Abstract
Abstract
Introduction
Green lacewings of the genus
The mass rearing of insects under laboratory conditions is typically time consuming and labor intensive, especially for insects that exhibit unusual behavior patterns. In the case of green lacewings, the females lay their eggs at the end of ∼10 mm long silk stalks that are connected to the oviposition substrate, thereby rendering the removal of eggs one of the most difficult steps in large-scale production (Pinto and Parra 2002; Carvalho and Souza 2009).
For small-scale rearing operations, eggs can be harvested by cutting the stalks with a sharp blade or thin long pointed scissors, but this method is not practical for medium- or large-scale production since it requires considerable time and effort. Furthermore, pedicels cannot be completely removed using blades or scissors, and the remaining parts may tangle during storage and transportation thereby inducing aggregation of the eggs. Such egg clusters cannot be distributed uniformly and this may hinder or block the release process. Complete destalking is, therefore, a crucial step in freeing eggs in a mass rearing operation (Finney 1948; Finney 1950; Carvalho and Souza 2009).
Destruction of the stalk may be achieved by treating the eggs with hypochlorite solution containing an appropriate concentration of the sodium (Krishnamoorthy and Nagar-katti 1981; Nordlund and Correa 1995; Ferreira 1997; Amaral 2011), potassium (Nasreen et al. 2004) or calcium (Bezerra et al. 2014) salt. However, this technique needs careful monitoring of the time of exposure of the egg to hypochlorite ion in order to prevent loss of viability of the embryo (Morrison 1977). An alternative method developed for the rapid and efficient collection of
While these techniques have proven to be useful for removing the stalks from Chrysopidae eggs, they are known to exert negative effects on viability egg (Nasreen et al. 2004; Carvalho et al. 1998). Thus, there remains a need for developing new methodologies, or improving existing ones, in order to facilitate the mass rearing of green lacewings through increasing the efficiency and decreasing the costs associated with the freeing of eggs. Within this context, the present study aimed at establishing an appropriate oviposition substrate and the best embryonic stage for the submission of C.
Materials and methods
Based on results obtained previously by Amaral (2011), the papers chosen as oviposition substrates for
Following incubation, substrates were placed individually in a white tray (30 x 40 cm) maintained at a slight angle to the horizontal, and the eggs were removed manually by brushing the substrates gently with a rectangle of soft muslin cloth folded so as to resemble a sponge. The pressure applied to the cloth was the minimum required to separate the eggs from their stalks. Eggs were collected and placed individually in microtiter plates (Sigma-Aldrich Ltda., SP, Brazil) with 96 wells with volume 400 µL, which were subsequently covered with polypropylene film and incubated in the growth chamber under the described conditions until hatching. The mechanical damage caused to the chorion was verified by examining the eggs under a stereomicroscope.
The percentages of eggs destroyed during harvesting and egg viabilities were evaluated for all treatments using a completely randomized experimental design comprising three repetitions of 40 eggs at the four different embryonic stages from both oviposition substrates, consisting on a 4 x 2 factorial design. Eggs destroyed were not used for the analysis of viability. Data were analyzed using R® software (R Development Core Team 2015) and the level of significance in all tests was established at P < 0.05. All the data were transformed to arcsine in order to meet the assumptions of normality and homogeneity of variances, and then submitted to an analysis of variance (ANOVA) with mean values being compared by the Scott-Knott test.
Results and discussion
Eggs harvested on the chamois substrate suffered significantly more damage during stalk removal as compared to those deposited on bond paper (F = 417.07, df = 1, 16, P < 0.001) (Tables 1 and 2). This effect was observed at all four embryonic stages and was attributed to the cotton-like texture of the chamois substrate. This type of paper does not allow the eggs to slide over the substrate, requiring greater pressure on the eggs to be destalked, and consequently, causing higher destruction. For chamois and bond paper, the significantly different percentages of destroyed eggs (87.8 and 44.1 %, respectively) were recorded when harvesting was performed at 24 hours (F = 1337.29, df = 3, 16, P < 0.001), indicating that immature eggs were more vulnerable to mechanical manipulation. Eggs aged 48, 72 or 96 hours suffered less damage during destalking, since the percentage of destruction was lower than 10 % in such treatments, although harvesting on the chamois paper significantly destroyed more eggs. These findings are in agreement with those of Carvalho and Souza (2009) who reported that
Results of the analysis of variance (ANOVA, α = 0.05) comparing the effect of manual destalking technique on two oviposition substrates at four different embryonic stages, on the destruction and viability of
Both parameters were transformed to arcsine in order to meet the assumptions of normality and homogeneity of variances.
Numerator, denominator (error).
(%) means (± SE) of destruction and viability of
For each parameter, uppercase letters compare the same embryonic stage on the different substrates, and lowercase letters compare the different embryonic stages on the same substrate;
Means followed by different letters in the comparisons are significantly different according to Scott-Knott test at 0.05 significance.
Eggs viability was significantly affected by the embryonic stages at the time of harvesting (F = 18.80, df = 3, 16, P < 0.001) (Tables 1 and 2), but not by the type of paper used as substrate (F = 0.14, df = 1, 16, P > 0.05). The eggs viability collected at 24 hours for both substrates was significantly different from the observed for the other embryonic stages (Table 2). These results suggest that the embryonic stages of the eggs at the time of destalking is of critical importance since those aged < 48 hours are more likely to be damaged during the process because the external membrane is more susceptible to abrasion.
Recent studies have focused on the destalking and harvesting from Chrysopidae eggs through immersion of the oviposition substrate in dilute hypochlorite solutions, although Morrison (1977) has long warned of problems arising from the use of this method in the mass production of natural predators. More than 40 years ago, Ridgway et al. (1970) reported the efficient use of a ball of nylon netting for scraping C.
Conclusions
Bond paper is a better substrate than chamois paper for the production and subsequent harvesting of
Acknowledgements
We thank to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support of the study and for MSc scholarship and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for DSc scholarship conceded to ALVS.