Life history response of the mustard erysimi to phenological changes in its host

B K AGARWALA* and N DATTA Ecology and Systematics Laboratories, Department of Life Sciences, Tripura University, Agartala 799 004, India *Corresponding author (Fax, 91-381-22 5434).

The mustard aphid, (Kaltenbach) is a serious pest of mustard in India and other tropical regions in the world. The population dynamics of this species is considerably influenced by immigrant alatae which migrate to the mustard crop from the off-season shelter. reproduce at a higher rate in the early vegetative stage of mustard plants when the developmental period is shortest and production of winged morphs is lowest. The population reaches an asymptote when the crop is 70 days old. The species regulates its developmental period, fecundity and intrinsic rate of increase in response to developmental changes of the mustard plant and maintains its dispersal throughout the duration of the mustard crop. In succeeding generations on a mustard plant new born nymphs took increasingly longer to develop into adults and over the same period these adults produced decreasingly fewer numbers of offspring. In the inflorescence and fruiting stages of mustard plants a higher proportion of the nymphs developed into alatae.

I. Introduction As the habitat becomes crowded or saturated, the rate of increase in the numbers diminishes. In other words, Phytophagous like aphids are pests of plants in population build-up and biological performance of aphids agriculture and horticulture (Blackman and Eastop 1984). vary in accordance with phenological changes of their The quality of host vegetation and other factors like hosts. With such basic knowledge of the dynamics of microclimate, inter-specific as well as intra-specific abun- aphid-host plant relations, a study was undertaken to dance and host plant density significantly influence the understand the life history responses of the mustard aphid, abundance of aphids (Dixon 1970; Van Emden and Lipaphis erysimi (Kaltenbach) in relation to biotic alte- Bashford 1971). Nutritional and biochemical changes rations occurring at periodic intervals in its food plant, the occur in these host plants during their growth from the mustard Brassica juncea (cv M 27). The aim of this seedling stage to maturity (Vereijken 1979). Aphids living investigation is to provide necessary inputs to the on such plants, therefore, regulate their population size decision-making process of control for this species which and structure in synchrony with changes in the quality of is a serious pest of cultivated plants of the in their hosts (Dixon 1969). Being polymorphic insects, the tropical and subtropical parts of the world (Blackman aphids are greatly helped by their ability to switch from and Eastop 1984; Prasad and Phadke 1984; Sekhon et al the reproduction to dispersal mode when host quality falls t989). below the minimum threshold for habitation (Dixon 1985a). However, exponential growth takes place in their 2. Materials and methods population when environmental factors are relatively constant (Honek 1987; Howard and Dixon 1992). Sub- 2.1 Field sampling sequent changes in abundance of aphids are the functions of host quality and population density, i.e., number of Immigration of L. erysimi was monitored by collecting aphids per unit area, which affect the reproduction of winged aphids from Yellow Pan Water Traps (YPT). Four individuals constituting the population (Solomon 1969). YPTs, each 51 x 30x 13 cm in size and painted with

Keywords. The mustard aphid; life history response; mustard plant; phenological changes

