar being assessed against O. furnacalis under field conditions. With few exceptions, release densities and the number of releases strongly influenced mean egg parasitism, plant damage and yield. Low levels of natural parasitism were recorded in control plots during the field releases in 2020, however, no parasitism was found in the open field and the control plots in 2021. This low field parasitism in the control plots may be
Conclusions
To summarize, egg parasitism, plant damage and yield were strongly influenced by the release density and number of releases during 2-years of field trials. Our field trials showed that releasing Trichogramma effectively controlled O. furnacalis (60–90% parasitism) and reduced the number of larvae (50–80%), ear damage (60–80%) and yield losses (30–50%) by this pest with achieving higher net return (15,000–18,000 CNY/ha). Moreover, applying biological control agents for pest control might improve
CRediT authorship contribution statement
Yee Yee Myint: Formal analysis, Conceptualization, Writing – original draft, Writing – review & editing. Shuxiong Bai: Formal analysis, Writing – review & editing. Tiantao Zhang: Formal analysis, Writing – review & editing. Dirk Babendreier: Formal analysis, Methodology, Writing – original draft, Writing – review & editing. Kanglai He: Writing – review & editing. Zhenying Wang: Project administration, Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no conflict of interest.
Acknowledgement
We would like to thank the Institute of Plant Protection, Jilin Academy of Agricultural Sciences for assisting in the field experiment. This study was supported by the Agricultural Science and Technology Innovation Program (ASTIP), the China Agriculture Research System of MOF and MARA (CARS-02), the Organization for Women in Science for the Developing World (OWSD) and the Swedish INTERNATIONAL COOPERATION Development Agency (SIDA).
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