Observations on three Bruchids associated with cowpea in Northern Nigeria

3. stored Prod. Res., 1967, Vol. 3, pp. 1-15. Pcrgamon Press Ltd. Printed in Great Britain.

Observations on Three Bruchids Associated with Cowpea in Northern Nigeria R. H. BOOKER Institute for Agricultural Research, Ahmadu Bello University, Zaria, Northern Nigeria* (First received 20 June, 1966 and inJina1 form 10 February, 1967) Abstract-Three species of Bruchidae, Callosobruzhus maculatus (F.), C. rhodesianus (Pit) and Bruchidius atrolineatus (Pit) were found associated with cowpea (Vigna zing&data) in Northern Nigeria. The infestation in the field was at a low level with an average of 2 per cent of the seeds being affected. B. atrolineatus was the most common of the three species and C. rhodesianus was comparatively scarce. Only C. maculatus continued infestation in store for any length of time. It is concluded that this was because B. atrolineatus and C. rhodesianus have a mean developmental period and a period of emergence which are longer than those for C. maculatus. In addition B. atrolineatus and C. rhodesianus have a much reduced level of egg production soon after the storage period begins. An account is given of the wild hosts of B. atrolineptus and it is concluded that Vigna triloba and possibly other legumes are primary hosts of this species whereas cowpea is not. Nearly kn per cent of the seeds of V. triloba were attacked in the field. Observations on the biology of C. maculatus in the laboratory showed that the period of pre-adult development is negatively correlated with temperature and is also affected to a lesser degree by humidity, low humidity lengthening the pre-adult _developmental period. The majority of eggs were laid during the early part of the female’s life and there was a decrease in the viability of eggs laid later. C. maculates has been bred on several hosts, but the only host comparable to cowpea is Phaseolus aureus, other hosts lead to heavy mortality and extension of developmental period, There are differences in the susceptibility to attack of different cowpea varieties, the larger seeded varieties of those tested being, in general, better ,as hosts. Tl-& was shown not to be due to larval competition within the seeds. There was a definite preference for oviposition on seeds with a smooth teSta when there is a choice of smdbth and rough, but there seemed to be no colour preference. One insect can cause a 3-5 per cent weight loss in a cowpea seed. The decrease in weight loss per individual insect is lower when there are several larvae per seed. Germination is adversely affected when there are three or more insects per seed. INTRODUCTION

COWPEA (Vigna unguiculata) is an important food crop in Northern Nigeria. It is sown during the rainy season, generally in July and August, and harvested in the dry season, generally November and December. In the area where the bulk of *Now at Cocoa Research Institute, Ibadan, Nigeria.

2

R. H.

BOOKER

the crop is grown, the dry season lasts about six months beginning in October. As soon as rainfall ceases the atmospheric relative humidity decreases rapidly. The mean temperature is lowest during December and January and highest during March and April. There is a pronounced diurnal fluctuation in temperature during the dry season. The crop is heavily attacked by insects in the field with severe loss of yield (BOOKER, 1965). Three species of Bruchid are found in association with the crop in Northern Nigeria. The most important is Culfo.&rucirus maculatus (F.) which causes severe losses in stored cowpeas. Callosobruchus rhodesiamu (Pit) and Bruchidius atrolineatus (Pit) are seldom found in cowpeas that have been in store for more than a month. All three species lay eggs on the pods when they are approaching maturity in the field but emergence usually occurs after harvest. The investigations reported in this paper were carried out to assess the size of field infestations of the three species, to confirm that C. rhodesiunus and B. atrolineatus do not persist in stored cowpeas, and to study various aspects of the biology of C. maculatus. INFESTATION IN THE FIELD During 1961 and 1962 sowings of cowpea were made in mid-June, mid-July and mid-August. The variety of cowpea used was a local rough-coated, white-seeded type which averages ten to eleven seeds per pod. Ripe pods were harvested from each crop at weekly intervals and samples of each of the pickings were kept to The number of Bruchids emerging enable emergence of Bruchids to be determined. from all the samples examined for each sowing date given in Table 1, show that B. atrolineatus was the most numerous of the three species. Similar observations were made in Kano, a hundred miles north of Samaru (PREVETT, 1961). C. rhodesiunus, a species not previously recorded in Northern Nigeria, was comparatively scarce. The overall seed infestation by Bruchids was 2 per cent and there was little Comparable levels of infestation by variation from this in any of the sowings. Cullosobruchus chinensis (L.) in freshly harvested cowpeas have been recorded in Natal (PHELPS and OOSTHUIZEN, 1958), but a higher level is usual in the Kano area of Northern Nigeria (PREVETT, 1961). After emergence was complete, the damaged pods were examined and the Of the number of Bruchid emergence holes in each pod and each seed counted. damaged pods 73 per cent had only one hole (Table 2) and when there were more, it was very unusual to find more than one to a seed. Although infestation was slight, it is sufficient to provide a dangerous nucleus for the storage period. DEVELOPMENT OF C. MACULATUS, C. RHODE!ZAAWS ON DRY COWPEA