J. Biosci., 24, No. 2, June 1999, pp 223-231. © Indian Academy of Sciences 223 224 B K Agarwala and N Datta bright yellow enamel paint inside and bottle green phenoiogy on population dynamics and the dispersal of outside, were placed at a height of 90 cm from ground the aphid population from the crop. Field studies were level, on the periphery, one per side, of a study plot conducted during three cropping seasons in the years measuring 20x 15 m 2 in which mustard plants were 1990, 1991 and 1993. grown. The winged aphids were picked up from the YPTs every alternate day at 8 a.m. using a '0' hair brush and preserved in 70% alcohol. Collection of aphids in the 2.2 Laboratory experiments YPTs was started three weeks in advance from the date of sowing of mustard seeds and continued till the harvest of Stock culture of L. erysimi was raised on B. juncea the mustard crop. This time-lag was chosen apriori to (cv M 27) in a greenhouse (20°C, -23°C and 16/8 h compare incidence of winged aphids between the pre- L/D). New born nymphs (< 1 day old) were used from the sowing and post-sowing weeks. Individuals of L. erysimi stock culture. These were gently transferred, one each, to were separated from these samples using a stereoscopic the lower surface of a leaf freshly detached from the binocular microscope. mustard plant. It was then placed in an aphid culture box Another study plot was prepared for raising a crop of of the type described by Blackman (1975). Ten such the mustard cultivar B. juncea (cv M 27). Seeds of this boxes were arranged at an angle of 45 ° in a tray holding cultivar were sown in linear rows. The spacing between water to 2 cm depth, to facilitate maximum run of rows and plants was kept at 40 x 10 cm. Fertilizers were moisture to a detached leaf. Each box was observed at applied in two split doses. One part of nitrogen and total 24 h intervals for recording moulting of the growing amount of phosphorus and potash were applied in the nymph and any mortality during development till every form of urea, superphosphate and muriate of potash as surviving nymph became an adult aphid and laid its first basal dressing. The remaining part of nitrogen was progeny. This was repeated ten times with each applied as top dressing after first irrigation which component of the plant, each time using the young leaf of followed sowing of mustard seeds. Subsequent irrigations early vegetative stage, inflorescence or fruiting stage of were scheduled at 15-day intervals (Nath and Mishra the mustard plant separately for the winged and the 1986). No insecticide was applied to the plants during the wingless morphs. This provided data relating to the period of this study. Applications of insecticides in the developmental time (DT), generation time (D) and adjoining fields used for cultivation of potato and cabbage developmental mortality (DM), where DT= interval from were kept to a minimum in order to avoid insecticidal birth to adult moult (approximate time in days), D = time drift to the experimental plot. Infestation of L. erysimi in from birth to first reproduction, DM = number of nymphs the crop was monitored by recording the number of suffering death during DT, expressed as a percentage. nymphs and adults, both non-winged and winged, on the Individual adults (< 1 day old) from the above experi- terminal 10 cm shoot portion (because aphids generally ment were gently released at the base of a fresh detached colonize growing twigs and young leaves) of each of 100 leaf placed in an aptiid culture box. Each box was randomly selected plants in the plot. This was done at observed at frequent intervals to record the first nymph 7-day intervals between 8 a.m. and 12 noon. Aphid born to an adult, subsequent birth of new nymphs which monitoring was started in the third week after the sowing were soon removed, last nymph born and the post- of mustard seeds when the first leaf appeared. This was reproductive period. This was repeated 10 times each continued till the week of harvest. Population growth rate using a young leaf of early vegetative stage, inflorescence was determined following the logistic growth model given or fruiting stage of the host in the case of winged and by the Verhulst-Pearl equation: wingless morphs. This led to procurement of data relating to repro[luctive period (RP), fecundity (F), intrinsic rate dNIdt = rN (1 - N/k ), of increase (r,,) and adult survival (AS), where RP = interval from the birth of the first nymph to the last where dN= change in population, dt = change in time, nymph in respect of an adult aptera, F = total number of r = specific growth rate offspring produced by a female during its reproductive period, AS = number of days an adult survived from the dNxN), adult moult, r,, = the combined effect of pre-reproductive time and reproductive rate which was calculated following I Wyatt and White (1977): N = present population density, K = the maximum number of individuals that were counted in the population or the rm = 0-738(1og e MD)lD(per day), carrying capacity of the habitat. The data obtained from the field were used to determine the influence of where 0.738 is a constant, D = time from birth to first immigrant alatae (winged aphids) on development of the reproduction, MD = number of nymphs laid during the aphid population in the mustard crop, effect of crop time equal to 'D'. Response of aphids to phenological changes of host 225

