Development of Spodoptera frugiperda on different hosts ... - UFRPE

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238 Barros et al. was under investigation in this study. Coordinated planting to create a host-free period between crop seasons is recommended as a preventive practice to manage pests. However, in areas with irrigation and a favorable climate, as is the case in the Brazilian Cerrado, corn, soybean, and cotton can be double cropped. The revenue obtained per area cultivated overcomes at first glimpse the risks imposed by pest infestation, overcoming therefore the benefits <strong>of</strong> adopting ‘sanitation windows’ or ‘host-free periods’ between cropping seasons. As a result, polyphagous species such as armyworm species may exploit a diversity <strong>of</strong> hosts to build populations outside <strong>of</strong> crop fields and migrate in high numbers into major crop areas (Nagoshi et al., 2008). Studies <strong>of</strong> FAW populations have shown that cropspecific severity <strong>of</strong> FAW infestations is linked to host-related strains: the corn strain and rice strain (Pashley et al., 1985; Pashley, 1986; Busato et al., 2004; Meagher & Nagoshi, 2004). The corn strain, for which corn is the preferred host, is the strain that also colonizes cotton fields (Martinelli et al., 2007). Thus, FAW can migrate among fields <strong>of</strong> corn and cotton and develop and reproduce in these unrelated crops (Nagoshi, 2009). The pest status <strong>of</strong> FAW is usually associated with specific developmental stages <strong>of</strong> the host plant. In corn, FAW initially colonizes plants during the vegetative whorl stage. During this period, the larvae are protected while feeding on the young leaves forming the leaf whorl. In host plants other than Poaceae that do not <strong>of</strong>fer a whorl as a preferred and protected feeding site for FAW, the reproductive structures are targeted. Thus, in cotton plants the squares, blooms, and bolls are the preferred location <strong>of</strong> FAW. According to Ali et al. (1990), and our observations, newly eclosed FAW larvae start feeding on underside leaf surface <strong>of</strong> cotton, subsequently moving to squares and blooms, and finally move downward within the plant canopy to feed on bolls before pupating. Thus, the cotton plant becomes a favorable host due to the abundance <strong>of</strong> flowering and fruiting structures produced systematically from ca. 40 to 120 days after emergence. Thus, although low FAW densities occur in cotton fields, FAW is a very destructive pest in cotton because it feeds directly on reproductive structures rather than on leaves. Thus, we can hypothesize that FAW colonizing cotton fields at the blooming stage will complete a generation in approximately 40–60 days after plant emergence, and develop a second generation during the predominance <strong>of</strong> boll production from 60 to 90 days, whereas a third generation may occur during the boll maturation stage. The development <strong>of</strong> a large FAW population would not be expected early in the cotton season, as only leaves would be available for feeding. Therefore, the largest outbreak is expected to occur during the second ⁄ third generation within cotton fields or from mass migration <strong>of</strong> adults from nearby corn fields or from large populations generated from winter ⁄ cover crops. Thus, this study had two objectives: (1) to investigate the life history characteristics <strong>of</strong> FAW fed corn [Zea mays L. (Poaceae)], millet [Pennisetum glaucum L. (Poaceae)], cotton [Gossypium hirsutum L. (Malvaceae)], and soybean [Glycine max L. Merril (Fabaceae)], and (2) to characterize the damage to cotton reproductive structures caused by FAW larvae during different developmental stages <strong>of</strong> cotton. Materials and methods Insects The FAW colony was initiated with larvae collected from corn fields (8 o 1¢S, 34 o 57¢W) at the Universidade Federal Rural de Pernambuco (UFRPE), Recife, Pernambuco State, Brazil, and reared with corn leaves to pupation. The emerged adults were used to establish a laboratory colony reared with artificial diet adapted from Greene et al. (1976). The larvae were reared using vials (2.5 cm in diameter · 8.5 cm high) containing the artificial diet and closed with cotton pads, and maintained at 25 ± 2 °Cand L12:D12 photoperiod. The adults were reared using PVC cages (15 cm in diameter · 22 cm high) with the inner surface covered with paper as an oviposition substrate and the upper opening closed with PVC plastic film. The cages were placed on plastic plates (17 cm in diameter) lined with paper towels. The adults were fed a 10% honey ⁄ water (wt/vol) solution soaked on cotton pads <strong>of</strong>fered in small plastic caps inside the cages and replaced every other day. All insects used in the studies were reared on artificial diet for more than five generations to avoid any potential conditioning to a specific plant food. Life history characteristics <strong>of</strong> FAW reared on different hosts in the field The development and reproduction <strong>of</strong> FAW were investigated using corn, millet, cotton, and soybean plants cultivated in microparcels in the field. The microparcels consisted <strong>of</strong> cement rings (100 cm in diameter · 50 cm high) filled with soil up to 5 cm below the upper edge. Each microparcel was established with five plants <strong>of</strong> the same host species. The plants were planted on different dates to obtain plants <strong>of</strong> the desired ages that could be infested at the same time. This procedure was adopted to allow for the simultaneous infestation by FAW <strong>of</strong> the optimal phenological stage <strong>of</strong> the three test plant species. Thus, corn plants (variety BRS Caatingueiro and millet variety ADR 500) were infested 40 days after planting, whereas cotton plants (variety Deltapine Acala 90) were infested at
