Abstract
Soybean looper (Chrysodeixis includens) is an important defoliating pest, which has caused significant losses in Brazilian soybean crops. The present study evaluated the foliar consumption, feeding period and mortality of small (< 1.0 cm), medium (1.0 to 2.0 cm) and large (> 2.0 cm) C. includens larvae after infection by the virus Chrysodeixis includens nucleopolyhedrovirus (ChinNPV), isolate Chin-IA (I-A). The bioassay was performed in a completely randomized design organized in a 3 × 2 factorial combination (three size of larvae fed on soybean leaf discs, either treated or not with 4.0 × 1011 PIB ha-1 suspension of virus) with ten replicates per treatment. The average consumption of all three sizes C. includens larvae were significantly reduced after ingestion of soybean discs treated with virus, compared to the larvae from control treatment. The total consumption reduction was 95.6%, 69.4% and 45.9% for the small, medium and large larvae, respectively. Feeding period was significatively reduced for small and medium larvae infected by the virus, but not for large larvae. The corrected mortality level of soybean loopers ranged from 70 to 90% and was not significant different between the three larval sizes. The behavior and physiological alterations of larvae started on the third day, and the mortality occurred between fifth and sixth day after ingestion of infective particles of virus, therefore reducing their damage abilities. Based on the results obtained, ChinNPV can be considered as an important tool within integrate management to control C. includens, mainly when small larvae were predominant in soybean crops.
Highlights
Brazil is the largest soybean exporter in the world [Glycine max (L)
C. includens larvae were collected in soybean fields near to Dourados
Adults were transferred to wooden cages and the C. includens larvae obtained were used in the experiments
Summary
Brazil is the largest soybean exporter in the world [Glycine max (L). Merr.], and the second largest grain producer with an estimated production of 115 million ton within an area of 35 million ha in 2018/2019 season (Conab, 2019; Lima et al, 2019). Chrysodeixis includens (Walker, 1858) (Lepidoptera: Noctuidae: Plusiinae) is a polyphagous insect, which is found from Northern USA to Southern South America (Wagner et al, 2011) It is an economically important pest, which causes damage in soybean crops by feeding on its leaves, especially between the veins, displaying a lacy appearance, which is the pest’s fingerprint (Bueno et al, 2011; Ávila & Grigolli, 2014). The soybean looper has become a serious problem in Brazil, especially in Cerrado’s region, where population outbreaks were reported causing significant losses in the productivity of soybean (Baldin et al, 2014; Bortolotto et al, 2015; Specht et al, 2015) This might be associated with incorrect and indiscriminate use of chemical products that decreased the incidence of natural enemies (pathogens, predators and parasitoids), causing a biological imbalance in soybean agro-ecosystems. Control of this species has been difficult because of its resistance to some synthetic insecticides and less exposure of larvae to the insecticides sprays due to their location on the low and middle part of the plants canopy (Oliveira et al, 2010)
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