2.3 Effect of clonal rearing of aphids on development three years of field study the YPT-collection in the first and reproduction four weeks of crop-age was found to be positively and significantly correlated with the rising phase of aphid Adult apterae (wingless morphs) and alatae (winged population in the crop by a lag of two weeks in the years morphs) were collected from the early vegetative stage 1990 (r=0.89, P<0.01) and 1991 (r=0.75, P<0.05) (28 days old) of field-grown mustard plants. Ten of both and by a lag of three weeks in the year 1993 (r = 0.91, P the morphs were individually placed on the apical leaf of < 0.01). Apterous adults of the first generation aphids pot-grown mustard plants of about the same age in a low contained a significantly higher number of embryos in the temperature cabinet at 20~ and the photoperiod was early vegetative stage of the mustard plants. Each adult adjusted to 16 h light and 8 h darkness. After 24 h only aptera at this stage of the plant contained, on an average, one of the progeny yielded by each adult aptera or alate 3.50 times more embryos than an immigrant alate or an was retained and the rest were removed from each plant. alate of the first generation (ANOVA: df 2,42; F = 75.99, Observations on developmental time, reproductive period P < 0.01) (table 1). and fecundity of this individual were recorded and such information on these aphids was obtained for the next six succeeding generations on the same plants until the 3.3 Population growth rate fruiting stage, care being taken to start each generation Population growth rate was recorded to be the highest in with only one first instar nymph. the early vegetative stage of the mustard plant. The growth rate gradually declined in inflorescence and 2.4 Induction of alatae in the mustard aphid flowering stages and the lowest rate was recorded in the fruiting stage (table 2). The population attained an upper Ten immigrant alatae, collected on mustard plants in threshold of- 10711 aphids when the crop was about 70 fields, were individually placed on pot-grown mustard days old (figure 3). This estimation is based on an average plants in the early vegetative stage. The proportion of of population counts for three years, Aphid population alatiform and apteriform individuals produced by them declined in the following weeks and eventually crashed and in six succeeding generations on the plants until the out of the crop. fruiting stage was noted. Variation in number of alatae Number of aphids/100 shoots recorded in this study that developed in each succeeding generation was showed a significant and positive correlation with the determined in relation to the growth stage of the mustard plant. All the collected data were subjected to statistical tests and regressions Using SPSS and Cricket III softwares in a PowerMac computer. Only regressions and tests whose values or correlation coefficients were significant at the 5% or 1% level were considered.

3. Results

3.1 When immigrant alatae become air-borne

In the three years of this study the first YPT-collection of immigrant alatae of L. erysimi was made in the third week of sampling. This time coincided with the date of sowing of mustard seeds in the month of October. Apterous aphids were, however, first recorded in the seedling stage of the plants which was two weeks after the date of sowing or the fourth week after the first YPT-sampling (figure 1).

3.2 Immigration of alatae into the crop and population development

The YPT-collections of immigrant alatae in the weeks Figure 1. Incidence of immigrant alatae in YPTs prior to first prior to settling on crops was higher than in the collection of aphids in the mustard crop. Bars show standard succeeding weeks after settling on crop (figure 2). In the errors of means expressed in log values. 226 B K Agarwala and N Datta

percentage of mustard plants that were found infested in at about 40% of crop plants infested in the inflorescence each of the three years (1990: r = 0.72, , P < 0.01 ; 1991: stage (figure 4). r = 0.92, P < 0.01; 1993: r = 0.87, P < 0.01). Regression analysis between these two variables showed curvilinear 3.4 Aphid density and its relation with nymphs and relationships with a threshold density of aphids recorded alatae

Aphid density on a host, at a considered point of time, is the observed number of total individuals constituting the

0 Crop sample sample of a population present in an unit area. These 4 YPT sample could be adults and nymphs of apterous and alate morphs. Variation in their number can influence aphid 3 density (Ghosh 1980). Therefore, it is important to know 8' the relationship between aphid density and constituent 2 morphs in the population. In the present study aphid t- e-i density showed a strong correlation with the number of m 1 nymphs born in the population (1990: r = 0.98,, P < 0.01; "6 1991: r=0-96, P<0.01; 1993: r=0.92, P<0.01) and 6 Z 0 ¸ alatae produced on the crop (1990: r = 0.89, P < 0.01; 1991: r=0.86, P<0.01; 1993: r=0.87, P<0.01). Regression analyses of aphid density showed linear -1 .... i .... i .... i .... i .... i .... ! 0 20 40 60 80 1 O0 120 relationships with the number of nymphs (figure 5) and alatae (figure 6) present in the population in all the three years of this study.

1991 b 4- 3.5 Dispersal of population

YPT-collection indicated that alatae morphs of the mustard aphid floated continuously into the study area v during the period of the crop. Regression analysis, however, suggested no relationship between the alatae '-Q. 2' collected in the YPT and on the crop (1990: "6 y = 0-837 + 0.251x, R2=0.31; 1991: y= 0-927 + 0.187x, 6 1 Z R 2 = 0.14; 1993: y = 0.339 + 0.343x, R 2 = 0.44).

