Hiding behaviour of the predator under screen house
The hiding behaviour of R. longifrons revealed that the percentage of predator adults hiding under pebbles was higher at 6 a.m. (F = 42.53; df = 3; P < 0.000) and 10.30 a.m. (F = 8.60; df = 3; P < 0.05) than other hiding places (Fig. 1). As day light increased, the predator moved under the plants (F = 41.60; df = 3; P < 0.005) and then moved either under pebbles (F = 8.63; df = 3; P < 0.05) or under the fallen leaves (F = 8.62; df = 3; P < 0.05) for hiding. Fifth-instar reduviids preferred to hide under pebbles (F = 42.63; df = 3; P < 0.0005), whereas fourth-instar (F = 8.60; df = 3; P < 0.05) predator first hid under fallen leaves and latter moved into pebbles, again left the place and returned to the fallen leaves (F = 8.61; df = 3; P˂0.05) to hide (Table 1).
Hiding area of R. longifrons fourth and fifth stadium nymphs and adult (male and female) (%) released from morning 6 a.m., 8.30 a.m., 11.00 a.m., and to 3.30 p.m. and observation made in 2-hours intervals under screen house conditions.
Biocontrol potential of R. longifrons in relation to day and night hours
Rhynocoris longifrons life stages significantly consumed more P. solenopsis (F = 32.563, df = 1, p ≤ 0.000) and A. gossypii adults (F = 6.696, df = 1, p ≤ 0.05), D. cingulatus nymphs (F = 49.439, df = 1, p ≤ 0.000), and H. armigera larvae (F = 40.119, df = 1, p ≤ 0.000) compared to the control category. No significant differences in the predation rate of R. longifrons life stages was recorded on A. gossypii (F = 1.022, df = 23, p ≥ 0.05), D. cingulatus (F = 1.410, df = 23, p ≥ 0.05), P. solenopsis (F = 0.794, df = 23, p ≥ 0.05), and H. armigera (F = 1.092, df = 23, p ≥ 0.05) during dawn hours. Similarly, there was also not much significant difference in the activity of R. longifrons life stages on D. cingulatus (F = 0.667, df = 23, p ≥ 0.05), P. solenopsis (F = 0.426, df = 23, p ≥ 0.05), H. armigera (F = 2.037, df = 23, p ≥ 0.05), and A. gossypii (F = 1.098, df = 23, p ≥ 0.05) during the dusk hours. A similar trend was observed when compared predation rate between dawn and dusk hours of R. longifrons life stages on the four preys species (Table 2). When we considered total predation rate (dawn and dusk hours), no significant difference of predation rate was observed between R. longifrons life stage when feeding on D. cingulatus (F = 1.422, df = 47, p ≥ 0.05), P. solenopsis (F = 1.081, df = 47, p ≥ 0.05), and H. armigera (F = 0.839, df = 47, p ≥ 0.05). However, A. gossypii was significantly consumed more preys by predator third-instar (F = 3.017, df = 47, p ≤ 0.05).
Bioefficacy of the predators under cotton field conditions
In the cotton fields, when the total number of insect pests was considered, the most predominant insect pests were A. gossypii (81%), P. solenopsis (10%), D. cingulatus (6%), and H. armigera (3%), during rain fed condition. However, during irrigated condition, the predominant insect pest was P. solenopsis (60.2%), A. gossypii (32.8%) and D. cingulatus (8.2%). Therefore, we mainly concentrated on these pests throughout our observations. A significant reduction in A. gossypii population was observed in R. Longifrons-released plots compared to control after the first (F = 58.571, df = 299, p ≤ 0.000) and second (F = 22.415, df = 299, p ≤ 0.000) predator releases during south-west monsoon (Table 3). Yet, during the post-monsoon season a significant reduction in A. gossypii population was observed in R. Longifrons-released plots after the first (F = 22.761, df = 299, p ≤ 0.0005) and third (F = 5.596, df = 299, p ≤ 0.05) predator releases. During the rain fed condition, significant reduction of D. cingulatus (F = 6.951, df = 249, p ≤ 0.05) and P. solenopsis (F = 32.147, df = 299, p ≤ 0.0005) populations were recorded respectively during the first and third releases of R. longifrons life stages. A significantly lower incidence of H. armigera (F = 19.930, df = 299, p ≤ 0.000) was reported only during the first release of R. longifrons life stages.
