As a Potential Control Agent of Tetranychus Urticae (Acari: Tetranychidae): Effect of Pest/Predator Ratio on Pest Abundance on Strawberry
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Experimental and Applied Acarology (2005) 37:57–66 DOI 10.1007/s10493-005-0067-7 Ó Springer 2005 -1 Neoseiulus californicus (Acari: Phytoseiidae) as a potential control agent of Tetranychus urticae (Acari: Tetranychidae): effect of pest/predator ratio on pest abundance on strawberry NANCY M. GRECO*, NORMA E. SA´NCHEZ and GERARDO G. LILJESTHRO¨ M Centro de Estudios Parasitolo´gicos y de Vectores (UNLP-CONICET), CEPAVE, 2 No. 584 (1900), La Plata, Argentina; *Author for correspondence (e-mail: [email protected]; phone: +54- 021-233471; fax: +54-21-232327) Received 21 February 2005; accepted in revised form 20 June 2005 Key words: Conservation biological control, Neoseiulus californicus, Predator/prey ratios, Straw- berry, Tetranychus urticae Abstract. Neoseiulus californicus (McGregor) is a promising agent for successful Tetranychus urticae Koch control through conservation techniques, in strawberry crops in La Plata (Buenos Aires, Argentina). In prey–predator interaction, initial relative densities have an important effect on system dynamics. The economic threshold level (ETL) used for this pest in the present study was 50 active mites per leaflet. In our laboratory experiments, initial T. urticae to N. californicus ratio had a significant effect on the population abundance of T. urticae at a 7-day period. When pest/predator ratio was 5/1 (at initial pest densities from 5 to 15 females/leaflet) the final number of active T. urticae/leaflet was significantly lower than the ETL, while at 20 females/leaflet this number did not differ from the ETL. At 7.5/1 ratio, the final number of active T. urticae/leaflet, at initial pest densities from 5 to 15 females/leaflet, reached the ETL without surpassing it. At 10/1 and 15/1 ratios, pest densities exceeded the ETL only at 15 initial T. urticae/leaflet. Most green- house and field observations were consistent with the predictions of a graphical model based on experimental results. This predator was very effective in limiting pest densities at a 7-day period and within the range of pest–predator ratios and absolute densities used in this study. Conservation of N. californicus promoting favorable pest/predator ratios may result in early control of T. urticae. Introduction Predaceous phytoseiid mites are important biological control agents of pest mites affecting many crops in different parts of the world (Helle and Sabelis 1985). In some agroecosystems, these natural enemies may drive spider mite numbers below economically damaging levels (Nyrop et al. 1998). Phytoseiulus persimilis Athias-Henriot, Neoseiulus fallacis (Garman) and N. californicus (McGregor) are commonly used through augmentative techniques, to control the two-spotted spider mite Tetranychus urticae Koch, in several countries (Hussey and Scopes 1985; Waite 1988; Raworth 1990; Zalom et al. 1990; Coop et al. 1997; Steinberg et al. 1999; Zalom 2002). 58 In commercial strawberry crops in La Plata (Buenos Aires, Argentina), the two-spotted spider mite is the most serious arthropod pest and N. californicus is its main established predator. It has a broader food range and a higher capacity to stay in patches with low pest density than other phytoseiids (Wei-Lan and Laing 1973; Croft et al. 1997; Pratt et al. 1999). Moreover, in this area, N. californicus was observed exhibiting a high spatial coincidence with T. urticae and a high ability to detect leaflets with prey (Greco et al. 1999). All these attributes suggest that it would be a promising natural enemy for successful T. urticae control through augmentative or conservation techniques. A current control method for this pest consists in the use of miticides on a calendar basis. Many growers have recently started to decrease their use in order to cut costs or with the purpose of obtaining chemical-free production. Conservation biological control would be the most appropriate alternative considering the need to reduce inputs in this region. Indeed, the natural occurrence of N. californicus on strawberry crops in Spain, due to the selective use of pesticides, was sufficient for T. urticae control (Ribbes 1990; Garcı´a Mari et al. 1991). In prey–predator interactions, initial relative densities have an important effect on system dynamics (Shaw 1985). The levels of control of T. urticae attained by phytoseiid mites depend on prey to predator ratios favorable to the predator (Strong and Croft 1995), among other factors. Therefore, investi- gating the effect of T urticae/N. californicus ratio on the pest abundance would be very important to develop a biological control protocol for T. urticae in La Plata strawberry crops. The knowledge of effective T. urticae/N. californicus ratios would allow growers to make control decisions based on weekly moni- toring schedule of pest and predator, using miticides only when N. californicus predation cannot