Impact of crop-crop diversity on insect pests
In castor based intercropping systems, the incidence of leafhopper, Empoasca flavescens, semi looper, Achaea janata, whitefly, Trialeurodes ricin, tobacco caterpillar Spodoptera litura and shoot and capsule borer Conignethes punctiferalis were predominantly noticed. The impact of castor based intercropping systems on the incidence of these insects is discussed here under.
The population of adult leafhopper fluctuated across different intercropping systems during the crop period and temporal variation were much evident.The infestation of leafhopper was observed from 31st – 52nd SWK. It can benote that the leafhopper population was low in the majority of intercropping systems up to 42nd SWK and thereafter increased till 51 SWK. A high population of leafhoppers/plant was observed during 49 – 51 SWK in castor +greengram, Castor + sunflower and sole Castor (in the range of 2.68-5.43 per plant). The intercropping systems viz., Castor + cluster bean, castor + sorghum and castor + cowpea recorded low level of incidence over time. The mean population was also significantly lower in these systems (2.05-2.62per plant) (Fig 1b), while a moderate level of pest incidence was noticed in Castor + black gram and Castor + groundnut systems.
The incidence was noticed during 31 – 38 SWK in the majority of intercropping systems and later it was found to be very sparse and low (plate 1). The incidence of semi looper was unimodal with peak infestation noticed during the 32-37 SWK (range of 3.5-4per plant), coinciding with the formation of primaries and the incidence varied across the intercropping systems. A perusal of data illustrated that the intercropping systems viz., castor+ cluster bean, castor + cowpea and Castor + pigeon pea recorded low level of pest incidence. The higher level of semi looper population was observed in castor+ green gram, castor+ black gram, castor+ groundnut and sole castor. A similar trend was observed with means incidence of A.janata across intercropping systems and was significantly less (0.78-0.88 per plant) in castor+cluster bean castor+cowpea and castor+pigeonpea.
The initial incidence of whitefly (plate 2) occurred during 30th SWK and continued till 52nd SWK. The whitefly population fluctuated widely among all the intercropping systems from early stage to harvest the crop. Castor + cowpea, Castor + soybean, castor + sunflower and castor + pigeon pea recorded higher population than sole castor. The intercropping systems such as castor + sorghum, castor + black gram and Castor + groundnut recorded low level of whitefly incidence.
The incidence of S.litura occurred during 35th SWK and continued till 48th SWK and later the incidence was reduced. The pest population fluctuated widely among all the intercropping systems from early stage to harvest the crop. The incidence of S.litura was unimodal and peak incidence occurred during 39-46 SWK in castor with intercrops like sorghum, green grams, and sole castor recorded higher larval population followed by castor+ sunflower. Low level of the population was noticed in castor+ cowpea, castor+ groundnut than the sole crop of castor.
The effect of different intercropping systems on the population of capsule borer was significant. The data recorded on damage caused by capsule borer across intercropping systems was collected individually spike order wise (primaries, secondaries and tertiaries). The damage was less in Castor crops with sorghum, cluster bean and pigeon pea compare to other systems. Black gram, cowpea and groundnut as intercrops recorded significant low level of capsule damage and were the next best.
Impact of crop-crop diversity on natural enemies
The occurrence of endo-parasitoid Microplitis on semi looper was monitored at weekly intervals and neonate larvae were collected and reared in laboratory and formation of cocoon at the posterior region of larvae were recorded and adult emergence of braconid was represented as percent parasitism.
The occurrence of the common predators of insect pests of castor was monitored at regular intervals in different intercropping systems. Among various species of coccinellid predators, viz., Menochilus sexmaculatus (F), Brumoides suturalis (F). Illois indica Timberlake, Coccinella transversalis (L) and Coccinella septempunctata (L), M. sexmaculatus was found most dominant accounting for more than 80% of the total coccinellid population. The coccinellids were considered as a group and their presence was recorded in all the intercropping systems. Four species of spiders were observed in the intercropping systems under study. These belong to the families Clubionidae, Araneidae, Linyphilidae, and Thomisidae. Among various spiders recorded, Clubiona spp was dominant. All the spiders irrespective of the family to which they belong were recorded together as one unit.
