**2. Insect pests and their associated natural enemies**

### **2.1 Qualitative and quantitative abundance**

Methodology used to quantify major insect pests of green gram and the associated natural enemies:

Study on population dynamics of insect pests and natural enemies:

i. Population of jassids (nymphs & adults) and white flies (adults) can be estimated by the visual count technique during early hours of the day from requisite plants per replication (usually counting from 1/5th of total plant population per plot), selected at random and tagged. The top, middle and bottom parts of the tagged plants should be given due consideration. Alternatively, the sudden trap method using a cubical ironframe trap of 45cm x 45cm base and 60cm height clothed in high density polythene can be used to trap the adult jassids and whiteflies for easy counting. The nymphs can be counted adopting the visual/sight-count technique from the plants by gently turning the leaves. Similarly, for aphids, the nymphs and adults can be counted on the plants directly taking observations from at least a 10cm top shoot/twig for sampling. Yellow pan-traps or sticky traps can also be used for counting the jassids and whiteflies.

operations are not feasible; hence, the use of pre-plant incorporation of fluchloralin or

The common crop rotations followed in India include: green gram – mustard; green gram – safflower; green gram – linseed; and green gram – wheat. In *kharif*, intercropping with maize, pearl millet, sesame, pigeon pea, and cotton is common. Spring or summer green gram is grown as a catch crop. The crop sequences that have been successful are green gram – maize – wheat, green gram – rice – wheat, green gram – maize – toria – wheat, green gram – maize – potato – wheat. In spring planted sugarcane, it is also grown as an intercrop. During *rabi*, it is grown in rice fallows of southern and south eastern region

**Years Area Production Productivity** 

2004-05 3.34 1.06 415

2005-06 3.10 0.95 428

2006-07 3.19 1.12 440

2007-08 3.43 1.52 510

2008-09 3.30 1.24 425

Methodology used to quantify major insect pests of green gram and the associated natural

i. Population of jassids (nymphs & adults) and white flies (adults) can be estimated by the visual count technique during early hours of the day from requisite plants per replication (usually counting from 1/5th of total plant population per plot), selected at random and tagged. The top, middle and bottom parts of the tagged plants should be given due consideration. Alternatively, the sudden trap method using a cubical ironframe trap of 45cm x 45cm base and 60cm height clothed in high density polythene can be used to trap the adult jassids and whiteflies for easy counting. The nymphs can be counted adopting the visual/sight-count technique from the plants by gently turning the leaves. Similarly, for aphids, the nymphs and adults can be counted on the plants directly taking observations from at least a 10cm top shoot/twig for sampling. Yellow pan-traps or sticky traps can also be used for counting the jassids

pendimethalin at the rate of 0.75 to 1.0 kg per hectare is recommended.

The area (m-ha), production (m-t) and Productivity (kg/ha) in India

Source: Directorate of pulses development Bhopal

**2.1 Qualitative and quantitative abundance** 

**2. Insect pests and their associated natural enemies** 

Study on population dynamics of insect pests and natural enemies:

**1.6.3 Cropping systems** 

(ICAR, 2006).

enemies:

and whiteflies.

The population of jassds and whiteflies can be estimated by the sudden trap method using a cubical iron-frame trap of 45cm x 45cm base and 60cm height clothed in high density polythene or covered by muslin cloth on three sides, while one side and the top can be clothed with high density polythene for viewing the adults captured in the trap. This is possible when the crop is 35 to 40 day-old; thereafter, we have to rely on visual count technique as it is not feasible to use the sudden trap because as per agronomical practices the spacing is recommended as 30cm between the rows and 10cm between two pants in a row; though, we may go for a spacing of 45cm (row to row) by 15cm (plant to plant in a row). Yes, an aspirator should be used to suck the adults for a reliable count. The few adults that might escape while counting from each tagged plant or replicate is a common error and will be taken care of if the sample size is more. The jassid nymphs move sideways but do not leave the foliage while observing, hence their count is reliable.

Data from visual counts can be homogenized by following the square root transformation method, especially when zero counts are recorded. We add 0.5 or 1.0 to the visual count data and find the square root before analysis that has to be retransformed after analysis for interpretation. In case of percentage data, if the percentage values range between 30 and 80, usually arc sine (angular) transformation would not be required; however, if the percentage values happen to be less than 30 and/ or more than 80, then angular transformation is required before analysis of data.

Green gram being an indeterminate plant, vegetative growth and flowering go together when the plant is 35 to 40 days of age; hence, using a suction trap (battery operated/electric) might not be successful, as it will cause more harm to the plant and at the same time disturb the insects. We can use the suction trap if the field/plot size is large so that sampling can be taken from distant areas within the field being observed without disturbing the insect species on adjacent plants.


It is true that correlations do not establish the cause and effect relationships and must be interpreted with caution. Therefore, further working out the simple regression lines or linear regressions through regression equations enables us to know the effect of the abiotic factors (independent variable) on the population (dependent variable). Data so collected and collated over many years (at least for 5 years in succession) will give a good understanding of the population trend.

