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II. Seed Setting and Production

II. Seed Setting and Production

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and concentrated; the remaining five are more extensive but production

of seed is less intensive.” Even within major seed growing areas violent

interannual and interfield yield fluctuations occur. I n Utah, for example,

in 1926, the production amounted to 20,000,000 lbs., but in each of

several recent years it has only been about 4,000,000 lbs. (Tysdal, 1946).

Investigations over the past several decades have served to reveal the

multiplicity of factors influencing seed setting and seed yield, and contributing to the variability from area to area, field to field, and year to

year. T o understand and interpret the role of various factors it has

been necessary first to gain a knowledge of the biology and functioning

of the alfalfa flower. To this fundamental information the influences of

soil, climate, beneficial and injurious insects, disease, and management

practices can be added.

1. Tripping and I t s Necessity

The anthers, anther filaments, stigma, style and ovary, collectively

called the staminal or sexual column, are enclosed by the two keel petals

which are united along one edge and held firmly together along the other

two free edges. The filaments of nine of the ten anthers are united t o

form a tube which practically surrounds the ovary and style and exerts

a strong forward pressure. Whenever a force separates the two keel

petals even slightly along their free edges, the restraining mechanism is

released and the staminal column is violently snapped (tripped) forward

from the pressure exerted by the tube. Upon tripping the upper end

of the staminal column makes a strong impact with the standard (banner) petal and comes to rest on it several degrees from the original

upright position. The process of release of the staminal column from

the keel is known as tripping.

Although tripping has been observed for many decades the fundamental nature of the process to seed setting has been a matter of controversy even fairly recently. Carlson (1935) and Brink and Copper

(1936) maintained that a considerable proportion of flowers set seed

without tripping. Recently Tysdal (1946) drew attention to the fact

that t.he procedure used by Brink and Cooper was open to question.

Ufer (1932), Armstrong and White (1935), Hadfield and Calder (1936),

Knowles (1943), and Tysdal (1940, 1946) concluded that a t most only

a very small percentage of flowers set pods without first tripping. Both

Tysdal (1940) and Knowles (1943) report on detailed observations

covering many individual flowers on large populations of p1ant.s over

extensive periods of time and a variety of soil and climatic conditions.

Their data show that about 1 per cent of untripped flowers may set pods.

These observations and conclusions are further supported by the high



correlations found between percentage of flowers tripping and setting

pods (Tysdal 1940, 1946; Knowles 1943).

Vansell and Todd (1946) observed one plant on which 10 per cent

of the flowers examined had the sexual column growing out of the top

of the keel. Carlson (1946) recorded some pod setting without tripping

and by histological examination established that pollen tubes and embryos were present in 13 of 84 untripped flowers. He, however, considered that the occurrence of pod setting without tripping was not high.

Tysdal (1946) pointed out that it was possible to select rare plants

in which tripping was unnecessary for pod setting. The progeny of one

such plant was in fact included in the study reported by Knowles (1943).

The accumulated masr of evidence establishes the fact that tripping

is almost an obligatory requisite to seed setting. The extent to which

tripping occurs is consequently the fundamental factor in determining

seed setting and seed yield. As Tysdal (1940) states "although tripping

will not insure seed production at least seed will not set to any great

extent without tripping.''

The essential function which tripping performs in rupturing the stigmatic membrane has been shown by Armstrong and White (1935).

They established that in untripped flowers a membrane covering the

stigma retained the stignlatic fluid. Upon tripping, the impact of the

stigma against the standard petal or other obstacle ruptured the membrane and released the fluid thus inducing pollen germination. Occasionally the membrane may rupture in untripped flowers as indicated

by the observations of Vansell and Todd (1946) and Carlson (1946),

referred to above.

Seed setting wit.hout tripping results in self-pollination, the consequences of which will be discussed in Section 111-1. Conversely while

tripping does not insure cross-pollination it is essential for its occurrence.

