Biology and Nutritional Indexes of Spodoptera frugiperda ( Lepidoptera : Noctuidae ) in Saccharine Sorghum

Saccharine sorghum has been analyzed as a supplementary prime matter for ethanol production, especially during the sugarcane off-season period. However, it has proven to be highly susceptible to insect attacks during the cultivation cycle. The fall armyworm should be emphasized due to its voracity and high damage capacity enhanced by feeding-caused decrease in photosynthetic area. Current analysis studies the biology and determines the nutritional indexes of Spodoptera frugiperda in saccharine sorghum. Cultivars of saccharine sorghum BRS 506, BRS 509 and BRS 511were evaluated. Duration and survival of the egg, caterpillar, pre-pupal and pupal phases were determined, coupled to weight of pupae and caterpillar, life span, fecundity and pre-egg laying period. Although S. frugiperda completed its life cycle on cultivars BRS 506 and BRS 511, egg-laying and egg feasibility rates were low, whereas insects did not lay eggs on cultivar BRS 509. There was no significant difference in feeding intake by S. frugiperda among these three sorghum cultivars. Results suggest that saccharine sorghum is not a suitable host for S. frugiperda. Biological data reveal that the three saccharine sorghum cultivars are recommended for the grain production system since the number of specimens of the next generation is low or null.


Introduction
Saccharine sorghum [Sorghum bicolor (L.) Moench] is one of the most promising alternatives as a renewable prime matter for the production of ethanol due to its similarity with sugarcane (Saccharum officinarum L.) with its saccharine fleshy stem.It may be planted in places where sugarcane has failed to adapt itself, in rotation with other yearly crops and potentially planted in non-cultivated areas.Further, it may be a supplementary culture in offseason in areas with sugarcane culture, with a harvest extension of four months (Emygdio, 2010).
It should be underscored that sorghum culture is totally mechanized, from planting to harvest; grain production is around 2.5 t ha -1 ; and the product may be used for human and animal food or for the production of biofuel.Bagasse is a source of energy for industrialization, co-generation of electricity or animal forage.In fact, it contributes for favorable energy balance, with primary strategy as a crop in renewable areas, with grinding period within 45 days (Teixeira et al., 1997;Emygdio, 2010;Parrella et al., 2011).
Pest insects are highly relevant among biotic factors since they attack the plants from seeding to harvest.Damage mainly comprises decrease in stand, plant vigor and radicle system, with high lodging and drastic reduction in harvest indexes.Although several pests attack saccharine sorghum culture, the fall armyworm Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) is one of the most damaging species for sorghum vegetative segment.The caterpillars feed on new leaves which, in turn, are damaged within the chrysalis.When the new leaves expand, symmetric lesions are detected on both sides of the foliar limbo.Caterpillars cause most damage during the last instars due to the great amount of leaves they consume (May et al., 2012;Mendes et al., 2012).
Feeding involves several aspects in Biology, including Ecology, Behavior and Physiology.Quantitative nutrition (amount of food), qualitative nutrition (nutrients required from the chemical point of view) and secondary compounds (allelochemicals) impact insect biology in very different ways (Parra, 1991).
The study of the biology, intake and utilization of ingested food are highly relevant since they are a basic condition for insect growth, development and reproduction.In fact, quantity and quality of feed in the larval phase influence adult activity (Scriber & Slansky, 1981;Parra, 1991).
Due to lack of information on the development of S. frugiperda in sorghum culture, current assay is an in-depth analysis on the biology and life table of this insect species, and on the determination of nutrition indexes.

Methods
The experiments were conducted at the Bioefficiency Centre of Embrapa Clima Temperado-Low Lands Experimental Station, located in the county of Capão do Leão/RS, in the period April/2014 February/2016.Three sorghum cultivars, BRS 506, BRS 509 and BRS 511, used in the assay, were seeded in a greenhouse in 20L-pails, and management followed technical instructions of the culture (Reunião, 2013).The caterpillars derived from a cultivation bred on artificial diet (Greene et al., 1976).

Study of Biology and Life Table
One hundred and thirty newly test tube-hatched caterpillars were selected for each cultivar, with pieces of sorghum leaves (12 cm²) at phenological phase V4.Tubes were buffered with cotton wool and maintained in an air-conditioned room at 25±1 °C, humidity 70±10% and 14 h photophase.The caterpillars were fed on sorghum leaves till the pre-pupa phase.The width of the cephalic capsule was measured daily by ocular micrometer locked to a stereoscopic microscope to determine the number of instars (Dyar, 1890).Experimental design was totally randomized.
The following variables were assessed for the analysis of biological aspects: Duration and viability of caterpillar phases, pre-pupa, pupa, adult, pre-egg laying and weight of pupa after 24 hours (males and females).Couples up to 48 h after emergence were formed to assess adult phase, and longevity and duration of the pre-egg-laying period were determined.
A fertility life table Silveira Neto et al. (1976) was prepared from data on the duration of the development period (egg-adult), total viability, sexual ratio, pre-egg-laying period, number of egg per day and daily mortality of males and females.Mean duration of one generation (T), net reproduction rate (Ro), intrinsic increase rate (r m ) and finite increase ratio (λ) were calculated.