The presence of Microplitis cocoon (Plate 3) at the posterior end of larvae and adult emergence was recorded and noted as an indicator of parasitism. The percent parasitism varied across intercropping systems and in the range of 5-11 percent. A Higher level of parasitoid attack on semi looper was recorded in castor+ cluster bean (11%) followed by castor+groundnut, which was significantly higher than the rest of the systems. The activity of parasitoid was more on castor with cowpea, black gram and sorghum as intercrops
The activity of coccinellids (plate 4) was recorded within a month after sowing (31 SWK) and continued up to 44 SWK and later sparse level of the population was noticed. The peak activity of coccinellids (0.5-0.8 per plant) was recorded during the formation of primaries in all intercropping systems during 33-38 SWK periods. The coccinellid population varied significantly across intercropping systems throughout the crop growth period. Systems like castor+ cluster bean, castor+ cowpea, castor+ black gram and castor+ green gram nurtured significantly higher population of coccinellids than the rest of intercropping systems and was reflected in the mean occurrence also (0.39-0.48 per plant) The intercropping systems viz., castor+ sorghum, castor+sunflower and sole castor recorded low level of 2-4 coccinellids per ten plants.
The activity was noticed from 31 – 51 SWK period and variation of the population was evident across intercropping systems (plate 5). The data were analysed and mean population was depicted in figure 5 data The fluctuation of the population was significant among intercropping systems and the spider activity was significantly higher in castor+ cluster bean(0.78- 0.88 per plant) over time. A similar trend was observed with the mean occurrence of spiders during crop growth period.
The occurrence of other parasitoids viz., egg parasitoid Telenomous and Trichogramma sp are noticed on eggs of semi looper. The level of parasitism was sparse (< 5% ). The other larval parasitoid Euplectrus sp is gregarious ecto parasitoid attacking semi looper larvae from outside and nearly 5-10 feed on a single larva.
Microclimate (Abiotic) factors
Variation in microclimate variables (plate 7) across castor based intercropping systems was noticed. The variables like canopy air temperature varied during the crop period. Higher Tc values were recorded during the early stages of the crop (34-38th SWK) and were in the range of 30-40oC with a variation of 3-5oC across intercropping systems. During early stages of the crop, castor + green gram, castor + black gram and Castor + cowpea recorded higher temperatures. Long stature intercropping systems like castor+ maize, castor + sunflower and castor+ sorghum recorded lower temperatures during the middle of the crop period.
Fresh and dry weights of castor The fresh weights of Castor were recorded across the intercropping systems at regular intervals. Distinct variation in dry matter accumulation was observed from 75DAS and trend was maintained till the harvest of the crops. Castor + cluster bean recorded the highest fresh weight till the end of the intercropping system. This was followed by castor + black gram, castor + green gram and castor+ groundnut. Lowest fresh weight was recorded in Castor + sorghum because of tall and lanky growth of plants which might be due to competition between main and inter crops.
A similar trend was observed with the dry weights also. Highest dry weights were recorded in Castor + cluster bean, castor sole, Castor + black gram, castor + green gram and Castor + cowpea in descending order. The lowest dry weights were recorded in Castor + sorghum followed by castor + sunflower and castor + pigeon pea.
Number of capsules in castor The number of capsules borne on three different spike orders varied markedly across intercropping systems. A prominent difference was observed in a number of capsules on primaries from secondaries and tertiaries. Primary capsules were in the range of 15-60 in number where as secondaries and tertiaries were in 10-40. In all other intercropping systems except castor+ sorghum system, a number of capsules was more than sole castor. A perusal of the results indicated that a number of capsules in primaries was more in castor+ cluster bean system followed by castor+ groundnut (35- 60per plant).