AND B. ATR0LIiVEAlZr.S

Cultures of the three species (three of B. atrolinecrtus, two of C. rhodesianus and four of C. muculatus) were started in early December 1962 as soon as sufficient adults This is the time of year when storage under emerged from the harvest samples. For each culture of each species five natural conditions would normally begin. males and five females were placed in a jar containing 25 dry newly harvested, undamaged cowpea seeds and left until they died. The cultures were kept in an

1961 1962 1962

1961 1962 1961

1022 150 380 393 I

1339 750 370

--

1187 27 597

229 154 62

No. damaged

Pods --_----

11.5 4.7 9.0 15.2

17.1 20.6 16.8

%

163 8 27 839

333 230 78

No. damaged

_---

1961

1962

1962

JUiP2

June

August Totals

----

Sowing date

.--

2.

1339 750 370 150 1022 300 3931

examined

pods

Total No.

TABLE

229 154 62 7 118 27 597

damaged -_I_---

No.of

INFE~TATXON

1 161 107 46 6 90 27 437 1’3 0 93

2 45 22 12

OF COWPEA

3 12 18 2 0 8 0 40

4 4 5 1 0 3 0 13

0 0 0 6

5 5 :,

! 0 4

6 2 0 0

--

; 3

7 0 1 0 0

0 I

8 0 0 1 0

AT DIFPERENT

917 17 567

217 183 52

B. airolincatus

DATES BY THREE

SEEDS BY BRUCHIDS

I.5 0.5 0.9 2.0

2.4 2.9 1.6

%

SoWINC

No. of Bruchids/pod

PODS AND

seeds

AT DIFFERENT

Bruehid damage

FKLD INFESTATION OP COWPEAS

*These are Bruchids that were too damaged for positive identification.

August July August Totals

July

J-e

June

Sowing date

I.

Total No. pods examined

TABLE

B~ucwm

333 230 78 8 163 27 839

No. of seeds examined

COWING

21 4 128

54 23 25

74 8 150 27 809

1 327 223

03 11 0 26

2 5 7

-

~

6: 6 147

20 9

50

Unidentified*

0 2 0 3

3 :

: 0 0 1

>3 0 0

No. of Bruchids/seed

40 0 36

2

19 11

c. rkodesianw

DATES

-??l&%l&lS

c.

No. of Bruehids

SPBCIES OF

w

4

R. H.

BOOKER

uncontrolled room and the seed was allowed to equilibrate with the ambient relative humidity before use in the cultures. The moisture content of the seed was approximately 7 per cent. As soon as emergence began, the jars were examined daiIy and emerged adults removed and counted. This was continued until there had been no emergence for a seven-day period. The number of days from the start of egg production to emergence for each adult was recorded and the emergence curves are shown in Fig. 1. The means of this period for the three species were 57 - 9 days for B. atrolineatus, 41 *3 days for C. rhodesianus and 37 - 6 days for C. maculatus, for 329, 144 and 775 individuals respectively. A further series of cultures was started in mid-January. One male and one female were placed in a tube containing 10 whole, dry cowpea seeds and left until they died. There were ten such pairs for B, atrolineatus, three for C. rhodesianus and five for C. maculatus. The number of days from the start of egg production to emergence of each adult was again recorded and the emergence curves are shown in Fig. 2. The means of this period for the three species were 44 *9 days for B. atrolineatus, 40 -4 days for C. rhodesianus and 32 .8 days for C. maculatus for 268, 6 1 and 322 individuals respectively. Egg laying by B. atrolineatus and C. rhodesianus in further cultures started after mid-January was poor. In the period February to March none of 22 female C. rhodesianus and only 15 of 62 female B. atrolineatus laid eggs. A culture of the latter started with three females and three males at the end of February gave rise to only 42 individuals with a mean developmental period of 48.6 days compared with

-

START FIG.