8. Self- and Cross-Pollination and Seed Setting

The functioning in fertilization of pollen from the same plant is

known as self-pollination as contrasted to cross-pollination which involves the functioning of pollen from another unrelated plant. As early

as 1914 Piper et al. showed that cross-pollination resulted in more seeds

than self-pollination. Investigations reported by Hadfield and Calder

(1936), Tysdul (1940), Cooper and Brink (1940), Jones and Olson

(1943), and Bolton (1948) all have shown that on the average crosspollination results in a t least three to four times as much seed as does

self-pollination. These studies have revealed that the higher seed yield

upon crossing is due to the combined effect of a higher proportion of

flowers sett.ing pods and a larger number of seeds per pod.



Plants vary widely in the extent to which they will set seed upon

selfing. Tysdal and Kiesselbach (1944) have shown th a t the interplant

variation in percentage of flowers setting pods upon selfing ranges from

0 to 100 per cent. Bolton and Fryer (1937) present data showing a

similar range. When, however, the number of seeds per pod is taken

into account there seems to be no reported case in the literature of a

plant which sets an equal or higher amount of seed on selfing than on

crossing. I n the general population of plants as indicated above crosspollination results in a markedly higher seed yield.

The explanation for the higher seed setting upon crossing as contrasted to selfing was established by Cooper and Brink (1940). They

conducted a very detailed study of the progress of pollen tube growth,

fertilization, and embryo development in seven plants. Upon selfing

14.6 per cent of the ovules were fertilized as compared to 66.2 per cent

upon crossing. Restricted pollen tube penetration of thc ovary and

failure of pollen tubes to enter the ovules accounted for the low percentage of fertilized ovules on selfing. Furthermore they found that 34.4

per cent of the fertilized ovules collapsed in the selfed series within 144

hours after pollination whereas only 7.1 per cent collapsed in the crossed

series. I n their study the combined effect of these two factors resulted

in about 5.5 times as much seed setting on crossing as on selfing. They

concluded that “one of the basic phenomena involved in reproduction

in alfalfa is partial self-incompatability.”

A study by Brink and Cooper (1939) revealed that the rate of

endosperm development was significantly higher on crossing than on

selfing. They postulated that compet.ition for nutrients occurred between

the inner integument and the developing endosperm, and that when the

growth rate of the latter was slow, as it is on selfing, the balance in the

competition was tripped in favor of the integument which resulted in a

hyperplasia in the latter tissue and in time terminated the ovule development. Ovule collapse due to this course of events was termed somatoplastic sterility by these authors.

Structurally and functionally the alfalfa flower is thus adapted to

tripping and cross-pollination. The extent to which seed setting is dependent on tripping has been shown in the previous section of this paper.

While in a random population of plants seed setting will take place to

a certain degree from self-pollination, yet high seed setting is dependent

upon a high cimount of cross-pollination. The extent to which crossand self-pollinstion occurs under natural field conditions will be discussed more fully in Section 111-1. Briefly, however, it has been shown

that the crop is naturally cross-pollinated to a high degree.



3. Tripping and Cross-Pollinating Agencies

a. Rain, Wind, Automatic and Mechanical Tripping. Tripping may

be induced by a number of factors. The role and relative importance

of wind, rain: temperature, insect activity, and mechanical treatment

have been the subject of a number of investigations.

Knowles (1943) and Tysdal (1946) noted that during rain a certain

amount of tripping occurred. Tysdal (1946) showed that the extent

varied with the intensity of the rain but as an average of five rains only

8.3 per cent of the flowers were tripped. B y sprinkling to simulate rain

and artificially tripping Tysdal (1946) demonstrated that sprinkling

materially reduced pod setting. Sprinkling, then tripping followed by

sprinkling, a sequence of events similar to rain tripping, resulted in only

21 per cent as much pod setting as did tripping with self-pollination in

the absence of sprinkling. The above sprinkling treatment gave only

14 per cent as much pod setting as did cross-pollination without sprinkling. Knowles (1943) also observed that rain tripping resulted in low

pod setting. In tripping induced by rain no provision is made for crosspollinat.ion and, therefore, the number of seeds per pod set is low. As a

consequence of the low percentage of rain tripped flowers which set pods

and the low number of seeds per pod, which is likely to result, tripping

by rain is undoubtedly of insignificant importance in seed production.

Wind action also appears to be of very minor significance as a tripping agent. Tysdal (1946) records that numerous observations have

failed t o show any appreciable degree of tripping due to this factor.

