Introduction of two Biocontrol Agents Euxestonotus error and Platygaster tuberosula for the Management of Wheat Midge Population in Montana

Principle Investigator: Gadi V.P. Reddy

Project Personnel: Govinda Shrestha, Dan Picard and Ramadevi L. Gadi

Western Triangle Agricultural Research Center, Montana State University, 9546 Old Shelby Rd., P.O. Box 656, Conrad, MT 59425, USA

Aim of the Study

The aims of this study were: 1) to develop rearing methods for Euxestonotus error and Platygaster tuberosula under WTARC laboratory conditions and 2) to release the E. error and P. tuberosula at wheat midges infested fields in the Golden Triangle area of Montana

Material and Methods

The release process of parasitoids were illustrated in Fig 1. Collection and storage of parasitoids

Prior to collection of parasitoids, a federal import permit (Permit#P526-141217-033) for Euxestonotus error and Platygaster tuberosula (Hymenoptera: Platygastridae) was issued on March 10, 2015 by USDA- Animal and Plant Health Inspection Service. In July 2015, approximately 20,000 wheat heads were collected from spring wheat fields of Langenburg, Saskatchewan (Canada) where E. error and P. tuberosula had been released in 1993 and 1994 and are now known to establish in this region. Immediately after the collections, parasitoids were transported to Western Triangle Agriculture Research Center, Montana State University, where the wheat heads were spread out in an even layer and left at room temperature (19–22°C) to dry for approximately 2 weeks in the laboratory. A small-sample de-awning machine was used to gently thresh dried heads. Midge larvae were separated from the seeds and the chaff with an air cleaner (Fig.1). Approximately 1500 larvae were harvested in this manner from 20,000 infested wheat heads. Harvested larvae were then placed in a soil-less mixture of vermiculite and sphagnum and stored at 2–4°C.

Fig 1. Whole release process of parasitoids

Literature review for rearing of parasitoids

Florent Affolter (1990) reported that emergence of E. error and P. tuberosula can occur within 4-5 weeks under laboratory conditions at 22oC. Although it has not been clearly stated whether the incubation of parasitoids began from egg or larval stage, we speculated that the rearing of parasitoids had been done from the larval stages. This study further indicated that puparium can be formed after 18 days of incubation and the pupal stage could last for 12 ± 1.5 days (n=9) and 21 ±2 days (n=7) respectively for male and female. We are therefore motivated to use 22oC as an incubation temperature for parasitoids and expected to see emergence in 4-5 weeks.

Regarding to the emergence of wheat midges, Thompson and Reddy (2016) reported that emergence of wheat midge adults may occur from June 16 to June 30 at the Golden Triangle area of Montana. The study further showed that 10% of wheat midge adults will be emerged by June 23 followed by 50% on June 26 and 90% on June 30. Although there might be a possibility of variation in emergence dates of wheat midge, the highest midge emergence could appear in last week of June.

Protocol development for morphological characteristic of E. error and P. tuberosula

The protocol to determine the morphological characteristic of E. error and P. tuberosula was developed with the great assistance of Peeter Neerup Buhl, a taxonomist working within the field of platygastridae. The criteria to identify the two parasitoid species are highlighted in Fig 2.

Fig 2. Morphological characteristic of parastioids Taking out parasitoids larvae from the refrigerator

After a protocol was developed for rearing of parasitoids, the soil mixtures containing the parasitoid larvae were taken out of the insect refrigerator on June 3rd, 2016. The parasitoid larvae were placed in plastic round deli containers. However, the containers were first filled with garden soil and afterward the larvae were placed in the top layer of the soil. These containers were sprayed (hand sprayer) with distilled water (3-4 ml) to moisten the soil. When the parasitoids were expected to emerge (3 weeks after incubation), all these containers were taken out from the growth chamber and placed in insect cages where they were further sprayed with distilled water at 1-2 days intervals. In each insect cage, about 5-7 plastic containers were placed and the emergence of parasitoids were observed every day. After 1-2 days of parasitoid emergence in an insect cage, they were taken out with the help of an aspirator and kept in Petri dishes (1 parasitoid per Petri dish). These Petri dishes were then kept in a cooler growth chamber (10°C) for 2 hours and subsequently, the parasitoid species were identified under a stereomicroscope.

Selection of wheat midge infested fields for parasitoids release

Two highly infested wheat midge fields were selected during the third week of June, 2016 in Valier, Montana and it was based on midge trap count data. Immediately after the fields’ selection, sweep netting activity (1-2 times before the parasitoids release) was performed to assess whether there were presence of E. error and P. tuberosula prior to their release at field sites. Selected field growers were further asked not to spray insecticide or inform us of their insecticide spray timing. As a result, we could better plan for release dates of the parasitoids.

Parasitoids release

The parasitoids species were identified under a stereomicroscope and released in wheat midge infested fields on different dates.

Results

Rearing of Euxestonotus error and Platygaster tuberosula

The parasitoids E. error and P. tuberosula were successfully reared under WTARC laboratory conditions. However, the number of parasitoids emerged was found extremely low as compared to the number parasitoids cultured (Fig 3) and, only 13 % of parasitoids adults were emerged from the cultured parasitoid larvae. It is suspected that many of parasitoids larvae could have died when they were stored at 2–4°C for 5-6 months.

Fig 3. Parasitoids rearing history

Pre-release survey of Euxestonotus error and Platygaster tuberosula

This pre-release survey clearly depicted no presence of E. error and P. tuberosula at our selected field locations (Table 1) and it therefore allowed us to release these two parasitoids at our study sites.

Table 1. The information regarding to pre-releases of parasitoids

Release of Euxestonotus error and Platygaster tuberosula

Parasitoids P. tuberosula and E. error were released on several dates at two field locations from June 29 to July 14, 2016 since there was no consistency on emergence pattern of both parasitoids under our WTARC lab condition (Table 2). 136 parasitoids were released at Cory Crawford’s field from June 29 to July 8 and 65 parasitoids were released at Jody Hobel’s field from July 10-14

(Table 2). Altogether 211 parasitoids were released and 50 parasitoids escaped from the cages during the release process.

Table 2. The number of parasitoids released on different dates on two spring wheat fields

(CM= Connie, Miller, CC= Cory Crawford and GS= Govinda Shrestha)

Acknowledgements

We would like to thank Connie Miller for assistance with field work. This work was supported by Montana Wheat and Barley Committee. This material is also based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of

Agriculture, Multistate Project W3185, The Working Group Biological Control of Pest Management Systems of Plants under Accession # 231844.

References

Affolter, F. 1990. Structure and dynamics of the parasitoid complex of the wheat

midges Sitodiplosis mosellana (Géhin) and Contarinia tritici (Kirby). International Institute for Biological Control, Delémont.

Thompson, B.M., and G.V.P. Reddy. 2016. Status of Sitodiplosis mosellana (Diptera: Cecidomyiidae) and its parasitoid, Macroglenes penetrans (Hymenoptera: Pteromalidae), in Montana. Crop Protection 84: 125-131.