C. maculatus

....,.....,C.

rhodesianus

--e------B.

atrolineatus

OF

EGG

PRODUCTION

1. Emergence curves for three species qf Bruchids. Experiment commenced early Decemk;1962.

Observations on Three Bruchids Associated with Cowpea in Northern Nigeria

-C.

maculatus

‘...,.....C.

rhodesianus

---------B.

atrolineatus

5

20-

8 5 15z k$ w a S I- lo. i‘

5-

i0

4'0 NUMBER OF DAYS FROM START OF EGG PRODUCTION

FIG, 2. Emergence curves for three species of Bruchids. Experiment commenced mid-January, 1963.

30.4 days for C. maculatus and a further culture started with 12 females and 10 males at the end of March gave only 161 individuals with a mean developmental period of 31.4 days compared with 26 *5 days for C. maculatus. (The last two figures for C. maculatus are taken from the pre-adult development studies described later.) B. atrolineatus in cultures started after the beginning of April failed to produce eggs. These observations have shown that, at the beginning of the ilormal storage period, B. atrolineatus has a considerably longer developmental period than C. maculatus Furthermore, B. atrolineatus and C. rhodesianus emerge over a and C. rhodesianus. longer period than C. maculatus, which has a well defined peak of emergence, and egg laying in these two species diminishes or stops shortly after storage begins. The differences in the biology of the three species are reflected in their relative importance as storage pests with C. maculatus causing the most damage. WILD

HOST OF B. ATROLINEATUS

From the foregoing observations and those of PREVETT (1961) we may conclude that hosts other than cowpea may be required to enable B. atroliniatus to survive from one cowpea growing season to the next. At Samaru this species was bred

6

R. H.

BOOKER

from several wild legumes of which the most important is Vigna triloba Walp. Pods of this species were collected in January, 1964, examined daily and 52 adults were obtained from 1172 seeds, (an infestation rate of 4.4 per cent.) Further pods collected in February gave 230 adults from 2397 seeds (an infestation rate of 9.6 per cent) and adults were still emerging from both collections when observations ceased in March. It seems likely that B. atrolineutw continues to breed on V. triloba throughout the dry season. The plant carries dry pods throughout this period and occasionally the pods remain on the plant until the next dry season. BIOLOGY

OF C. MACULATUS

Pre-adult development One freshly emerged male and female were placed in a tube containing ten undamaged cowpea seeds. The seeds were changed daily and kept in separate, labelled tubes. This process continued until the female died. Nine such pairs were set up for each individual experiment and thirteen experiments were completed during 1963. Eggs laid by each female were counted and daily records of The ‘active’ form emergence of males, females and ‘active’ forms recorded. (CASWELL, 1960) is only recognizable as females. These experiments provided the data on the number of eggs laid, rate of oviposition, egg laying periods, egg viability and developmental periods given in Table 3.

OF

FIG. 3.

Daily egg production

EGG PRODUCTION

(broken line) and viability (continuous line)

in c. maculatus.

More than 50 per cent of the total number of eggs were laid during the first three days after emergence (Fig. 3) and there was no loss of viability up to the sixth day. After this very few eggs were laid and further, the mean length of life of females in these experiments was only 6.8 days, so that this loss of viability was of little importance. The mean length of life of males was 6.9 days.

51.63 5.2.63 5.3.63 2.4.63 3Oe4.63 205.63 11.6.63 6.763 1.a.63 31 .a.63 30,963 1.11.63 4e12.63

Start of experiment

71.4 56.0 71.0 64.8 66.0 74.8 68.9 76.7 81.8 87.4 67.0 74.2 45*9

Mean No. of eggs per female (S.E. =&7*0)

3.