Knowles (1943) found that no correlation existed between wind velocity

and percentage tripping. I n common with rain tripping, there is lit.tle or

no opportunity for cross-pollination following wind tripping.

The occurrence of automatic (self) tripping has been frequently

observed over a considerable period of time. Armstrong and White

(1935) and Wexelson (1946) have concluded that a high amount of

automatic tripping occurred. By excluding tripping insects by means

of screen cages, paper or cotton bags, however, it has been possible to

measure the extent of automatic tripping. Knowles (1943) reported

that 26 per cent of the flowers inside cages set pods as compared to 55

per cent of flowers of the same plank outside cages. It should be noted

that a number of the plants included in this study were selected for

ability to trip automatically. Tysdal (1940) found that from 2 to 4 per

cent of flowers inside nainsook cotton bags set pods as contrasted to

15 to 35 per cent outside. Hughes (1943) observed 5.4 per cent of flowers

setting pods inside of cages. Lejeune and Olson (1940), Silversides and

Olson (1941), and Vansell and Todd (1946) reported very low pod setting



inside cages. Carlson (1946) found from 4.6 to 12.1 per cent of flowers

setting pods in paper bags and Tysdal (1946) reported an average of 7

per cent pod-setting one year and 5.8 per cent the next year inside

paper bags. Vansell and Todd (1946) also gave data showing a low

pod setting inside cages as compared to outside. Furthermore, individual flower histories based on frequent or continuous observation

have been reported by Tysdal (1940) to show a low incidence of automatic tripping. While about 5 per cent of flowers setting pods due to

tripping of this nature is of some consequence in seed setting and yield

it is of relatively minor significance in relation to the potential pod

setting when tripping insects populations are adequate to trip 70 to 100

per cent of the flowers.

Although in the general population t.he incidence of automatic tripping

is low, certain individual plants may be found which trip freely (Armstrong and White, 1932; Carlson, 1946). Tysdal (1946) states that less

than 1 per cent of the population have this characteristic. Tysdal 1942,

1944, 1946) has repeatedly stressed that highly self-tripping-self-fertile

plants are undesirable for use in a breeding program because of the high

degree of selfing which occurs and the resulting depressing effect on the

progeny yield. The work of Stevenson and Bolton (1947) with such

plant material has borne out Tysdal’s contention.

Theoretically automatic tripping could result in cross-pollination,

assuming that the pollen was wind borne or deposited by insects and

lodged on t.he standard petals of untripped flowers. Hadfield and Calder

(1936) found an average of 28.7 pollen grains per square inch on greased

slides. Unpublished studies a t Saskatoon have shown that comparatively

little pollen is wind-borne and has indicated that the quantity of pollen

adhering to standard petals is totally inadequate to effect cross-pollination to any significant degree. Knowles’ (1943) data showing that pods

set outside of cages contained twice as many seeds as pods set inside

cages from automatic tripping is evidence that automatic tripping results

in selfing. He showed further that in a random lot of 17 plants under

field conditions hand tripped flowers, which would correspond to flowers

automatically tripped, set 0.42 seeds per flower tripped as compared to

2.55 seeds per flower tripped by bees on t.he same plants. This differential

corresponds closely with that shown to exist upon self- as compared to


Acceptance as a fact that rain, wind and automatic tripping result

very largely in self-pollination and that under natural conditions a high

degree of cross-pollination occurs, evidence of which will be discussed in

a later section, leads to the conclusion that there is a low incidence of

tripping from the above causative factors under average field conditions.

21 2


In the absence of tripping and cross-pollination from other caiises the

seed set and seed yield therefore will be low.

Recognition of the necessity of tripping and the general deficiency of

tripping agents has led to the investigation of the effectiveness of mechanical tripping by such mean3 as ropes, chains, harrows or specially

constructed devices drawn through the fields. Hadfield and Calder

(1936) reported that with the peveral implements tried the results were

negative. Silversides and Olson (1941) showed that while tripping was

increased the seed yield was not increased by mechanical treatment.

