Phosphine Susceptibility of Adult Megaselia scalaris (Loew) (Diptera: Phoridae)

Megaselia scalaris was originally purchased from Sumika Technoservice Corp. (Hyogo, Japan) and maintained on a 1:1:2 mixture of rodent diet (Oriental Yeast Co. Ltd., Tokyo, Japan), wheat bran, and water (10), under 24–28 °C. Twenty unsexed adults (2–3 d after emergence) were released on 4 g of feed in 29 mm ∅ × 95 mm polystyrene vials, and the vials were capped with a polyurethane foam plug. After 24–48 h, the adults were removed, and another 48–72 h later, ca. 1 g of wheat bran was used in the diet for pupation. The pupae were collected from the bran and maintained under rearing conditions. The adults were collected within 24 h of emergence and fed a 10% glucose solution.

The third instar larvae and pupae for the evaluation of phosphine susceptibility were collected from rearing vials. Adults within 24 h after emergence were collected after being kept at 5 °C for 5–10 min to suppress their moving. To prepare eggs, 30 unsexed adults of 48–72 h after emergence were enclosed in 25 mm ∅ × 50 mm vials and covered with #80 nylon mesh. A piece of absorbent cotton soaked in 1% solution of beef extract (Nacalai Tesque, Kyoto, Japan) was put on the mesh cover, and the vial was kept at 27 °C for 12–14 h under darkness. The deposited eggs on the mesh and cotton were picked up with a fine brush. Fifty eggs, 30 larvae, 30 pupae, or 30 adults were placed as a group into polystyrene vials measuring 29 mm in diameter and 95 mm long with 1 g feed. The vials were then placed in gas-tight acrylic resin container measuring 160 mm × 295 mm × 100 mm. Three vials for each developmental stage were used in the experiment. Phosphine fumigation was performed using the protocols described by HORI and KASAISHI (11). Phosphine was generated from aluminum phosphide (Tyvek^®, Degesch Japan Co. Ltd., Saitama, Japan) and introduced into the containers after adjusting the concentrations using a phosphine gas analyser (Komyo Rikagaku Kogyo K.K., Kanagawa, Japan).

To assess the susceptibility of developmental stages, the containers with 200 ppm phosphine were maintained at 25 °C for either of 2, 4, 8, or 24 h in an MLR-352 plant growth chamber (PHC Corporation, Tokyo, Japan). For untreated control, the container prepared in the same manner without phosphine were kept at 25 °C for 24 h. For dose-response assay, the containers with/without phosphine were maintained at either of 15, 20, or 25 °C for 6 h. After exposure, the insects in vials were held at 27 °C until their survival was checked. Life or death of adults was determined 1–2 h after treatments and those of the other stages were determined 14 d after treatments.

Probit analysis was performed using the PriProbit (ver. 1.63) computer program developed by SAKUMA (12) which was downloaded from http://www.ars.usda.gov/Services/docs.htm?docid=11284. The phosphine concentrations needed to achieve probit 9 mortality (≈LC_99.9968) were read from the data table for linear graphic display.

Table 1 shows the lethality of phosphine to each developmental stage. All adults exposed to 200 ppm phosphine at 25 °C died within 4 h, and the most pupae survived the same treatment for 24 h exposure. These results show that the pupa is the most tolerant and that the adult is the most susceptible stage to phosphine. Fortunately, life stages other than adults are not associated with cured tobacco, and therefore a phytosanitary treatment should be effective against adult, the least tolerant stage.

No adult flies died that were not treated with phosphine. The mortality data obtained in this test were well fitted with the probit model (Figure 1). The phosphine concentrations required to achieve probit 9 mortalities (≈LC_99.9968) in the adult were calculated to be 636.2 ppm at 15 °C, 565.9 ppm at 20 °C, and 280.1 ppm at 25 °C (Table 2). The concentration × time products (Ct, ppm·d) at the respective temperatures were calculated to be 159.1 ppm·d at 15 °C, 141.5 ppm·d at 20 °C, and 70.0 ppm·d at 25 °C. These values are lower than those recommended for the control of insect pests of cured tobacco leaves by CORESTA, which are 1800 ppm·d (300 ppm × 6 d) at 16–20 °C and 800 ppm·d (200 ppm × 4 d) at temperatures higher than 20 °C (8). Phosphine is much more effective at lower concentrations over longer exposure periods than at higher concentrations for a shorter period; the relationship can be expressed by the modified form Cⁿt = k (constant), where n is less than 1 (13). Such a relationship has not been confirmed in this species. However, it appears that the CORESTA recommendations for cured tobacco fumigation will also control the adults of M. scalaris.

Figure 1

Large-scale confirmatory tests should be further performed to validate the efficacy for quarantine treatment. Theoretically, a minimum of 93,613 insects are required to verify the probit standard at the 95% confidence level (14). The estimated probit 9 values obtained in this study can be utilized for the development of phosphine-based quarantine and preshipment treatments for this species.