BKXQ~~CAL DATA

55.37 50.00 55-97 49.65 57.61 54.27 54.96 58.34 6060 56.59 51.25 53=51 51.72

(S.E*=~3~OO)

Angular transfbrmations of mean percentage of eggs giving rise ta adults

TABLE

C. maculafus

0 0 0 4.5 13.8 7.7 36.0 10-3 17-f 4.0 4.7 0 0

‘Active-’ forms as percentage of total adults hatching

mm

2fa

9 203 14-4 220 168 187 171 142 t93 247 265 179 202 t20 2514

normal

199 265 189 222 137 2441

PERIODS

;; 22 18 0 0

0 0 0 15 58 34 158

active B

No. of adults

DIFIT.RENT

234 142 262 154 175 238 139

&

AT

OF

YEAR

32*4 30.5 30*5 26.6 2643 27*1 28.9 30.7 30.6 27.7 27+9 37.1 42.2

32.4 29.5 30.2 26.4 26.0 26.9 28~6 .31*t 30.4 27-6 27.7 36.7 41.0

normal CT P

(days)

34.1 34*0 33.8 35*8 37-g 38.2 35.9 35.3 -

active 9

Mean developmental period

THE

22.6 26.2 26.2 28.3 27.4 25~7 24.4 23.6 23.7 24,6 24.3 21.9 21.2

(“C)

MeZiu temperature to 50% emergence

8

R. H.

%OKER

There was significant variability (P=O *05) in the mean numbers of eggs laid per female at different times of the year, but these differences could not be related to any particular factor. There were no significant differences between the mean percentages of eggs giving rise to adults at different times of the year. If the ‘active’ females are excluded, the sex ratio did not differ from unity overall but variation occurred in individual experiments. There were only slight differences between the developmental periods of normal males and females with the period for the females being generally slightly shorter than for the males. The ‘active’ form appeared from April to October with a peak during July, These observations agree with previous work at Kano (PREVETT, 1961), but differ from that at Ibadan where the main emergence was from March to June (CASWELL, 1960). In general the mean developmental period for this form is seven to eight days longer than that for the normal forms, an observation recorded previously (CASWELL, 1960). It has been suggested that the ‘active’ form appears when the food source is in poor condition and is a distributive form flying to fresh cowpeas, possibly to cowpea pods in the field. At the time of peak emergence however there will be few cowpea pods suitable for oviposition in the field. The significance of this form remains uncertain. E$ect of humidity and temperature on @e-adult

development

The effect of humidity was studied in two experiments. In the first, ten tubes each containing 20 undamaged cowpea seeds were placed in humidity cabinets at four humidities obtained by using solutions of sulphuric acid (SOLOMON, 1951). After six weeks, to allow for conditioning, one newly emerged male and female were placed in each tube. Eggs were counted and emergences noted daily. The second experiment was similar except that the same range of humidities was achieved in desiccators containing different salt solutions (SOLOMON, 1951). The results from the two experiments are given in Table 4. There were no significant differences between number of eggs produced or between percentages of eggs giving rise to adults in either experiment. No actual experiments on the effect of temperature were completed, but during the course of the pre-adult development work maximum and minimum temperatures were recorded daily and the results are included in Table 3. The mean temperature to 50 per cent emergence is the mean of the averages of maximum and minimum temperatures for the period up to the day that half of the adults (excluding ‘active’ forms) emerge. The developmental periods were affected more by temperature than by humidity. In the first experiment there were no significant differences between developmental periods at different humidities. At a mean temperature to 50 per cent emergence of normal forms of 28”C, the mean developmental period was 24 -5 days varying by only 0 * 7 days between humidities of 10 to 70 per cent. In the second experiment the developmental period at 10 per cent humidity was significantly longer than at any other (P=O .OOl). At a mean temperature to 50 per cent emergence of normal forms of 21°C the mean developmental period was 38 a6 days with a variation of 31 days between 10 and 70 per cent humidities. The effect of temperature is also seen from Table 3. A negative correlation P(=O*OOl) was shown between mean temperature to 50 per cent emergence of

65.7 62.3 65.3 66.9 f 3 -2

expt 1

35.2 33.9 38.4 39.3 k2.2

expt 2

4.

