Effect of Potassium Bicarbonate (MilStop®) and Insecticides on the Citrus Mealybug, Planococcus citri (Risso), and the Natural Enemies Leptomastix dactylopii (Howard) and Cryptolaemus montrouzieri (Mulsant)

Both laboratory and greenhouse experiments were conducted to determine if the fungicide, MilStop (BioWorks, Victor, NY), which contains the active ingredient, potassium bicarbonate, has direct activity on the citrus mealybug, Planococcus citri Risso. Spray applications of four different rates (4.5, 5.9, 7.4, and 14.9 g·L) were applied to green coleus, Solenostemon scutellarioides (L.) Codd. , plants infested with citrus mealybugs. In addition, experiments were conducted to assess both the direct and indirect effects of MilStop on two natural enemies of the citrus mealybug: the parasitoid, Leptomastix dactylopii (Howard), and the coccinellid beetle, Cryptolaemus montrouzieri (Mulsant). MilStop provided between 56% and 86% mortality of citrus mealybug; however, the highest rate (14.9 g·L) was phytotoxic to coleus plants. Percent mortality associated with the second highest rate (7.4 g·L) was 82%, which was comparable to acetamiprid (84%) applied at 0.05 g·L. For the natural enemies, MilStop treatment rates of 1.5 and 3.5 g·L resulted in 16% mortality, whereas the 5.5and 9.0-g·L rates resulted in 33% mortality of L. dactylopii adults. MilStop treatment rates of 3.5, 5.5, 9.0, and 12.0 g·L resulted in 30%, 60%, 40%, and 90% mortality, respectively, of C. montrouzieri adults. Therefore, depending on the application rate, this fungicide may inadvertently kill citrus mealybugs when used to control fungal plant pathogens. It should not disrupt biological control programs targeting citrus mealybug in greenhouses that involve releases of L. dactylopii when used at low application rates, whereas MilStop applications should be properly timed when using C. montrouzieri. Pesticides are used to manage arthropod (insect and mite) pest populations and diseases in greenhouses (Bethke and Cloyd, 2009; Tauber and Helgesen, 1978). Fungicides are applied to control plant pathogenic fungi, although certain fungicides have been shown to directly affect populations of arthropod pests including spider mites (Biggs and Hagley, 1988; Bower et al., 1995). However, no efficacy data are available for other arthropod pests including the citrus mealybug, Planococcus citri Risso, a major insect pest of greenhouses and other interior plantscapes throughout the United States (Kole and Hennekam, 1990; Kosztarab, 1996; McKenzie, 1967). If fungicides are efficacious against both plant pathogens and insect pests such as citrus mealybugs, the number of pesticide applications required during a cropping cycle could be reduced. Furthermore, because greenhouse producers may use biological control to regulate arthropod pest populations including the citrus mealybug (Bartlett, 1978; Doutt, 1952; Pilkington et al., 2010), it is important to determine if fungicides could potentially disrupt biological control programs. In general, specific fungicides are considered to be less harmful to natural enemies than insecticides and miticides (Wright and Verkerk, 1995). However, fungicides used in greenhouse production systems may negatively affect pest-natural enemy interactions, thus compromising the success of biological control programs (Cloyd, 2012; Echegaray and Cloyd, 2012). Potassium bicarbonate (MilStop ; BioWorks, Victor, NY) is a fungicide registered for use in agricultural and greenhouse cropping systems for the control of various foliar plant pathogens (Abd-El-Kareem, 2007; Cushman et al., 2007; Ziv and and Hagiladi, 1993; Ziv and Zitter, 1992). The effects of potassium bicarbonate have not been extensively evaluated against arthropod pests and there is minimal information on any direct or indirect effects on natural enemies. Gradish et al. (2011) showed that potassium bicarbonate and other pesticides evaluated were not directly harmful to the natural enemies, Orius insidiosus (Say), Amblyseius swirskii (Athias-Henriot), and Eretmocerus eremicus Rose & Zolnerowich, under laboratory and greenhouse conditions. Therefore, the objectives of this study were to 1) determine the efficacy of MilStop and other pesticides against the citrus mealybug; and 2) assess any direct and/or indirect effects on the natural enemies of the citrus mealybug, Leptomastix dactylopii (Howard) and Crytolaemus montrouzieri (Mulsant), also known as the ‘‘mealybug destroyer,’’ under both greenhouse and laboratory conditions. Materials and Methods Experiments were conducted to determine the direct effects of MilStop (active ingredient = potassium bicarbonate) on the citrus mealybug, Planococcus citri (Expt. 1) and the direct and indirect effects on two natural enemies of the citrus mealybug, Leptomastix dactylopii and Cryptolaemus montrouzieri (Expts. 2 through 4). Expt. 1: Effects of potassium bicarbonate (MilStop ) on the citrus mealybug. Sixty plants of green coleus, Solenostemon scutellarioides (L.) Codd. , were started from cuttings taken from stock plants and transplanted into 15.2-cm containers (Kord Products, Toronto, Canada) in a growing medium (Universal SB 300 Mix; Strong-Lite Horticulture Products, Pine Bluff, AR) consisting of 50% composted pine bark, 20% Canadian sphagnum peatmoss, 20% medium coarse vermiculite, 10% perlite, a starter nutrient charge, and a wetting agent. Plants were fertilized with 20N–8.3P–8.8K at 250 ppm nitrogen in a constant liquid feed program and grown in a glass greenhouse on 12 · 3-m wire-mesh raised benches and arranged in a completely randomized design. There were a total of 10 treatments, which included four rates of potassium bicarbonate and six other compounds including sesame oil, pyrethrins, polyetherpolymethylsiloxane, acetamiprid, alkyl aryl polyoxyethylene, and azadirachtin (Table 1). There were five replications per treatment. In addition, a water control and an untreated control (no water applied) were included. Effects of treatments on the citrus mealybug were evaluated 7 d post-application. Received for publication 2 Aug. 2013. Accepted for publication 10 Oct. 2013. We acknowledge Drs. Mary Beth Kirkham (Department of Agronomy) and Kun Yan Zhu (Department of Entomology) of Kansas State University, Manhattan, KS, and Dr. Randy Martin of BioWorks, Inc., Victor, NY, for reviewing previous versions of the manuscript. We also wish to thank three anonymous reviewers. The feedback we received from everyone was very much appreciated. To whom reprint requests should be addressed; e-mail rcloyd@ksu.edu. HORTSCIENCE VOL. 48(12) DECEMBER 2013 1513 DISEASE AND PEST MANAGEMENT