prevent pest damage. Considering that the pest population has an exponential growth we would expect that different T. urticae/N. californicus ratios be required to achieve a satisfactory control at different initial pest densities. Our objectives were: (1) to determine the effect of initial T. urticae and N. californicus ratio on T. urticae density after a week interval, in laboratory experiments, and (2) to examine the experimental laboratory results in relation to field samples. Materials and methods Laboratory experiments Tetranychus urticae and N. californicus colonies were reared in an experimental greenhouse on strawberry plants (Fragaria X ananassa). The short-day cultivar ‘Camarosa’ was used. Experiments were conducted under controlled conditions of 25 ± 2 °C, 60– 70% relative humidity and 14L/10D photoperiod. The experimental unit was a 59 new and completely expanded trifoliate leaf. Each petiole was placed in water- filled tubes (height 7 cm, diameter 2 cm) to keep the leaves turgid. Tubes were placed in plastic containers (500 ml) covered with plastic film. The experiments were planned as a completely randomized design and each treatment was replicated between 8 and 15 times. One day old T. urticae females and recently mated N. californicus females were used for the experiments. Spider mites and predators were transferred to the leaflets with a very fine brush. The experi- mental initial pest/predator ratios as well as the pest and predator densities assayed are summarized in Table 1. Pest and predator densities used in the experiments ranged between values frequently found in undamaged field crops. Initial density of 20 T. urticae per leaflet was assayed only at 5/1. After 7 days, the active number of pest and predator on each leaflet was recorded and named final density. This period was set because it represented the time customarily interval used by growers for monitoring this pest. The effects of T. urticae/N. californicus ratios on pest and predator abundances after 7 days were tested using MANOVA. A significant MANOVA was fol- lowed by a univariate analysis (ANOVA). Data were transformed by square root when appropriate. Differences between active T. urticae densities after 7 days and the economic threshold level (ETL) were analyzed by the two-tailed t-test for differences between a population mean and a hypothesized popula- tion mean (Zar 1996). The ETL of this pest has not been determined in Argentina. However, observations made in La Plata area indicated that the yield was not reduced at infestation levels ranging from 50 to 100 active mites per leaflet, during the spring (Tito unpublished data). Taking into account this information and considering that the most common cultivars grown in La Plata are short-day (Rosa Linda, Milsei Tudla, Camarosa, Sweet Charly), we used the ETL of 50 active mites per leaflet reported by Wyman et al. (1979) for another short-day cultivar. A graphical model was constructed with experimental data. Initial active T. urticae and N. californicus densities were represented on the y-axis and x- axis, respectively. Then, a line was drawn joining the greatest number of pests at a given predator density such that pest final pest density would not Table 1. Initial pest and predator densities and the pest/predator ratios used in the experiments. Treatments: Initial pest/predator Pest–predator densities per leaflet (n) Experiment 1 Experiment 2 Experiment 3 Experiment 4 5:1 5–1 (10) 10–2 (10) 15–3 (10) 20–4 (10) 7.5:1 5–0.67 (15) 10–1.33 (9) 15–2 (10) 10:1 5–0.50 (8) 10–1 (9) 15–1.5 (8) 15:1 5–0.33 (10) 10–0.67 (8) 15–1 (12) Control 5–0 (24) 10–0 (10) 15–0 (14) 20–0 (9) 60 exceed the ETL. This line separates two regions: the lower one represents the densities of both species at which the pest will never exceed the ETL after a week, while the upper one represents those densities that will exceed the ETL. Field sampling During 1999 and 2000 growing seasons, one greenhouse and one open-field commercial strawberry crop (of 600 and 900 m2, respectively) located in La Plata were sampled. Plants of ‘Camarosa’ and ‘Rosa linda’ cultivars were planted in both, greenhouse and field, in autumn. Crops with low or no miticide applications were chosen. During spring season, from September to December, densities (i.e. the number of active mites per leaflet) of both pop- ulations were determined once a week. A leaflet was selected at intervals of 5– 10 m in each row (the number of leaflets in a sample varied from 70 to 160). The number of eggs, immatures and adults of T. urticae and the number of active individuals, immatures and adult stages of N. californicus were recorded by examining each leaflet with a pocket lens of 10 · magnification. The mean density of active mites for both species was determined. To examine the relationship between field samples and experimental results, mean weekly densities were superimposed on the graphical model and were color coded to indicate whether or not pest density the following week was greater or less than the ETL.