OF HUMIDITY

46.33 46.85 46.23 44.91 *1*70

expt 1 48.11 50.03 54.96 56.21 &240

expt 2

DEVELOPMENT

15.0 23.1 30.4 62.6

expt 1 24.8 24-5 24.4 24.1 f0.3

expt 1

40.6 37.9 38.3 37.5 -&0*5

expt 2

32.6 30.7 29.8 29.4

expt 1

0 0 0 0

expt 2

Mean developmental period. ‘Active’ forms (days)

Expt 2. December, 1963. Mean temperature to 50%

0 0 0 0

expt 2

Mean developmental period. Normal forms (days)

OF C. mnculafus

Percentage ‘active’ forms

ON PRE-ADULT

Mean percentage eggs giving rise to adults (angular transformations)

EFFECT

Expt 1. April, 1963. Mean temperature to 50% emergence 28”C, range 19-38°C; emergence 21X, range lO-33°C.

10 30 50 70 Standard error

Mean r.h. (%)

Mean No. of eggs/female

TABLE

10

R.H.

BOOKER

normal forms and the mean developmental period under the temperature conditions prevailing. It is difficult to compare these results with those of other workers. HOWEand CURRIE (1964) found that at a constant temperature of 22 -5°C the developmental period was 20 days longer at 25 per cent relative humidity than at 20 per cent whereas at 30°C the difference was only 6 days. Differences caused by temperature were greater: they showed that at 70 per cent humidity the developmental period was over 50 days longer at 20°C than at 30°C. Although the temperatures used in these experiments were similar to the mean temperatures prevailing in the present work, the latter represent the means of large ranges. In the first experiment the range was from 19 to 38°C and in the second 10 to 33°C. These figures show the pronounced diurnal fluctuation of temperature in Northern Nigeria. Comparisons with data obtained under constant conditions are therefore difficult, but the trends reported are similar. In the first humidity experiment the proportion of ‘active’ forms increased with rising humidity, but in the second experiment no ‘active’ forms were produced, but the first experiment was done when ‘active’ forms normally began to appear and the second when there were none (Table 3). This suggests that factors other than external ones are involved in their production, a conclusion reached by a previous worker ( CASWELL, 1960). Hosts of C. maculatus In addition to cowpea, C. macuiatus has been bred in the Samaru laboratory on Phaseolus aureus Rox, Cajanus cajan (L.) Druce, Voandzeia subterranea (L.) D.C., Sphenostylis stenocarpus Harms. and Kerstingielia geocarpa Harms. In experiments to compare the development of C. maculatus on different hosts, only P. aureus consistently proved satisfactory as a food source. Breeding on the other hosts was poor with extension of developmental period and heavy mortality. C. maculatus would not breed on Phaseolus lunatus L., P. acutifolius A. Gray, P. mungo L., Glycine max (L.) Merrill, C&r arietinum L. and Dolichos lab-lab L. on all of which it has been bred elsewhere (EL-SAWAF, 1956; HOWE and CURRIE, 1964; LARSONand FISHER, 1938). Varietal susceptibility in cowpeas Eight varieties of cowpea were used to test whether there were differences in susceptibility of different varieties to attack by C. maculatus. For each of the varieties, a newly emerged male and female were introduced into eleven tubes each containing 10 undamaged seeds and left until they died. All the varieties were newly harvested and had comparable moisture contents of approximately 7 per cent. Eggs were counted and emergences noted daily (Table 5). There were no differences in the number of eggs laid per female. There were some significant differences between the developmental periods (P=O *05) but these were small. There were however appreciable differences in the number of eggs producing adults and in general it was the larger seeds that produced the highest emergence rates. This suggested that larval competition for food within the smaller-seeded varieties rather than varietal differences in food value was a

Blackeye No. 7 ex-Maiduguri ex-Kontagora Alabama bunch Banta-B California blackeye Black Ushiki-C Standard errors

of seed size)

Variety (in order

0.178 0.174 0.172 0.141 0.122 0.113 0.112 0.093

(6) 51.2 49.3 43.1 50.3 47.6 54.8 54.4 46.3 k3.8

expt 1 34.4 39.2 39.0 43.5 42.3 40.1 39.5 34.6 f2.5

expt 2 45.86 54.50 51.03 5040 36.79 38.84 41.17 40-25 k2.71

expt 1 44.77 58.63 5844 57 a28 42.79 40.94 51.43 38.86 f2.38

expt 2

Mean percentage eggs giving rise to adults (angular transformations)

41.2 41.2 40.6 41.5 42.1 43.6 42.3 42.3 *O-6

expt 1

42.0 41.3 40.7 42.7 44.1 44.4 41.6 44.7 f0.4

expt 2

Developmental period (days)

ON DIFFERENT COWPEA VARIETIES IN TWO EXPERIMENTS

Mean No. of eggs/female

DEVELOPMENT OF C. maculatus

Mean individual seed weight

TABLE 5.