0 Table 1. Mean (+ SE) number of embryos dissected out of L. 0 20 40 60 80 100 erysimi collected from the mustard crop in the year 1993. Aphid N No. of embryos

Immigrant alatae 15 7.40 + 0.24 (a) 5- 1993 c First generation apterae 15 25.40 + 1-60 (b) First generation alatae 15 9.00 + 1.14 (c) 4, ANOVA (one way) df = 2, 42 F=75.9 P < 0-019 Means in rows followed by different letters differ significantly at the 1% level by Duncan's multiple range test.

Table 2. Population growth rate* of L. erysimi on B. juncea (cv M 27)

Growth rate (aphids/day/aphid)

"1 • Crop age 0 20 40 60 80 1O0 Plant stage (in days) 1990 1991 1993 Crop age (days) Early vegetative 28 0.90 0.36 0.36 Inflorescence 42 0.47 0-16 0.27 Flowering 56 0.23 0.04 0.24 Figure 2. Graphs showing incidence of aphids in the mustard Fruiting 77 0.17 - 0.09 - 0.03 crop and in YPTs sampled at equal intervals of crop age in (a) 1990, (b) 1991 and (c) 1993. *Determined by the method of Odum (1971): Nt-NolAT.No. Response of aphids to phenological changes of host 227

3.6 Performance of the mustard aphid on different growth stages of mustard plant

y=0.172+1.871xR==0.91 • 1990 3.6a Early vegetative stage: New born nymphs of y = 0.184 + 1.817 x R2 = 0.82 O 1991 apterae and alatae took an average of 4.72 and 5.05 days y=-0.781+2.494xR==0.94 • 1993 respectively to develop into adults (table 3). These adults produced a mean total of 45.27 (by apterae) and 43.75 (by alatae) nymphs during a reproductive period of about 10 s I days. Mean duration of adult longevity lasted 12.09 and ~" 4 • •m O 12.75 days in apterae and alatae respectively. The intrinsic rate of increase was found to be lower in alatae (0.23) in comparison to apterae (0.36). g' °o~ 3.6b Inflorescence stage: Duration of development from new born nymphs to adults was longer in this stage. 1 °o°.f ° These were 5.88 days and 6.50 days for apterae and alatae ojf..Oo respectively (table 3). Adults of both the morphs produced a significantly lower number of nymphs in this stage, these being 30.18 aphids in case of apterae and 27.75 aphids in case of alatae during a reproductive period of 8.18 days and 7.12 days respectively. Mean duration of adult longevity lasted little above 10 days in both apterae and alatae. The intrinsic rate of increase in 0 20 40 60 80 100

apterae declined significantly whereas it remained nearly Percentage plants infested the same in alatae. Figure 4. Graph showing regression relationship between 3.6c Fruiting stage: Mean durations of development number of aphids collected from 100 shoots and per cent of were found to be 7.18 days for the nymphs of apterae and plants infested in the three years of the study. 7.80 days for nymphs of alatae (table 3). These were significantly longer in comparison to durations that were y = 0.129 + 0.658 x R2 = 0.98 x 1990 y = 0.119 + 0.766 x R= = 0.93 o 1991 y = 0.003 + 0.784 x R ~ = 0.95 • 1993 15000 - / 8 10000. J~ 8W ! o ;,///..

s0(x)-

J~

A 0 =-- | 20 100 0 1 2 3 4 5 Crop age (days) Nymphs/100 shoots (iog x) Figure 3. Population growth of the mustard aphid based on collection of aphid samples at 3-day intervals, expressed as Figure 5. Linear regressions between number of nymphs and three year cumulative average. Curve fitted on the basis of number of adult aphids present in samples collected from 100 visual assessment. shoots of the study plot in the three years of the study. 228 B K Agarwala and N Datta recorded in early vegetative and inflorescence stages. ficant difference in comparison to that obtained for Mean fecundity was recorded to be the lowest at this counterparts produced by apterae and alatae that fed on stage, 17.63 aphids and 23.62 aphids for apterae and plants during the inflorescence stage. The intrinsic rate of alatae respectively. Adult longevity did not show a signi- increase in the two morphs declined significantly in this stage.