Nutrition Indexes
Twenty-five newly-hatched caterpillars from test tubes (8.5 cm × 2.5 cm) were selected to study feed intake and use.Caterpillars were kept in an air-conditioned chamber at 25±1 °C, humidity 70±10% and 14 h photophase, weighed and fed daily on leaves from saccharine sorghum BRS 506, BRS 509 and BRS 511.Feed wastes and feces were dried in an oven at 55-60 °C (24 h) and weighed on a precision electronic scale.
When they reached maximum developmental stage (last instar), the caterpillars were removed from the test tubes, killed by freezing and taken to the oven together with diet remnants and feces in the test tube.The weight of dry matter from in-taken food and weigh gain of the caterpillars were thus obtained.A 10-test tube aliquot with diet and without caterpillars was separated to determine initial dry weight of the diet (Parra, 1991).
Methodology by Waldbauer (1968), modified by Scriber and Slansky Jr. (1981), was employed to determine quantitative nutrition indexes of the larval phase, comprising the following parameters: -Relative Intake Rate (TCR): amount of food ingested per mg body weight of the insect per day; -Relative Metabolic Rate (TMR): amount of food spent on metabolism per mg body weight of the insect; -Relative Growth Rate (TCrR): indicates biomass gain by the insect in relation to its weight; -Efficiency of Feed Conversion Ingested in Biomass (ECI): percentage of ingested food that is transformed into biomass; -Efficiency of Feed Conversion Digested in Biomass (ECD): percentage of digested food that is converted into biomass; -Approximate Digestibility (DA): percentage of the digested food that is effectively assimilated by the insect.
Parameter rates were calculated by gravimetry (Panizzi & Parra, 2009).Experimental design was totally randomized, with each caterpillar considered a replication.

Statistical Analysis
Data of biological parameters underwent analysis of variance; means were compared by Tukey's test (p>0.05)with program Genes (Cruz, 2010).
Temporal data were compared for phase duration.Variables related to time were calculated by Survival Analysis (Lee, 1992).
Statistical analysis of consumption test was given by transforming sen arc (x/100) ½ and data may be approximated to binomial distribution and compared by Tukey's test (p > 0.05).

Results and Discussion
The incubation period of S. frugiperda eggs fed on sorghum cultivars BRS 506 and BRS 511 during the larval phase lasted three days, with no significant differences (Table 1).Results corroborated those by Rosa et al. (2012) who reported rates between 2.8 and 3.3 days when they studied the biological aspects of S. frugiperda fed on corn strains.There was no time duration for insects fed on cultivar BRS 509 since the adults failed to lay any eggs.Egg viability of insects fed on cultivars BRS 506 and BRS 511 was 57.0% and 17.0% respectively, with significant difference (Table 1).Viability from cultivar BRS 511 was similar to that by Fernandes (2003), between 18.4 and 24.0% when insects were fed on conventional corn, and between 16.0 and 19.3% with modified corn.However, the above differs from results by Sarro (2006) with viabilities over 79.0% when corn leaves and several cotton cultivars were supplied to S. frugiperda.Since adults deposited few eggs coupled to low viability, it should be highlighted that the nutritional content of saccharine sorghum cultivars was not adequate for the fall armyworm.The above corroborates the fact that several species acquire nutrition resources for reproduction during the larval phase and invested energy resources may contribute towards vitellogenesis and egg maturation process (Chapman, 1998).Caterpillars with the smallest ingested feed conversion efficiency (ECI), which is , the percentage of ingested feed by the insect and transformed in biomass, and low efficiency of digested food conversion (ECD), which is, the percentage of digested feed converted into the biomass of the insect, had a higher metabolic rate.Results (Table 8) are similar to those registered by Crócomo and Parra (1985) who studied food consumption and use in different hosts, reporting lower ECI and ECD and higher metabolic rates for caterpillars fed on corn leaves.

Conclusions
Although S. frugiperda completed its life cycle when fed on cultivars BRS 506 and BRS 511, egg laying is rare and proves that hosts are scantily fit for the development of the insect.Moreover, cultivar BRS 509 is not fit for the development of the fall armyworm.Current analysis confirmed that S. frugiperda caterpillars (though polyphage) fed on leaves of saccharine sorghum cultivars evidenced low capacity of compensating nutritional contents supplied by the plant.
Based on S. frugiperda biological data, the use of the three saccharine sorghum cultivars (BRS 506; BRS 509; BRS 511) may be recommended for the production of grain.If fall armyworm occurs, it may be inferred that, although the insect completes its life cycle feeding on cultivar BRS 506 and 511, sorghum cultivars are not favorable feed for S. frugiperda.If a severe attack at the onset of crop development occurs, heavy liabilities may occur due to stand decrease.

Table 1 .
Duration and viability of egg phase of Spodoptera frugiperda in saccharine sorghum cultivars in laboratory (Martinez & Emden, 2001) letters in the column do not differ by Tukey's test (p > 0.05). 2 Data transformed to (x + 0.5) ½ .3cient of variation.The larval phase for insects fed on sorghum BRS 509 and 511 lasted for 31.8 and 32.4 days, which was significantly different from duration with cultivar BRS 506 (Table2) at 19.5 days.Different from results byGiolo et al. (2002)who assessed fall armyworm populations from several places and obtained larval duration of 16.5 days for caterpillars fed on corn.Delay in the duration of the larval phase is normally verified by reduced intake of feed, frequently due to one or more inhibitors in the food or to nutritional inadequacy of the feed substrate(Martinez & Emden, 2001).

Table 2 .
Duration, viability and weight of larval phase of Spodoptera frugiperda in saccharine sorghum cultivars in laboratory Note.1Means followed by small letters in the column do not differ by Tukey's test (p > 0.05). 2 Data transformed to (x + 0.5) ½ . 3oefficient of variation.Survival analysis showed that caterpillars fed on cultivar BRS 511 may prolong the feed period up to 40 days; BRS 506 up to 60 days and BRS 509 up to 80 days.The above facts evidence that even if sorghum cultivars are jas.ccsenet.