During the irrigated condition, a low D. cingulatus population appeared in cotton fields after the first release of R. longifrons life stages (Table 3). However, no significant reduction of D. cingulatus population (F = 0.000, df = 299, p ≥ 0.05) was observed in R. Longifrons-treated plots compared with the control. Similarly, during post-monsoon season no significant reduction of P. solenopsis population was observed after the first (F = 0.020, df = 299, p ≥ 0.05), second (F = 0.000, df = 299, p ≥ 0.05) and, third (F = 1.198, df = 299, p ≥ 0.05) releases of the predator life stages. When we take in account the total mean population after all three-predator releases, there was a significant reduction of A. gossypii (F = 41.908, df = 899, p ≤ 0.000), P. solenopsis (F = 4.949, df = 899, p ≤ 0.05), and H. armigera (F = 12.734, df = 899, p ≤ 0.000) during south-west monsoon. While, only A. gossypii population was significantly reduced after release of R. longifrons life stage during post-monsoon season (F = 12.167, df = 899, p ≤ 0.05).
In general, the release of R. longifrons in cotton fields was capable to reduce the populations of H. armigera (50%), P. solenopsis (28%), D. cingulatus (18.8%), A. gossypii (11.8%) during south-west monsoon season (Fig. 2). However, during post-monsoon season, populations of D. cingulatus were reduced by 26%, followed P. solenopsis (20.6%) and A. gossypii (16.8%).
Augmentative releases of R. longifrons on the natural enemies population (number/plant) under rainfed condition. Plots without reduviid predator (A) and Plots without reduviid predator (B).
Effect on natural enemies’ populations
Other natural enemies like ants, coccinellids, wasps, other reduviids, and spiders were observed in the cotton fields. During the rainy season, the populations of reduviids (F = 13.223, df = 299, p ≤ 0.000), spiders (F = 20.926, df = 299, p ≤ 0.000), and coccinellids (F = 4.015, df = 299, p ≤ 0.05) had significantly increased in R. Longifrons-treated plots from the first and second releases respectively compared to control (Table 4). However, a significant reduction of ants (F = 4.056, df = 299, p ≤ 0.05) and wasps (F = 5.279, df = 299, p ≤ 0.05) populations was observed after the second release of R. longifrons life stages. Similar trend was also observed during post-monsoon season where significant reduction of ants (F = 4.983, df = 299, p ≤ 0.05) and wasps (F = 4.388, df = 299, p ≤ 0.05) populations was observed after the third release of R. longifrons life stages.
When we considered the total mean population after the three releases, there were significant increasing in the population of coccinellids (F = 3.906, df = 899, p ≤ 0.05), reduviids (F = 14.206, df = 899, p ≤ 0.000) and, spiders (F = 8.171, df = 899, p ≤ 0.05) compared with the control plots during south-west monsoon. It is the same trends for the reduviids populations (F = 14.764, df = 899, p ≤ 0.000) during post-monsoon season (Table 4). However, ants populations were significantly reduced as well during south-west monsoon (F = 5.403, df = 899, p ≤ 0.05) than post-monsoon seasons (F = 5.700, df = 899, p ≤ 0.05) in R. longifrons treated plots.
Cotton production, Cost Benefit Ratio (CBR) and Percent avoidable loss
Cotton production was higher in the predator-release plots (837.0 and 753.4 kg /Hectare−1 for south-west monsoon and post-monsoon seasons respectively). Similarly, the cost benefit ratio was higher in the predator-release treatment (1:1.28) than in the control treatment (1:1.17) during the south-west monsoon season, as well as post-monsoon (Table 5). Cost of cultivation was less in the control [= 4872.0 (73,08 US Dollar) and 26652.0 (399,78 US Dollar) for South-west monsoon and post-monsoon seasons respectively)] compared with the predator-released field [= 29096.0 (436,44 US Dollar) and 28134.0 (422,01 US Dollar), for South-west monsoon and post-monsoon seasons respectively)]. A higher PAL was recorded in the South-west monsoon season (14.5%) than in the post-monsoon season (4.9%) (Table 5).
Source: Ecology - nature.com