108 x13 129

eggs

50 53 53

52 79 86 75 46.3 46.9 41.1

52.5 a1 -4 72-8 70.8 319 231 24#

235 271 202 324

%@

133 97 98

113 182 140 227

adults

%

DIFFERENT

adults

macufalus FROM SEEDS WITH

4-6 Eggs/seed

OF C.

1-3 Eggs/seed

OF ADULTS

Banta-3 California bfackeye Wshiki-C

PRQDUCTION

99 97 118 106

6.

Blackeye No. 7 ex-Maiduguri ex-Kontagora Alabama bunch

TABLE

41.7 42.0 39.5

48.1 67.2 69.3 70.1

%

NVMIWRS

75 97 55

95 100 109 95

eggs

36 44 22

46 70 80 60

adults

> 6 Eggs/seed

48.0 45”4 4o*O

48.4 70.0 73.4 63.2

%

OF EOGS USING SEVEN COWPEA

I.28 0.80 0.05

0*55 8.54 0.72 I.94

X”

VARIETIES

f

!?

.F .3:

Observations on Three Bruchids Associated with Cowpea in Northern Nigeria

13

factor reducing the proportion of eggs producing adults. To test this possibility the experiment was repeated, but in addition the seeds were marked with indian ink to show the number of eggs thereon to enable the number of adults produced for seeds with different numbers of eggs to be compared (variety Black was excluded because of its colour). The seeds were divided into three groups for each variety; seeds with l-3 eggs per seed, seeds with 4-6 eggs per seed and seeds with more than 6 eggs per seed. Table 5 shows that most of the results of egg production, developmental periods and emergence rates were comparable to those obtained in the first experiment. Chi-square tests were applied to the data obtained from the seed marking (Table 6). In only one variety, ex-Maiduguri, was there any difference in the percentage of eggs producing adults from seeds with different numbers of eggs and this variety was a large-seeded type. It is concluded that all cowpea varieties are not of equal susceptibility to attack by C. maculatus. Preference of C. maculatus

for oviposition on daxerent ppes of cowpea

To test whether C. maculatus had any preference for oviposition on seeds of different sizes, testa types or colour, two experiments were carried out. In the first a series of forty tubes each containing 10 large seeds with rough testa was set up. In twenty these were mixed with 10 small rough seeds and in the other twenty with 10 small smooth seeds. Into each tube a newly emerged male and female were introduced and when oviposition was complete, eggs on the different types of seed were counted (Table 7). TABLE

7.

Seed type

OVIPOSITION

BY C. maculates ON COWPEA SEED OF DIFFERENT SIZE AND TESTA TYPE

Mean seed weight

No. of eggs

(g)

Proportion of eggs laid

Large rough

0.195

722

o-759

& 0.014

Small rough

0.081

229

O-241 h

0.014

Large rough

0.196

702

0.605

f

0.014

Small smooth

o-079

458

0.395

f

0.014

Mean No. of eggs/female 50.1

58.0

In all tubes most eggs were laid on larger seeds, but whereas the percentage laid on the small smooth seeds in one series was almost 40, the percentage laid on small rough seeds in the other series was only 24, a difference significant at the 0.001 per cent level. In the second experiment 60 tubes were set up to compare the numbers of eggs laid on rough white, smooth white and smooth black seeds. Twenty seeds were placed in each tube, ten of each of two kinds and twenty replicates were set up for each of the three possible combinations. Into each tube a newly-emerged male and female were introduced and, when oviposition was complete, the eggs were counted.