Undoubtedly the explanation of the failure of mechanical manipulation

lies in the fact that the indeterminate type of flowering habit of the crop

necessitates repeated treatment and also that any treatment severe

enough to induce tripping causes considerable injury (Silversides and

Olson, 1941; Jones and Olson, 1943). In addition no mechanical device

so far evolved mekes provision for cross-pollination.

b. Tripping Insects. Some of the earlier and many of the more recent

investigations have produced a wealth of data showing the fundamental

part which insects play in tripping. The higher tripping and pod setting

which occurs outside as compared to inside cages and bags is evidence

of the part which insects play. Knowles (1943) reported 0.11 seeds per

flower observed inside and 0.90 seeds per flower observed outside cages.

The presence of bees outside and their exclusion inside the cages is the

major treatment difference in his study. From detailed observation of

individual flowers Tysdal (1940) concluded that relatively little tripping

occurred except from insect activity. Further evidence of the important

role of insects in tripping is found in the high positive correlations between percentage tripping and population of tripping bees (Knowles,

1943; Peck and Bolton, 1946). The latter authors found the multiple


correlation between tripping and population of all bees to be

.63 the next year, bot,h of which values were highly signifiyear and


The significance of bees in tripping has seemed almost incredible to

many since frequently their numbers appear very low. Actually many

investigators have agreed that the bee populations are inadequate and

have concluded that low seed yields are the consequence. When it is

realized, however, that several efficient tripping species visit from 10 to 20

flowers per minute (Knowles, 1943; Peck and Bolton, 1946; Linsley,

1946; Vansell and Todd, 1946; Linsley and MacSwain, 1947), and trip

80 to 100 per cent of the flowers they visit, their role and importance can

be more fully appreciated. With suitable weather for their activity over

a period of time on plants in a thrifty condition and in the absence of

insects or disease destruction of buds, flowers, pods or seed, a relatively





few bees can be responsible for a considerable amount of seed. Knowles

(1943) estimated that one bee working for 100 hours during the flowering

season could effect sufficient tripping and cross-pollination to set one

pound of seed. Yet 200 t o 300 bees dispersed aver an acre could be

almost unnoticed to the casual observer.

Tripping bees are undoubtedly of fundamental importance as crosspollinating agents as well. In the act of tripping the stamina1 column

generally strikes them and pollen is deposited on their bodies and is

transferred from flower to flower. Their habits in respect to concentrating

on the flowers of one raceme or one plant as contrasted to skipping

rapidly between racemes and plants may influence the degree of crosspollination. Intergeneric differences in the working habit of bees have

been noted bv Linsley and MacSwain (1947a) and Vansell and Todd

(1946), and have been considered to be a possible factor influencing the

extent of cross-pollination.

While a wide variety of insects visit alfalfa flowers i t has been observed and generally accepted that only those in search of pollen are instrumental in tripping to any appreciable extent. Many nectar gatherers

can attain their end without disturbing the flowers enough to cause

tripping. A few large insects, such as some of the bumble bees, may

occasionally induce tripping apparently by their weight or clumsiness.

Butterflies, thrips, and flies, while often present, are generally conceded

to be “unable to trip or at the most very unimportant as trippers”

(Linsley, 1946). A wide variety of pollen-collecting bees are recognized

R S being primarily responsible for tripping.

The species of bees which are found tripping varies widely from area

to area and even from field to field within a relatively small area.

Linsley (1946) and Linsley and MacSwain (1947a) reported that in

California the following species tripped alfalfa flowers: leaf cutter bees

(Megachile s p . ) , bumble bees (Rombus sp.), alkali bees (Nomia sp.),

metallic sweat bees (Agapustemon s p . ) , true sweat bees (Halactus sp.)

and (Lasioglussum sp.) , cotton bees (Anthidium sp.) , osmiine bees

(Diceratosmia sp.) , long horned bees (Melissodes s p . ) , anthophorid bees

(Exornolapsis s p . ) , furred bees (Anthophora sp.) , carpenter bees (Xylcopa sp.), and honey bees (Apis inellifera L.). Tysdal (1946) lists the

following additional genera : Auguchlora, Andrenids, and Calliopsis.

Crandall and Tate (1947) drew attention to the efficiency of species of

the latter genera. Peck and Bolton (1946) reported in addition Osmia

s p . , Coelioxys sp.; and Psithyrus as of some value as trippers. Tysdal

(1946) also noted that the soldier beetle (Chauliognathus basalis) had

been observed to trip flowers.