The equation of a line of regression (Y on X) is given as:

Y = a + bX

Insect Pests of Green Gram *Vigna radiata* (L.) Wilczek and Their Management 205

vii. For the estimation of the population of soil dwelling predators, especially carabids, pitfall traps (500ml capacity glass jars) should be laid out in each replication and at least 3 traps should be randomly placed in each plot of 18 sq. m. (6m X 3m as length X breadth). For instant killing of the predatory insects and to avoid cannibalism (as in carabid grubs), ethylene glycol or formalin (1-2%) can be used in the traps.

**2.2 Loss estimation and establishment of economic threshold for the pod borer** 

In order to asses the losses caused by insect pests of green gram the paired plot experiment, as suggested by Leclerg (1971), can be adopted. The method involves growing the crop in 26 plots, each measuring preferably 6m X 3m. Each plot should be separated by a buffer strip of one meter all around. One set of plots has to be kept protected from insect infestation by regular need-based application of recommended insecticides. The other set of plots has to be exposed to natural infestation and thus called unprotected. Observations on the plant height, number of primary branches, pod length, pod and grain damage (%), and any other yield attributing parameter recorded from five randomly selected plants from each plot at maturity should be taken. Loss in yield can be calculated by comparing the yield obtained

X -X Loss in yield(%)= ×100 <sup>X</sup>

Sum of squareof thedeviation from themeandifference Standarddeviations (s)= Number of paired plots-1

Standarddeviation(s) Standarderrorof meandifference(Sd)= Numberof pairedplots(n)

Y -Y 1 2 t= sd

In order to calculate the economic injury level for the pod borer, losses in grain weight due to various levels of larval density of the pod borer has to be estimated. Green gram can be sown in pots of suitable size and the neonate larvae can be released on the developing

Y1 = Average yield in treated plot; Y2 = Average yield in untreated plot

**2.3 Determination of economic threshold level for the lycaenid pod borer** 

sd = Standard error of mean difference

The yield data can be analyzed statistically and significance tested using the "t" test.

1 2 1

Comparisons among the treatments can be accounted for.

from protected and unprotected plots using the following formula:

Where

Where

X1 = Yield in treated plot X2 = Yield in untreated plot

The data can be entered and processed in "MS-Excel" using the correlation function and make a chart in excel using the custom type Classic Combination Chart (either line-column on two axes or lines on two axes) that also enables to have the regression equation and coefficient of determination (R2).

iv. Estimation of the population density of insect pests and their natural enemies in the different treatments can be made and expressed as a percentage after comparing the data from the control treatment or the standard check. Wherever applicable, diversity indices can be computed using suitable techniques (Shanon-Weiner or Simpson Diversity Index).

The following mathematical/ statistical analysis can be made towards estimating the species richness and diversity:

Mean density:

$$\text{Mean density} = \Sigma \frac{Xi}{\text{N}} \times 100 \text{ J}$$

Where,

*Xi =* No. of insects or natural enemies in ith sample

*N =* Total number of plants sampled.

Shannon-Weiner diversity index (H'):

Shannon-Weiner diversity index

$$\text{(H') = } -\Sigma \text{ pi } \ln \text{ pi}$$

Where, p*i* = the decimal fraction of individuals belonging to ith species.

However, along with the Shanon Diversity Index the Simpson's Index can also be computed.

Simpson's index is calculated using the equation:

$$\mathrm{Ds} = \frac{\mathrm{N}(\mathrm{N-1})}{\sum \mathrm{n}(\mathrm{n-1})} \times 100$$

N= Total number of individuals of all species

n= Number of individuals of a species


vii. For the estimation of the population of soil dwelling predators, especially carabids, pitfall traps (500ml capacity glass jars) should be laid out in each replication and at least 3 traps should be randomly placed in each plot of 18 sq. m. (6m X 3m as length X breadth). For instant killing of the predatory insects and to avoid cannibalism (as in carabid grubs), ethylene glycol or formalin (1-2%) can be used in the traps. Comparisons among the treatments can be accounted for.

### **2.2 Loss estimation and establishment of economic threshold for the pod borer**

In order to asses the losses caused by insect pests of green gram the paired plot experiment, as suggested by Leclerg (1971), can be adopted. The method involves growing the crop in 26 plots, each measuring preferably 6m X 3m. Each plot should be separated by a buffer strip of one meter all around. One set of plots has to be kept protected from insect infestation by regular need-based application of recommended insecticides. The other set of plots has to be exposed to natural infestation and thus called unprotected. Observations on the plant height, number of primary branches, pod length, pod and grain damage (%), and any other yield attributing parameter recorded from five randomly selected plants from each plot at maturity should be taken. Loss in yield can be calculated by comparing the yield obtained from protected and unprotected plots using the following formula:

$$\text{Loss in yield} \left( \% \right) = \frac{\chi\_1 \cdot \chi\_2}{\chi\_1} \times 1000$$