R. H. RJOKJZR

14 TABLE 8. OVIP~STION Seed type

BYC.

macdatus~~

Mean seed weight

COWPEASEED OF DIPPERENTTESTANPE

Number of eggs

Proportion of eggs laid

(g) Rough white

0.155

460

0.432

f

0.015

Smooth white

0.081

606

0.568

& 0.015

Rough white

0.148

551

0.435

& 0.014

Smooth black

0.124

716

0.565

& 0,014

Smooth white

0.086

536

0.504

& 0.015

Smooth black

0.126

528

0.496

& 0.015

ANDCOLOUR Mean No. of eggs/female 59.2

63.4

56.0

The results (Table 8) show that C. macuhtus, when offered a choice, has a preference for smooth testa seeds; this is still evident when the smooth seeds are smaller than the rough. There is no preference for a black or white testa. Weight loss caused by C. maculatus One hundred cowpea seeds were individually weighed, numbered with indian ink and placed in a tube. Ten newly emerged males and females were introduced. When emergence was complete the seeds were again weighed and the number of A further ten males and females were then introduced emergence holes counted. and the process repeated. When emergence was complete those seeds with several emergence holes were re-weighed again. TABLE 9.

No.of holes/seed

No. of seeds in group

WEIGHT LOSSCAUSED BY C. maculates Mean initial seed weight (g)

Mean average seed weight after emergence

o/o Weight loss

o/o Weight loss/insect

(g) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

11 22 25 21 13 9 12 10 11 7 7 2 2 1 0 1

0.2306 0.2050 0.2279 0.2380 0.2078 0.2072 0.2361 0.2462 0 - 2566 0.2413 0.2854 0.2870 0.2785 0.2720 0.2980

0.2300 0.1938 0 a2063 0.2074 0 - 1685 0.1561 0.1733 0.1736 0.1739 0 * 1479 0.1856 0.1760 0.1585 0.1500 0.1460

0.26 5.46 9.48 12.86 18.91 24.66 26.60 29.49 32.23 38.70 34.97 38.68 43.09 44.85 51.01

5.46 4.74 4.29 4.73 4.92 4.45 4.22 4.01 4.31 3.50 3.52 3.59 3.46 3.39

Observations on Three Bruchids Associated with Cowpea in Northern Nigeria

15

Percentage weight loss (Table 9) tends to decrease slightly with increasing number of insects per seed. This is because adults emergkg from seeds containing a large number of individuals tend to be smaller, so that the weight loss per insect is less. The e&t

ofC. maculatus

Groups of cowpea germinated in Petri recorded.

on germination of cowpea seed

seed containing varying numbers of emergence holes were The number of seeds germinating each day was dishes.

TABLE 10. EFFECT OF C. maculatus ON GERMINATION OF COWPEASEED No. of holes/seed

0 1 2 3 4 5 6 7

No. of seeds in group

0/0 Seeds germinated after 3 days

o,0 Seeds germinated after 7 days

100 100 100 100 100 32 14 7

82 83 82 66 56 50 7 14

88 87 87 69 58 50 7 14

With three or more emergence holes, the germination siderably reduced by C. maculatus (Table 10).

of cowpea

seed is con-

Acknowledgemenfs-My thanks are due to the Director, Institute for Agricultural Research, Ahmadu Belle University for permission to publish this paper and to Mr. M. A. CHOYCEfor his kind assistance during its preparation. All the work described was carried out at the Institute for Agricultural Research, Samaru. REFERENCES R. H. (1965) Pests of cowpea and their control in Northern Nigeria. Bull. ent. Res. 55, 663-672. CASWELL,G. H. (1960) Observations on an abnormal form of Callosobruchus maculatus (F.) Bull. en&.Res. 50, 671-680. EL-SAWAF, S. K. (1956) Some factors affecting the longevity, oviposition ;md rate of development in the southern cowpea weevil, Callosobruchus maculatus F. (Coleoptera: Bruchidae). Bull. Sot. ent. Egypte, 40, 29-95. HOWE, R. W. and CURRIE,J. C. (1964) Some laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses. Bull. ent. Res. 55, 437-477. LARSON,A. 0. and FISHER, C. K. (1938) The bean weevil and the southern cowpea weevil in California. Tech. Bull. U.S. Dept Agric. No. 593, 70 pp. I nsects injurious to cowpeas in the Natal region. PHELPS, R. J. and OOSTHUIZEN, M. J. (1958) J. eat. Sot. sth. Afr. 21, 286-295. PREVE~, P. F. (1961) Field infestation of cowpea (Vigna unguicufatu) pods by beetles of the families Bruchidae and Curculionidae in Northern Nigeria. Bull. ent. Res. 52, 635-645. SOLOMON,M. E. (1951) Control of humidity with potassium hydroxide, sulphuric acid, or other solutions. Bull. ent. Res. 42, 543-554. BOOKER,