y = - 0.274 + 0.847 X R = = 0.81 3.7 Effect of clonal rearing of aphids on development 3" and reproduction 1990 a In succeeding generations new born nymphs of both alatae and apterae took increasingly longer time to develop into 2 eo adults and these adults produced decreasingly fewer number of nymphs (table 4). In apterae and alatae, the developmental period in the sixth generation was 1.70 8 1 and 2.30 times, respectively, longer than in the first generation. Over the same period total fecundity. decreased by a factor of 2.52 and 2.74 times, respectively, in alatae and apterae. 0 o ; ; 3.8 Induction of alatae in the mustard aphid

y = 0.368 + 0.327 x R = = 0.53 There was a significant difference between the proportion 2- of alatae developed from offspring born to aphids 1991 b • colonizing the seedling stage and from progeny of 8' individuals settling on mustard plants during the " inflorescence or fruiting stage. A high proportion of the nymphs born to immigrant alatae in the first and second generations were apterous and these aphids on becoming 8 adults produced apterous nymphs alone. Apterous mothers in the third generation produced about a quarter of its nymphs as alatoid and thereafter the population of nymphs emerging as alatae increased reaching a maximum of 71.4% in the sixth generation (figure 7). o ;

4. Discussion y = - 0,161 + 0.604 x R = = 0.82 4 The mustard, the second important oilseed crop after -~, 1993 c groundnut in India (Rai 1976; Nayar et al 1979), suffers yield loss up to 93% due to infestation by L. erysimi (Phadke 1980; Bakhetia 1983; Prasad and Phadke 1984; Singh and Verma 1990). The aphid exhibits gregarious .g 2 colonization of the mustard in all stages of its growth (Ghosh 1980; Mathur etal 1987; Kishore and Phadke 1988; Singh and Singh 1988) but flowering shoots have been reported to prolong nymphal development, reduce / " fecundity and induce high production of alatae (Singh and

0"1- • Verma 1990; Singh etal 1990). Different species and o ; ; varieties of Brassica show differential responses to the Aphids/100 shoots (log x) infestation by L. erysimi. The rai varieties of mustard were found to be least susceptible to infestation by this Figure 6. Regression relationship between number of alatae aphid followed by the yellow and brown varieties in order and aphid density recorded in the study plot in (a) 1990, (b) of susceptibility (Prasad and Phadke 1980; Bakhetia et al 1991, and (c) 1993. 1983; Phadke and Prasad 1987). Bakhetia etal (1983) Response of aphids to phenological Changes of host 229

reported 30% infestation of the plants by L. erysimi as the plants. According to available estimates, an incidence of economic threshold in the rai variety (RLM 198), and 20-51 aphids/plant in the late vegetative stage can cause considered it to be a reliable criterion for the field economic injury to mustard plant in the riverine plains of estimation of aphid incidence on the mustard (Bakhetia north India (Prasad and Phadke 1984). In this study such and Ghorbandi 1989). an incidence of L. erysimi was observed in the early In the present study, the population size of immigrant vegetative stage when about 25% of mustard plants in the alatae influenced the growth of the L. erysimi population study plot were infested. during the first generation on mustard crop in the field. The positive correlations that were observed between The existence of a positive correlation between numbers aphid density and the numbers of nymphs and alatae of immigrant alatae in the pre-vegetative stage and the present on mustard plants provide information about the population of this species of aphid in the vegetative stage ability of this aphid species to respond to nutritional of mustard plant bears ample testimony to this statement. changes in plants that accompany growth. Overcrowding This suggests that the number of alatae immigrating to the in the late vegetative stage and possibly poor host quality fields of mustard in the first four weeks of sowing can be in the senescing stages (flowering and fruiting) influence a pointer for forecasting the status of the aphid population the production of alatae. However, there was a very poor during the vegetative stage of the mustard plant. Higher relationship between the alatae collected in YPTs and in population growth, in the present study, occurred in the the crop in this study. This suggests that L. erysimi vegetative stage leading to an asymptote at about 70 days remains air-borne for most of the time during which the in the inflorescence stage. From the plant-protection point mustard is cultivated in fields, and that the dispersal of of view, an appropriate control measure against L. erysimi these alatae is not entirely dependent on the alatae could be rewarding when the mustard crop is 3-4 weeks produced in the mustard of that area. A recent study has old since immigrant alatae would have then settled on the shown that small populations of L. erysimi are maintained

Table 3. Mean (-+ SE) duration of developmental period (DP), reproductive period (RP), fecundity (F), adult longevity (AL) and intrinsic rate of increase (rm) of the adult apterae and alatae of L. erysimi on the leaves of the mustard crop in different growth stages.