stage R4 (s<strong>of</strong>t bolls) and soybeans (variety BRS Sambaíba) at stage R3 ⁄ R4 (pods appearing). The five plants in each microparcel were confined using cylindrical cages (95 · 100 cm) made with nylon mesh screen (2 mm openings) fastened on a cylindrical iron structure set up over the cement rings. A zipper 60 cm long was fixed along the upper edge <strong>of</strong> the cage to allow access to the cages. Fall armyworm larvae were released at a rate <strong>of</strong> 250 larvae per cage (50 larvae per plant). The releasing procedure consisted <strong>of</strong> placing 50 larvae on leaf disks <strong>of</strong> the respective plant host. Then, the leaf disk was stapled onto the topmost fully developed leaf <strong>of</strong> the plant. This number may sound excessive, but one egg mass <strong>of</strong> FAW can produce over 200 eggs (Cruz, 1995). These larvae were reared for 48 h on the respective hosts in the laboratory prior to release to facilitate handling and counting the number <strong>of</strong> larvae to be released. The outer border <strong>of</strong> the microparcels was treated with tanglefoot BioStop Ò (Biocontrole, São Paulo, SP, Brazil) to build a barrier against predators inside the cages. Evaluations were conducted on day 10 and day 18 after FAW larvae were released. The relative developmental success <strong>of</strong> FAW infesting the plants was determined by the number and weight <strong>of</strong> larvae surviving on day 10 and pupation rate on day 18. The day 10 evaluation consisted <strong>of</strong> harvesting the plants and collecting the larvae, which were then counted and weighed. During this evaluation six, six, five, and eight microparcels (replications) cultivated with corn, millet, cotton, and soybean, respectively, were evaluated. The day 18 evaluation (pupation rate) consisted <strong>of</strong> digging and sifting the soil in the microparcels to collect the pupae. To facilitate collection <strong>of</strong> pupae, a plastic 2 mm mesh was placed 15 cm below the soil surface, encompassing the area <strong>of</strong> the cement ring. The mesh was then covered with sifted soil prior to planting. Five holes <strong>of</strong> 1cmindiameterweremadeinthemeshto receive the seeds and to allow the main plant stem to develop. On day 18, five microparcels (replications) were evaluated for each host plant treatment. The pupae were collected, separated by sex and divided into two groups <strong>of</strong> male and female pupae: one group was reared in the laboratory and the other group returned to the soil inside the cages in the field to determine pupal viability in the field (adult emergence rate). The group <strong>of</strong> pupae kept in the laboratory was monitored to assess reproductive parameters. Thus, in the laboratory 13 pairs (replications) <strong>of</strong> FAW adults were reared for each host plant. They were kept in PVC cages (10 cm in diameter · 15 cm high) lined with paper as an oviposition substrate, fed 10% honey ⁄ water solution and maintained in a climate chamber at 25 °C and L12:D12 photoperiod. Because <strong>of</strong> accidental escapes, data analyses were ultimately run on 13, 12, 11, and 13 Fall armyworm performance on different hosts 239 females from corn, millet, soybean, and cotton, respectively. The cages were inspected daily to record number <strong>of</strong> egg masses produced, eggs per egg mass, and adult mortality. Number and fresh weight <strong>of</strong> larvae on day 10, pupation rate (number <strong>of</strong> pupae per 250 larvae released) evaluated on day 18, adult emergence from field cages, preoviposition period, female longevity, and number <strong>of</strong> eggs produced were tested for normality (Kolmogorov D: normal test) and homogeneity <strong>of</strong> variance (Bartlett’s test), and square root (x + 0.5) or log(x + 1) transformations were used when necessary; however, untransformed means are presented in tables and figures. The results were submitted to one-way analysis <strong>of</strong> variance (ANOVA) and Tukey’s studentized range test, with Bonferroni’s corrections for 0.05 <strong>of</strong> significance level (a =0.05⁄ n, where ‘n’ represents the number <strong>of</strong> means in comparison to hold the error level equal or lower than 0.05; Abdi, 2007) using SAS s<strong>of</strong>tware (SAS Institute, 2001). Data on development and viability (rate <strong>of</strong> adult emergence from field cages), adult reproduction and survival, were used to estimate the net reproductive rate, mean generation time, and intrinsic rate <strong>of</strong> natural increase, adapting the procedure written by Maia et al. (2000) using SAS s<strong>of</strong>tware (SAS Institute, 2001), which relies on a Jackknife method to estimate confidence intervals and to allow comparisons between host plants tested. Loss <strong>of</strong> reproductive structures <strong>of</strong> cotton plants in different stages caused by FAW infestation Cotton plants (variety BRS Rubi) that produce light brown fibers were cultivated in the field using microparcels as describe above. In this assay, however, three plants were grown per microparcel instead <strong>of</strong> five. All seeds were sown on the same day, but with different FAW infestation dates. Caging <strong>of</strong> plants in the microparcels followed the same procedure as that used in the previous experiment and the infestation rate was 50 neonate larvae per plant (i.e., 150 larvae per microparcel). The experiment consisted <strong>of</strong> treatments evaluating the effect <strong>of</strong> FAW colonization at two different cotton plant stages (flowering and boll development). FAW colonization <strong>of</strong> cotton fields can occur early in the blooming stage; therefore, cotton plants are susceptible to FAW attack sufficiently long to allow at least two FAW generations before boll maturation. Thus, the experimental design was set up to infest plants at the blooming stage and evaluated 20 days later. All released larvae had completed larval development by this point (hereafter referred to as ‘blooming stage and one immature generation = Blooming 1’). The pupae evaluated were maintained in the same cage to determine the entire immature period required to com-
Page 1: Development <stron
Page 5 and 6: Table 2 Adult life history characte
Page 7 and 8: et al., 2004), we found lower larva
Page 9: Cruz I & Monteiro MAR (2004) Contro

Development of Spodoptera frugiperda on different hosts ... - UFRPE

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