Synergistic Effects and Toxic Mechanism of Phosphine with Ethyl Formate against Citrus Mealybug (Planococcus citri)

Methyl bromide (MB) has been used in a wide range of applications, but since it was determined to be an ozone-depleting compound, it has only been used for pre-shipment and quarantine purposes in trade. Phosphine (PH3) is currently the ideal fumigant as an MB alternative worldwide. However, the development of PH3 resistance in the target insect pest and longer PH3 fumigation treatment times raise questions about the continued use of PH3. This study attempted to shorten treatment time via combination treatment with ethyl formate (EF). Planococcus citri was used as the main quarantine pest in Korea, and the acute toxicity of EF, PH3, and EF + PH3 was determined at every developmental stage. EF treatment at 4 h showed LCT99 values of 45.85~65.43 mg∙h/L, and PH3 treatment at 20 h showed that of 0.13~0.83 mg∙h/L depending on the developmental stage. The efficacy of PH3 decreased after reducing the treatment time, but synergistic effects were observed at all stages of development of P. citri when both fumigants were used simultaneously for 4 h. After combined treatment, dihydrolipoyl dehydrogenase expression and the production of two phospholipids, PI(O-16:0) and PC(18:2), were significantly reduced in treated P. citri adults compared with the control. Therefore, combined treatments might be key to reducing the treatment time and resistance of PH3 in the field.

Ethyl Formate as a Methyl Bromide Alternative for Fumigation of Citrus: Efficacy, Fruit Quality, and Workplace Safety

Ethyl formate (EF) was evaluated as a potential alternative to methyl bromide (MB) for phytosanitary treatment of imported citrus fruit in the Republic of Korea. Planococcus citri (Risso) (Hemiptera: Pseudococcidae), a mealybug with known tolerance against EF and MB, was used as a representative pest to test efficacy of the two fumigants against eggs. In nine commercial-scale refrigerated container (67.5 m3) trials using imported orange, lemon and grapefruit, EF applied at the currently approved dose for citrus (70 g·m-3 at 5°C for 4 h, developed for Aspidiotus excisus Green (Hemiptera: Diaspididae), a species less EF tolerant than P. citri) resulted in 76.9–98.3% mortality of P. citri eggs. The EF treatment did not affect the sugar content or the color of peel and pulp of the treated fruit. When oranges were treated according to the current MB (64 g·m-3 at >5°C for 2 h) or EF treatment guidelines, the concentration of fumigant around the fruit fluctuated between 9.4 and 185.1 ppm for EF and 9.5–203.0 ppm for MB during the 72-h post-fumigation processes (venting [0–2 h], transportation to storage [2–24 h], and storage periods [24–72 h]) with both EF and MB maintained between 10 and 100 ppm during the storage period. Considering the efficacy of EF, its apparent lack of phytotoxicity, and its more manageable threshold limit value for humans (100 ppm EF compared to 1 ppm MB for an 8-h time weighted average exposure), our results suggest that EF may be a promising alternative to MB for the phytosanitary treatment of imported citrus in Korea.

Lethal and sub-lethal toxicity of Verticillium lecanii on the biology of Mealybug (Pseudococcidae; Homoptera) under laboratory conditions

The Mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae) is a polyphagous pest of Citrus, grapevine, coffee and ornamental plants. The entomopathogenic fungus Beauveria bassiana, Metarhizium anisopliae, Verticillium lecanii and Isaria fumosorosea are very effective and commonly used as a biopesticide against various insect pests of different crops. The present study was conducted to check the lethal and sub-lethal effects of V. lecanii on mortality and life period of adult mealybug. Firstly LC50 value for nymph and adult was calculated by applying four different concentrations 1×105 , 1×106 , 1×107 and 1×108 CFU/ml. For nymphs, the median lethal concentration was 5.57×106 CFU/ ml, and for adults, 2.66×107 CFU/ml. After this, two lethal, 3.99×107 and 2.66×107 CFU/ml, and two sub-lethal concentrations, 1.33×107 and 6.65×106 CFU/ml, was prepared and tested against adult mealybug. Dose-dependent mortality was observed, and the highest mortality was due to the highest concentration of V. lecanii. The percentage mortality was 72.23, 52.78, 27.78 and 19.45% due to 3.99×107 , 2.66×107 , 1.33×107 and 6.65×106 CFU/ml, respectively, after 10 days of application. The weight of adult females was highest in the control treatment while lowest in high concentration, and it was gradually decreased after every day in the first three concentrations. The fecundity of mealybug also had an inverse relation with concentrations of V. lecanii, and the numbers of eggs were more in the control treatment than the other four treatments. The fecundity in lethal concentrations was reduced after the 8 th day of application. The life duration was most extended due to control treatment, 27 days, while the lowest period was due to the highest concentration, 13 days. Keywords: Verticillium lecanii, mealybug, mortality, weight, fecundity, life duration

Intraguild Interactions between the Mealybug Predators Cryptolaemus montrouzieri and Chrysoperla carnea

The ladybird Cryptolaemus montrouzieri and the green lacewing Chrysoperla carnea have shown potential for use in augmentative biological control of mealybug pests in greenhouse crops. In the context of combining these predators within an integrated pest management system, the risk of negative intraguild interactions between both predators was evaluated in a laboratory setting. Different life stages of either predator were confronted in petri dish arenas containing a Ficus benjamina leaf, and after 24 h the incidence and direction of intraguild predation (IGP) was recorded for each combination. The effect of adding Planococcus citri nymphs or Ephestia kuehniella eggs as extraguild prey on the level of IGP was also studied. IGP was frequently observed between the two predator species and was asymmetrical in favour of C. carnea in most cases. The presence of extraguild prey reduced the number of IGP events between the predators to a similar extent. The relevance of the observed intraguild interactions for the combined use of these predators in protected cultivation is discussed.