Mustard crop DP RP F AL rm stage (days) (days) (no.) (days) (per day)

Apterae Vegetative 4.72 -+ 0-I 0 a 9.95 -+ 0.31 a 45-27 -+ 1.59 a 12.09 -+ 0.27 a 0.36 -+ 0.009 a Inflorescence 5.88 -+ 0.22 b 8.18 -+ 0.64 b 30.18 -+ 2.57 b 10.35 -+ 0.55 b 0.24 -+ 0.008 b Fruiting 7.18+0.26 c 7-16+0.79 b 19.38+1.63 ~ 9.50+0.42 b 0.11+0.008 c

Alatae Vegetative 5.05 -+ 0.25 a 10.00 -+ 0.54 a 43.75 -+ 0.47 a 12.75 -+ 0.25 a 0.23 -+ 0.007 a Inflorescence 6.50 -+ 0-28 b 7.12 -+ 0.47 b 27.75 -+ 2.52 b 10.25 -+ 0.25 b 0.22 -+ 0.006 a Fruiting 7.80 + 0.20 c 6.64 + 0.45 b 17.60 -+ 2-37 c 10.85 + 0-46 b 0.12 -+ 0.009 b

Means within each column followed by the same letter do not differ significantly at 5% level by Duncan's multiple range test.

Table 4. Mean (-+ SE) developmental period (DP), reproductive period (RP) and fecundity (F) recorded in succeeding generations which developed by cloning of individual apterous and alate females of L. erysimi on the leaves of pot-grown mustard plants.

Alatae Apterae

Generation DP RP F DP RP F

Parent 4.0 _+ 0.0 a 8.50 + 1.50 a 35.50 _+ 9.50 a 4.7 _+ 0.26 a 10.10 --- 0.43 a 43.30 _+ 1.85 a I* 5.06+0.21 b 8.16+0.72 a 26.22+3-12 b 5.2+0.32 b 9.77+0.52 a 36-00+1.86 b II* 5.58 + 0.19 b 8.24+0-48 a 22.17+2.45 c 5.44+0.33 b 8.88+0.69 b 32.22 + 2.87 c III 7.33 -+ 0.33 ~ 8.66 _+ 0.33 a 28.00 _+ 3.51 d 6'66 + 0"42c 6"00 -+ 0'36 c 19"50-+ 1"40 d IV 7.66 -+ 0.33 c 9.33 -+ 0-88 b 26.00 _+ 4.04 d 7"0 + 0-00c 8.66 -+ 0"33 b 18'33 -+ 1"66d V 7"66 -+ 0"33~ 6-75 -+ 0'85 c 14"25_+ 2.05 e 7"0 + 0"00c 7'00 -+ 0"00d 15"00-+ 2'64 e VI 9'2 + 0-37 d 7"0 + 0"31 c 14"2+2'08 e 8"00--+0"570 8"25+0"47 b 15'75 + 1"03e

*Data of apterae; no alatae developed in the first and second generations. Means within each column followed by the same letter do not differ significantly at 1% level by Duncan's multiple range test. 230 B K Agarwala and N Datta

any point in its life cycle is commensurate with pheno- logical changes in their hosts. y = 0.003 + 1.82~-3 x R = = 0.82 apteriform nymphs O.OZ - Y = 0.002 - 1.82e-3 x R = = 0.82 alatiform nymphs Acknowledgements ° This study was supported by the Council of Scientific and

¢- Industrial Research and Indian Council of Agricultural Research, New Delhi. We are thankful to Prof. A F G Dixon of the University of East Anglia, England and to '~ 001• Prof. S S Krishna of Entomology Research Institute, Chennai for reading the earlier drafts of this paper and g making useful comments which have helped in its improvement.

