Life Cycle of the Diamondback Moth Plutella xylostella L . ( Lepidoptera : Plutelli-dae ) , in Broccoli and Cauliflower under Laboratory Conditions

This research aimed to determine the biological cycle of Plutella xylostella (DBM), raised in broccoli var. Calabrese and cauliflower var. Snowball in laboratory conditions averaging 76.1 ° F and 65% RH. The mating of the moths was carried out in one-liter plastic containers and the postures were incubated inside 200 ml polypropylene cups, both covered with nets. To provide suitable food for larvae the plants were cultivated under cover to avoid infestation with pests. Fresh leaves were used to feed the larvae until pupation. The results were analyzed with the nonparametric statistical test of Kruskal-Wallis. The following biological parameters were obtained, reared with broccoli and cauliflower leaves respectively: incubation period 3 days; larval period 9.76 and 9.69 days; pupal period 5.1 and 5.3 days; biological cycle 19.5 and 19.9 days and 175 and 187 eggs as the capacity of oviposition of mated females. These results do not show significant statistical differences of the moth’s biological cycle between the two host plants.


Introduction
Vegetables, and within them Brassicaceae family, are very appreciated in human feeding due to its high nutritional and medicinal value, mainly for its properties in cancer prevention (Cohen, Kristal & Stanford, 2000;Keck & Finley, 2004).The chemo-protective effect of these vegetables is due to their high content of glucosinolates, anticancer substances that are responsible for detoxification (Lampe & Peterson, 2002).
Climatic conditions in Peru allow the growth of several species in the Brassicaceae; all of them are attacked by insect pests., (Sánchez & Vergara, 2014) cite Brevicoryne brassicae L., Hellula phidilealis Walk., Leptophobia aripa Boisduval, and Plutella xylostella L. The last one is considered the main pest; it is known as "Diamondback Moth", a cosmopolitan species with Mediterranean origin (Lingappa, Basavanagoud, Kulkarni, Patil, & Kambrekar, 2004) P. xylostella, is the key pest in tropical areas, given the numerous generations per year and the absence of effective natural enemies (García-Morató, 2000); this species reduces yields and product quality, especially in summer due to high infestation (Sánchez & Vergara, 2014).Due to the importance of this pest, the objective was to determine the biological cycle of Plutella xylostella L., in broccoli (Brassica oleraceae var.italika) Plenck, and in cauliflower (Brassica oleraceae var.botrytis) L. The results will provide important data for a better integrated pest management of Diamondback Moth in Peru.

Materials and Methods
This study was conducted in the laboratory of the Entomology Museum "Klaus Raven Büller" (MEKRB) of the Universidad Nacional Agraria La Molina (UNALM), from March to August 2013.All the rearing work was conducted under controlled conditions, averaged at 76.1 °F of temperature and 65.1% RH.In order to obtain more reliable data, the rearing was carried out until reaching three generations of P. xylostella.
Outside MEKRB, an area covered with canvas and tulle was installed, inside which pots were placed with broccoli (Brassica oleracea var.italika, Calabrese variety) and cauliflower (Brassica oleracea var.botrytis, Snowball va-riety), in order to provide fresh leaves, free of insect's pest and (Figure 1a and 1b).For each variety, twenty clay pots of four kg capacity were used, the pots were filled with 50% agricultural soil and 50% compost.To ensure the fertility and good development of the plants, two plants were placed per pot.
Fifty adults of P. xylostella were collected from an agricultural field located in the district of Carabayllo (Lima, Peru).These specimens were taken to the MEKRB Laboratory where they were conditioned in two five-liter glass jars, on the bottom of which paper towel and fresh "broccoli" leaves were placed, as an oviposition substrate.Twenty moths were placed in each bottle; to ensure a proper gas exchange, the opening of these containers was covered with nets secured with rubber bands.For the mass rearing, the leaves containing eggs of P. xylostella were extracted, and then transferred to Petri dishes of 15 cm in diameter x 1.5 cm in height.The larvae that emerged were fed broccoli leaves until their pupal stage.The pupae were placed individually in polypropylene containers of seven cm in diameter, five cm in diameter and four cm in height.Once the adults emerged, they were used as breeding stock for this research.

Adult breeding and egg production
For mating, twenty polypropylene containers of one-liter capacity were used (twelve cm in their highest diameter, nine cm in their lowest diameter and thirteen cm in height)  1c).Each container was covered with net to favor gas exchange.At the bottom of each container was placed paper towel, a container of two cm in diameter x one cm high, with diluted honey for adult feeding, and broccoli or cauliflower leaves for oviposition; a pair of adults of P. xylostella was placed inside each container.Of these containers, ten were used for the study in broccoli and ten for cauliflower.

Individual breeding of eggs
The eggs were removed from the mating containers, and these with the help of the marten hair brush, were individually placed in polypropylene rearing containers of seven cm in their highest diameter, five cm in their lowest diameter and four cm in height and covered with net at the top (Figure 1d).Each rearing container was conditioned with paper towel moistened in the bottom and a fraction of leaf of broccoli or cauliflower according to the case for the hatching and feeding of the larva.Sixty rearing containers were used for broccoli research and sixty for cauliflower, material that was used to determine the biological parameters of P. xylostella.
Once the rearing was started, eggs, larvae, and pupae were observed daily until adults were obtained.The incubation, larval, pupal, biological cycle length and oviposition capacity were recorded.
The results were analyzed using the nonparametric statistical test of Kruskall-Wallis, statistical analyzes were performed using the SPSS 19 Statistics software.