Where

204 Agricultural Science

The data can be entered and processed in "MS-Excel" using the correlation function and make a chart in excel using the custom type Classic Combination Chart (either line-column on two axes or lines on two axes) that also enables to have the regression equation and

iv. Estimation of the population density of insect pests and their natural enemies in the different treatments can be made and expressed as a percentage after comparing the data from the control treatment or the standard check. Wherever applicable, diversity indices can be computed using suitable techniques (Shanon-Weiner or Simpson

The following mathematical/ statistical analysis can be made towards estimating the species

<sup>N</sup> Mean density 100 *Xi*

H' p *i ln pi*

However, along with the Shanon Diversity Index the Simpson's Index can also be

N(N- 1) Ds= n(n- 1)

v. To record the incidence of blister beetles at flowering stage the numbers of beetles per plant for a fixed time interval during the morning hours (8 to 10am) or evening hours (3 to 5pm) of the day can be observed on the randomly tagged plants. However, some species visit during early hours while others late; hence, a preliminary observation on their behaviour shall become essential before standardizing the methodology for blister

vi. To note the damage of pod borers, the numbers of healthy and damaged pods can be counted from a known (pre-decided) sample of pods (say 100) taken from the tagged plants and the data expressed as a percentage of the total. Usually the pods are split

*Xi =* No. of insects or natural enemies in ith sample

Where, p*i* = the decimal fraction of individuals belonging to ith species.

*N =* Total number of plants sampled.

Simpson's index is calculated using the equation:

N= Total number of individuals of all species n= Number of individuals of a species

open to record the species of the borer under study.

Shannon-Weiner diversity index (H'):

Shannon-Weiner diversity index

coefficient of determination (R2).

Diversity Index).

richness and diversity:

Mean density:

Where,

computed.

beetle counts.

X1 = Yield in treated plot X2 = Yield in untreated plot

The yield data can be analyzed statistically and significance tested using the "t" test.

Sum of squareof thedeviation from themeandifference Standarddeviations (s)= Number of paired plots-1

Standarddeviation(s) Standarderrorof meandifference(Sd)= Numberof pairedplots(n)

$$\mathbf{f} = \frac{\mathbf{\color{red}{Y\_1 \cdot Y\_2}}}{\mathbf{sd}}$$

Where

Y1 = Average yield in treated plot; Y2 = Average yield in untreated plot

sd = Standard error of mean difference

#### **2.3 Determination of economic threshold level for the lycaenid pod borer**

In order to calculate the economic injury level for the pod borer, losses in grain weight due to various levels of larval density of the pod borer has to be estimated. Green gram can be sown in pots of suitable size and the neonate larvae can be released on the developing

Insect Pests of Green Gram *Vigna radiata* (L.) Wilczek and Their Management 207

potential vector of mungbean yellow mosaic virus (MYMV), can cause losses ranging from 30–70 per cent. The major insect pests, particularly those often cited, have been enlisted in

The insect pests that infest green gram are better classified according to their appearance based on crop phenology. Accordingly, they can be: (1) stem feeders, (2) foliage feeders, (3) pod feeders, and (4) pests of stored grains; which are also convenient to access their

At the seedling stage are the agromyzid flies, also known as bean flies (possibly few species), *Melanagromyza* (*Ophiomyia*) *phaseoli* (Tryon) being of more common occurrence. *Ophiomyia phaseoli* larva is a cortex feeder and pupates in the cortex mostly at the root-shoot junction. Sometimes pupae can be seen sticking under the membranous epidermis. In India the girdle beetle, *Oberiopsis brevis* (Swedenbord), a major pest of soybean, sometimes infests

(*M*. *s*. = *M*. *sojae*, *O*. *p*. = *O*. *phaseoli*, *0*. *c*. = *O*. *centrosematis*; please note, *O*. *phaseoli* does not lay eggs in

Fig. 1. Location of ovipositional and larval feeding sites in soybean plant

the cotyledons of green gram) [Ref.: Talekar, 1990]

economic importance so as to devise suitable management measures.

Table - 1.

mungbean locally (Talekar, 1990).

tender pods or flowers at different population densities (1, 2, 3 and 4 larvae per plant or in a geometrical progression as 2, 4, 8 and 16). A no-larval release control should also be taken side by side on the pot plants. The plants should be caged properly and the treatments replicated. Observations on the number of healthy and damaged pods, and grain weight per plant should be recorded. Taking the reduction in yield due to different levels of larval density release, the regression analysis can be worked out to quantify the damage. The economic injury level for the pod borer on green gram can be determined by using the method suggested by Hammond and Pedigo (1982).

ManagementCost(Rs/ha) Gain threshold(G.T.)= =kg/ha Marketed valueof Mungbean(Rs/kg)

Gain threshold(kg/ha) Economicinjurylevel(EIL) =insect/ha Lossperinsect(kg/insect)

The economic threshold level can be calculated by the method suggested by Johnston and Bishop (1987). They established economic threshold level as the population of economic injury level minus the increase in population of the pest concerned per day. The increasing rate of larval population under natural field conditions can be determined by recording the weekly population of the pod borer during larval activity. The rate of increase in population can be calculated arithmetically.