References o.oo ; ;; l,, v Agarwala B K, Bhattacharya S and Datta 1998 Survival of the Generations mustard aphid Lipaphis erysimi (Kaltenbach) on an alternate host Rorippa indica indica (Cruciferae); J. Aphidology 12 Figure 7. Proportion of apteriform and al~atiform nymphs produced by adult aphids in succeeding generations that were 75-80 reared in pot-grown mustard plants. Bhattacharya S, Datta N and Agarwala B K 1995 Immigration, population development and dispersal of Lipaphis erysimi (Kaltenbach) in relation to phenology and quality of mustard crop; Proc. Indian Natl. Sci. Acad. B61 265-274 Bakhetia D R C 1983 Losses in rapeseed/mustard due to on a wild cruciferous host, Rorippa indica indica, Lipaphis erysimi (Kalt.) in India--a literature study; in VI throughout the year (Agarwala etal 1998). This and International Rapeseed Conference (Paris) pp 1142-1147 possibly similar populations on some other cruciferous Bakhetia D R C, Brar K S and Sekhon B S 1986 Screening of hosts as well contribute to the persistent incidence of some Brassica species and their strains for resistance to Oilseed Res. alatae of L. erysimi. This characteristic of the mustard mustard aphid; J. 1 81-82 Bakhetia D R C and Ghorbandi A W 1989 Relationship aphid could enable the species to scan large areas on a between the parameters of aphid population per plant and continuous basis and seek suitable hosts when the season percentage of plants infested by Lipaphis erysimi (Kalt.) in is still favourable. This possibly explains why seasonal Indian mustard; J. Aphidology 3 119-124 crops generally have higher frequency of aphid incidence Bakhetia D R C, Labana K S, Sukhija H S and Brar K S 1983 Studies on the economic threshold of mustard aphid Lipaphis compared to biannual plants or tree-dwelling mono- erysimi (Kalt.) infestation; J. Res. P.A.U. 10 272-279 phagous aphids where their incidence is generally spaced Blackman R L 1975 The Aphids (Aylesbury: Ginn and Co.) throughout the year (Leather and Dixon 1981; Honek Blackman R L and Eastop V F 1984 Aphids on the world's 1991). crops: An identification guide (New York: John Wiley) Growth and reproduction of aphids is dependent upon Dixon A F G 1969 Population dynamics of the Sycamore aphid Drepanosiphum platanoides (Schr.) (: ): the state of growth of plants in terms of their soluble migratory and trivial flight activity; J. Anim. Ecol. 38 585- nitrogen content (Van Emden and Bashford 1971). There 606 is more nitrogen in the phloem sap of plants whose leaves Dixon A F G 1970 Quality and availability of food for sycamore are growing because nutrients are then actively trans- aphid populations; in populations in relation to their located into or out of leaves. The sap is not as nutritious food resources (ed.) A Watson (Oxford: Blackwell) pp 271- 287 in the mature leaves of ageing plants. Better development Dixon A F G 1985a Seasonal development in Aphids; in of nymphs, enhanced fecundity and longer survival of Aphids, their Biology, Natural Enemies and Control (eds) P adults and the higher rate of population increase in the Harrewijn and A Minks (Amsterdam: Elsevier) pp 315-320 mustard aphid on young leaves in the vegetative phase of Dixon A F G 1985b Aphid Ecology (Glasgo: Blackie) mustard crop. in this study might be attributable to the Ghosh M R 1980 Spread of infestation of Lipaphis erysimi (Kalt.) and its population composition at different growth high nitrogen content of plants at that stage (Bhattacharya phases of Indian mustard plant; Indian J. Agric. Sci. 50 869- etal 1995).This explanation is also vindicated by the 872 results of clonal rearing of aphids of both the morphs. Honek A 1987 Effect of plant quality and microclimate on Aphids in the later generations exhibited a gradual decline population growth and maximum abundances of cereal in efficiency in terms of increase in durations of aphids, Metopolophium dirhodum (Walker) and Sitobion avenae (F.) (Hom., Aphididae); Z Appl. Entomol. 1114 304-313 development and decrease in fecundity when maintained Honek A 1991 Environment stress, plant quality and abundance on ageing mustard plants. In the light of the above it may of cereal aphids (Hom., Aphididae) on winter wheat; J. Appl. be asserted that the performance of individual aphids at Entomol. 112 65-70 Response of aphids to phenological changes of host 231

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MS received 27 October 1998; accepted 19 January 1999

Corresponding editor: RENEE M BORGES