Results
In broccoli leaves, P. xylostella recorded an incubation period of 3 days, incubation period did not have significant differences between the three generations (Kruskal -Wallis test: X2 = 0.00, with GL = 2 and p ≥ 0.05); the larval stage was 9.76 days with a range of 8 to 11 days, the duration of this period fed broccoli leaves had significant differences between the three generations (Kruskal -Wallis test: X2 = 9.56, with GL = 2 and p ≥ 0.05); the pupal stage was: 5.13 days, with a range of 4 to 7 days, had significant differences between the three generations (Kruskal-Wallis test: X2 = 7.19, with GL = 2 and p ≥ 0.05); the biological cycle was 19.4 days; with a range of 16.8 to 21.8 days, there were no significant differences between the three generations (Kruskal-Wallis test: X2 = 0.96, with GL = 2 and p ≥ 0.05).The oviposition capacity was 187 eggs, with a range of 53 to 336 eggs, according to the analysis, there were no significant differences between the three generations (Kruskal-Wallis test: X2 = 2.98, with GL = 2 and p ≥ 0.05) (Table 1).
In leaves of cauliflower, P. xylostella recorded an incubation period of 3 days, incubation period did not have significant differences between the three generations (Kruskal -Wallis test: X2 = 0.00, with GL = 2 and p ≥ 0.05); the larval stage was 9.69 days with a range of 8 to 12 days, there were significant differences between the three generations (Kruskal-Wallis test: X2 = 8.39, with GL = 2 and p ≥ 0.05); the pupal stage was 5.29 days with a range of 4 to 7 days, there were significant differences between the three generations (Kruskal-Wallis test: X2 = 21.45, with GL = 2 and p <0.05); the biological cycle was 19.8 days with a range of 17.8 to 22.2 days, there were no significant differences between the three generations (Kruskal-Wallis test: X2 = 0.9, with GL = 2 and p ≥ 0.05).The oviposition capacity was 175 eggs with a range of 72 to 286 eggs, according to the analysis, there were no significant differences between the three generations (Kruskal-Wallis test: X2 = 1.26, with GL = 2 and p ≥ 0.05) (Table 2).

Discussion
In this study, the incubation period was three days, both in broccoli and in cauliflower.These results are similar to those obtained by (Hasanshahi et al., 2014) in three varieties of cauliflower; also, (Fernández & Alvarez, 1988) recorded 2.9 days in cabbage (Brassica oleracea var.capitata) and (Ebrahimi, Talebi, Fathipour, & Zamani, 2008) recorded 3 days in rapeseed cultivars (Brassica napus L.) The larval period showed similar results for both cauliflower and broccoli (9.76 and 9.69 days respectively).These results coincide with those obtained by (Niu, Li, Li, & Liu, 2013), 9.6 days in cauliflower, however, other researchers recorded different values for this parameter.These values range from 6.64 days (25 °C) (Saeed, Sayyed, Shad, & Zaka, 2010) to 11.09 days (23.5 °C) (Sarnthoy, Keinmeesuke, Sinchaisri, & Nakasuji, 1989).This variation may be due to differences in many factors under which these studies were carried out, such as the cultivar used and the strain of P. xylostella used, which, according to (Sarnthoy et al., 1989), shows variable results.
The pupal period recorded in broccoli and cauliflower (5.13 and 5.3 days respectively) was very similar for both varieties.The range of values recorded by other researchers varies from 3.86 in cauliflower (Golizadeh, Kamali, Fathipour, & Abbasipour, 2009) to 5.89 in broccoli and 6.0 days in cauliflower (Syed & Abro, 2003).This variation would be a function of the same variables that affect the duration of the larval status mentioned above.
In broccoli the biological cycle of P. xylostella was 19.49 days and in cauliflower it was 19.88 days.These slight differences are not significant, since in all the previous parameters there were no wide differences.
In broccoli, the oviposition capacity of P. xylostella was 187 eggs, and in cauliflower it was 175 eggs.These values do not differ significantly.

Conclusions
In broccoli leaves, the duration of the biological cycle of P. xylostella was 19.4 days, the egg state lasted 3 days, the larva 9.76 days, the pupa 5.13 days and the oviposition capacity was 187 eggs on average.
In cauliflower leaves, the duration of the biological cycle of P. xylostella was 19.8 days, the egg state lasted 3 days, the larva 9.69 days, pupa 5. 3 days and the capacity of oviposition was 175 eggs on average.
There are no significant differences between the biological parameters of P. xylostella bred in both broccoli and cauliflower.
In Peru, it is the first study of P. xylostella in these species broccoli and cauliflower.

Table 2 .
Average of biological parameters of Plutella xylostella L., during three successive generations (G) in cauliflower, under laboratory conditions.March -August 2013.La Molina, Lima -Peru.

Table 1 .
Average of biological parameters of Plutella xylostella L., during three successive generations (G) in broccoli, under laboratory conditions.March -August 2013.La Molina, Lima -Peru.