World Aquaculture December 2019

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2019 59 ( C O N T I N U E D O N P A G E 6 0 ) October, all market-size and larger cucumbers in each treatment were harvested and weighed. The predatory mites Amblyseius swirskii and Phytoseiulus persimilis (BioBest Group NV, Westerlo, Belgium) were distributed to cucumber plants in the greenhouse twice during the three-month experiment with one month between each application. Marigolds were planted as a source of pollen for A. swirskii populations. No biological agents were used to control the two-spotted spider mite Tetranychus urticae but bifenazate was sprayed for mite control before any biocontrol agent was introduced. Later during the growth period, imidacloprid was diluted in irrigation water to control a green peach aphid Myzus persicae infestation without significantly affecting populations of natural enemies. Twelve plants per treatment were randomly chosen and marked with a red flag (Fig. 2). Insect scouting on each of these 48 plants was performed weekly by identifying and counting every insect on six random leaves on each plant. The organisms counted were the green peach aphid M. persicae , the western flower thrip Frankliniella occidentalis , the silverleaf whitefly Bemisia tabaci and the two- spotted spider mite Tetranychus urticae. Statistical analysis of the data was performed using SAS (V.9.2, SAS Institute Inc., Cary, NC, USA). Simple ANOVA and the Student Newman-Keuls means separation test were used to determine significant differences among treatment means (α = 0.05) . Results and Discussion Fish tank effluent increased cucumber production. Plants that were irrigated with fish tank effluent supplemented with fertilizer produced a significantly greater biomass of cucumbers (368 kg) than plants in all other treatments. Plants irrigated with aquaculture tank effluent but not fertilized produced 274 kg of cucumber, more than plants irrigated with groundwater alone (257 kg) but less than plants irrigated with groundwater and fertilized (303 kg). Fish tank effluent improved plant growth by supplying nutrients for cucumber plants and by enriching the soil with organic matter, resulting in better water retention by soil. There were no significant differences in counts of thrips and whiteflies among treatments. Counts of both pests remained less than what is considered the economic threshold of 5 whitefly adults/ plant (Jeon et al . 2009) and 3.6 thrip adults/flower (Zepa-Coradini et al . 2010), with minimal fluctuations in counts during the growing period. These low counts are not typical of cucumber agriculture in Lebanon and are assumed to have remained low because of the predatory mites. Similar results were reported by Calvo et al . (2015). Accordingly, irrespective of irrigation water source, augmentative biocontrol on cucumber in greenhouses using predatory mites is effective against silverleaf whitefly and the western flower thrip. Counts of aphids and mites on plant leaves were affected by the water used for irrigation and by fertilization. Green peach aphid and two-spotted spider mite populations increased throughout the season on all plants. However, by the fourth counting event, aquaculture tank effluent water caused an increase in abundance of the two pests on plant leaves (Tables 1 and 2). Fish tank effluent always contains nitrogenous compounds and Gotyal et al. (2015) theorized that a large supply of nitrogen to plants strengthens their physiological health but renders them susceptible to herbivorous pests. Green peach aphids inflict their damage by feeding on phloem sap of host plants (Douglas 2006) while two-spotted spider mite feed by emptying cell content (Bensoussan et. al . 2016). Both target areas for insect pest feeding are involved in the nitrogenous pathway, where amino acids compounds are transported by the sap, and nitrogen is assimilated by chloroplastic enzymes in newly developed leaves (Masclaux-Daubresse et al . 2010). Consequently, they become nutrient rich areas for sustaining insect pest life. Turcios and Papenbrock (2014) suggest that high concentrations of nitrogen in fish tank effluent would attract insects and potentially increase infestation of commercial crops. The addition of the predatory mite Phytoseiulus persimilis was successful in controlling two-spotted spider mites, significantly reducing their numbers on crops. We started observing a gradual decrease in mite number at T5 until their population was close to non- detectable (Table 2). Simultaneously, counts of P. persimilis indicated that their population was increasing. The increase of two-spotted spider mite at the beginning of the experiment suggests that the first introduction of P. persimilis on July 7 was unseccussful. However, the second introduction on August 10 was deemed successful because it was followed by a significant decrease in two-spotted spider mite counts. Also, P. persimilis became well established in the greenhouse. The decreasing populations of P. persimilis towards the end of the plant growing period was probably caused by the exhaustion of feed sources (i.e. insect pests), followed by cannibalism (Walzer and Schausberger 1999). A periodic assessment of the predatory mite A. swirskii population indicated that the mite remained alive and even suggested an increase in numbers throughout the experiment, indicating the presence of sufficient numbers of insect pests to preserve their population and allow reproduction. They probably were feeding on whitefly eggs given that the cycle of whiteflies does not exceed 25 days at the prevailing experimental temperatures (Perring et al . 2017) and possibly feeding on thrips. Xu and Ekegaard (2010) reported that A. swirskii consumes nymphal stages of thrips twice as much as they do two-spotted spider mites. Hence, the present experiment focused on the efficiency of A. swirskii in controlling whiteflies and thrips while P. persimilis were allocated for mite control. Silverleaf whiteflies and western flower thrips remained below their economical threshold FIGURE 2. Greenhouse with growing cucumber plants. Note red flags placed at the base of randomly assigned plants for insect pest surveillance.