The lacewing Ceraeochrysa caligata as a potential biological agent for controlling the red palm mite Raoiella indica

Rearing of predators and collection of red palm mites

Eggs of the lacewing C. caligata were collected from abaxial surfaces of coconut leaves infested with the red palm mite R. indica at experimental fields of the Embrapa Tabuleiros Costeiros in Aracaju (10°56′46″37°03′12″W), Sergipe State, Brazil. These coconut plants were located in pesticide-free coconut plantations. The eggs were maintained in Petri dishes, and the emerged larvae were kept individually separated during development (i.e., each larva was placed in a separate Petri dish) and fed ad libitum with eggs of Anagasta kuehniella (Zeller) (Lepidoptera: Pyralidae). Adults were maintained in plastic containers (20 cm in diameter and 30 cm in height) with open tops covered with fine tissue (i.e., organza) for ventilation. Adults were fed ad libitum with an artificial diet consisting of a mixture of bee honey and brewer’s yeast (1:1) and provided with cotton wool soaked in distilled water, which was replaced every 2 days (Freitas, 2001). The rearing units were maintained under controlled conditions of temperature (27 ± 2 °C), relative humidity (65 ± 5%), and photoperiod (12:12 L:D). In order to avoid potential field-driven differences among individuals, we used only larvae that came from individuals that remained under the laboratory conditions for at least one generation.

On the day of the experiments, eggs, immature-stage individuals (larvae, protonymphs, and deutonymphs), and adult females of R. indica were collected from pesticide-free leaves of coconut plants at experimental fields of the Embrapa Tabuleiros Costeiros.

Functional response bioassays

The functional responses of the C. caligata individuals of three larval instars to R. indica eggs, immatures, and adult females were assessed under laboratory conditions, using experimental procedures described by Hassanpour et al. (2009) and Hassanpour et al. (2011). The assay arena consisted of Petri dishes (5 cm in diameter) containing a clean coconut leaflet piece (15 cm²) placed upside down on a layer of solidified agar (7 cm² of free area). Immatures stages and adult females of R. indica were gently transferred to the coconut leaflet piece using a fine brush, while pieces of leaflets containing eggs were cut and placed in the arena. Surplus eggs were removed to adjust the prey density. The prey densities were 20, 30, 40, 70, 100, and 150 individuals for the first instar of C. caligata and 100, 150, 250, 350, 450, and 600 individuals for the second and third instars. The maximum and minimum prey densities for each C. caligata instar were determined in preliminary tests. Ceraeochrysa caligata larvae (<12 h old) were starved for 12 h before the bioassays. Then, using a brush, C. caligata larvae were transferred individually to the experimental arenas with various prey densities. The Petri dishes were covered with perforated Parafilm to prevent the predators from escaping. Predator larvae that did not feed in the first 5 min were excluded from the experiment. The number of prey consumed was recorded 6 h after predator release, and prey were not replaced. Each prey density was replicated 10 times for each C. caligata larval instar. The Petri dishes with prey and predators were maintained at 27 ± 2 °C and 65 ± 5% relative humidity with a 12 h scotophase.

Statistical analyses

The functional responses were estimated by determining the general shape of each functional response curve based on logistic regression of number of prey consumed as a function of mite stage and density using the CATMOD procedure of SAS statistical software (SAS, 2008). The cubic model was initially tested owing to its ability to detect the most possible functional response graph variations (Juliano, 2001), and a polynomial function was fit: (1)${\displaystyle \frac{{\mathrm{N}}_{\mathrm{e}}}{{\mathrm{N}}_0}=\frac{exp\left({\mathrm{P}}_0+{\mathrm{P}}_1{\mathrm{N}}_0+{\mathrm{P}}_2{\mathrm{N}}_0^2+{\mathrm{P}}_3{\mathrm{N}}_0^3\right)}{1+exp\left({\mathrm{P}}_0+{\mathrm{P}}_1{\mathrm{N}}_0+{\mathrm{P}}_2{\mathrm{N}}_0^2+{\mathrm{P}}_3{\mathrm{N}}_0^3\right)},}$

where (N_e) is the number of mites attacked; (N₀) is the initial prey density; and P₀, P₁, P₂, and P₃ are the intercept, linear, quadratic, and cubic coefficients, respectively, associated with the slope of the curve. The signs of P₁ and P₂ are used to determine the type of functional response. A significantly negative (P₁ < 0) linear coefficient indicates that the predator displays a type II functional response, indicating that the proportion of prey consumed declines monotonically with the initial prey density. A significantly positive (P₁ > 0) linear coefficient indicates that the predator presents a type III functional response (Juliano, 2001).

As our experiments were conducted with prey depletion, we used the random predator equation (Juliano, 2001; Rogers, 1972) to describe the type II and type III functional responses: (2)${\displaystyle {\mathrm{N}}_{\mathrm{e}}={\mathrm{N}}_0\left\{1-exp\left[\alpha \left({\mathrm{T}}_{\mathrm{h}}{\mathrm{N}}_{\mathrm{e}}-\mathrm{T}\right)\right]\right\},}$ (3)${\displaystyle {\mathrm{N}}_{\mathrm{e}}={\mathrm{N}}_0\left\{1-exp\left[\left(\mathrm{d}+\mathrm{b}{\mathrm{N}}_0\right)\left({\mathrm{T}}_{\mathrm{h}}{\mathrm{N}}_{\mathrm{e}}-\mathrm{T}\right)∕\left(1+\mathrm{c}{\mathrm{N}}_0\right)\right]\right\},}$

where N_e is the number of prey attacked; T is the exposure time (6 h); N₀ is the initial prey density; α is the attack rate, a constant rate of successful search; and T_h is the handling time. The coefficients α, b, c, and d are constants associated with the attack rate. Subsequently, the functional response parameters T_h (handling time) and α (attack rate) were estimated with nonlinear least square regression using the PROC NLIN procedure of SAS (SAS, 2008) as described elsewhere (Juliano, 2001). Differences in prey consumption among predator larval stages were determined by nonparametric Kruskal–Wallis test (P < 0.05).