Moderate level (30-45per plant) of capsules on primaries were recorded in castor+ black gram, castor+ cowpea and castor+ green gram systems. While the intercropping systems viz., castor with sorghum and maize recorded minimum number (15- 30per plant) of capsules on primaries, the systems like castor+ cluster bean, castor+ black gram, castor+ green gram, castor+ groundnut and castor+ cowpea systems recorded the highest number of capsules from secondaries and tertiaries, The least number of capsules from secondaries and tertiaries were recorded in Castor + sorghum and castor+ sunflower systems
Castor equivalent yields and LER
The castor equivalent yields were significantly higher in castor with cluster bean (11.93qha-1) (plate 8) followed by the black gram (11.42 qha-1) as intercrops. Lower equivalent yields were recorded in Castor + sorghum (6.60 qha-1), Castor + sunflower (6.93 qha-1) and castor + pigeon pea (7.70 qha-1) than sole castor. The intercropping systems were evaluated with respect to land utilization point of view and land equivalent ratios were presented in. The LER values were highest in Castor + cluster bean (1.45) followed by the Castor+ black gram and Castor + cowpea i.e. more than one. All remaining systems recorded low LER values than sole castor. Lowest LER was observed in castor with sorghum (0.56) followed by pigeon pea (0.64) and green gram (0.73) as intercrops
Agronomical indices of intercropping systems
The agronomic evaluation of intercropping systems indicated that aggressivity (A) values for castor were positive and those of intercrops were negative showing the dominance of the main crop(castor) except castor+ sorghum and castor+ pigeon pea systems. Highest aggressivity was observed in castor + cluster bean (0.254) followed by castor + groundnut and castor + black gram. In these systems mainly, castor+ sorghum system, castor crop was smothered by sorghum and the values of the main crop were negative (-0.090) and intercrop was positive.
In other systems, the trend indicated that castor was vigorous in growth and was dominant in the mixture. The higher competitive ratio (CR) values for castor were observed with cluster bean (1.417) than black gram (1.271) and groundnut (1.257) The competitive ratio values showed that castor was more competitive than the intercrops and the difference between CR values of main and intercrops was more in Castor + cluster bean (0.715) and castor+ pigeon pea (0.818) and indicated that intercrops were not suppressed significantly by the main crop.
The differences between CR values of main and intercrops were less in castor+sorghum and castor+ sunflower systems. The relative crowd coefficient (RCC) values for castor were more than unity and were high in castor+clusterbean (4.699). However, the rest of systems like castor+groundnut, castor+ black gram, castor+cowpea and castor+ green gram recorded more than unity values. RCC was lower than one in castor+sorghum, castor+sunflower and castor+pigeonpea systems.
The economics of castor based intercropping systems revealed that highest costs on various components were incurred in Castor + groundnut (Rs.5444.7 ha-1) and castor + cluster bean (Rs.5370 ha-1) followed by Castor+ pigeon pea (Rs.5042 ha-1), castor+ black gram (Rs.4944 ha-1). The cost structure indicated that human labour and seed value were the most important factors contributing to the total cost and the relative expenditure on seed and labour was also more in case of intercropping systems compared to sole castor. The gross margin as indicated by returns over variable costs was found to be highest in case of castor+ cluster bean and castor+ black gram. The systems like castor+ sorghum and castor+ pigeon pea recorded fewer returns.
Low external input IPM modules
Three systems namely castor+ cluster bean, castor+ cowpea and Castor + green gram were found to perform better in terms of lower pest incidence, better LER and higher gross margin. On these three systems, three IPM modules as defined in section 2.3 were superimposed. All these modules consisted of the only farm generated inputs. The findings from a two factor RBD analysis showed that the three modules and the three intercrops differed significantly in terms of incidence of pests and equivalent yields.
The incidence of A.janata varied across treatments having different combinations of intercrops and IPM modules. The incidence in terms of both CPU and mean population per plant varied significantly across the treatments. The effect of both the factors was found significant. The incidence of A.janata was particularly low in castor when intercropped with cluster bean (22.73 CPU and 3.78 larvae/plant) and when protected with IPM module III (28.51 CPU and 4.75 larvae/plant). The interaction effect was also found significant. Similar results were obtained in case of leafhopper also. The incidence of Empoasca sp was low in castor +clusterbean when protected with IPM module I and III. The interaction effect was also found non-significant. These differences in pest infestation were also reflected in the seed yield.
The equivalent yields were more when cluster bean and cowpea were grown as intercrops in castor (14.50 and 11.09 q/ha) with IPM III (11.68 q/ha) The IPM III module (consisting of sequential application of botanical extracts, oils, erection of bird perches and mechanical collection of larvae) on Castor + cluster bean was found to suffer least pest incidence, attract more natural enemies, give higher yields and returns, followed by the castor+ cowpea system. These two systems can thus serve as a cultural component or platform on which the low external input or bio-intensive modules of crop protection can be adopted. These modules were also comparable to the recommended IPM package in terms of pest management and yields. However, the cost incurred in LEIPM modules was much less because of the avoidance of external inputs like bio agents and chemical insecticides.
- Central Institute of Dryland Agriculture