The leaf-cutter bees are widely distributed in North America (Tysdal,



1946; Knowles, 1943; Peck and Bolton, 1946; Linsley, 1946). Bumble

bees are also widely distributed, although Tysdal (1940) considers them

to be more important in the eastern United States than eleswhere. Nomk

sp. were reported by Tysdal (1940) to be partirularly important, pollinators in Wyoming, Idaho, Utah and Oregon.

The crop apparently is attractive to certain species of a genus and

not to others of the same genus. Peck and Bolton (1946) reported that

certain leaf-cutter species were not found in alfalfa fields. Between

genera and also between species within genera there are marked differences in speed of flight, rate of flower visitation, efficiency in tripping,

rapidity of transfer from raceme to raceme and plant to plant,

length of working day, etc., as shown in various aspects by studies reported by Tysdal (1940,1946),Knowles (1943), Peck and Bolton (1946),

Linsley (1946), and Linsley and MacSwain (1947a). The above and

possibly other considerations complicate the evaluation of the variour:

species as tripping agents. However, more intensive research on wild bee

populations would appear to be warranted.

Interannual fluctuations in populations of wild bees in alfalfa fields

have been observed in California by Linsley and MacSwain (1947) and

in Saskatchewan by Knowles (1943) and Peck and Bolton (1946).

The importance of the honey bee in tripping has been one of the most

controversial topics. Tysdal (1940, 1946), Knowles (1943), Peck and

Bolton (1946), Linsley (1946), Wexelsen (1946), Akerberg and Lesins

(1946) and Harrison et al. (1945) have reported them as being frequently

present in very large numbers but effecting little or no tripping. Lejeune

and Olson (1940) noted that over a period of 2 days a relatively small

number of honey bees tripped up t,o 28 per cent of the flowers visited,

but the following day 16 bees observed failed to trip a single flower, and

no honey bee tripping was observed for the balance of the season. On

the other hand, Hare and Vansell (1946) and Vansell and Todd (1946)

have shown the honey bee to be an important. tripper in the Delta area

of Utah. Knowlton and Sorenson (1947) have also stressed their value

in Utah. I n contrast to their 1945 studies Linsley and MacSwain (1947a)

considered the pollen-collecting honey bees to be of major importance in

1946 in California. Alfalfa plants in cages in which honey bees have

been confined have shown considerable seed setting (Hadfield and

Calder, 1936; Dwyer and Allman, 1933).

The confusion in respect to the value of the honey bee probably is

due largely to the diversity of ecological and environmental factors in

various areas. The most important single ecological factor is undoubtedly the abundance of competing preferred pollen sources for the bees.

I n California Linsley and MacSwain (1W7a) have concluded “that of



all bees important in alfalfa pollination in these areas the honey bee

is most readily diverted from alfalfa by this particular series (sweet

clover, mustard, carrot, tamarisk, sunflower, blue curl, and arroweed) of

competing pollen plants.” I n contrast these authors recorded that alfalfa

is the preferred nectar source both for wild and honey bees. Ecological,

environmental and humanly controlled factors influence the abundance

of competing pollen sources geographically and seasonally, and contribute

to the lack of agreement on the value of honey bees as trippers.

The desirability of utilizing honey bees for tripping and cross-pollinating has occurred t o many since their populations are controlled so readily

by man. The primary problem in so doing, as indicated above, is t o

force them to forage for pollen on the crop. I n areas where conditions

lend themselves to reduction or elimination of competitive sources by

the use of selective herbicides, mowing, or by other means, the possibility

seems to warrant further investigation. The further possibility exists

of influencing pollen collection by manipulation of the pollen supply in

the colony by means of pollen traps (Rubnev, 1941). However, Linsley

and MacSwain (19474 have indicated that such treatment, through adverse effects on brood development, may defeat its purpose.

4. Factors Influencing Bee Visitation

Competing pollen sources have already been cited as influencing the

visitation of honey bees. This has been shown by the work of Linsley

(1946), Linsley et al. (1947a), Hare and Vansell (1946), and Vansell

and Todd (1946). The plant species involved vary with the area and

the season and need to be determined for each locality. The preference

of wild bees for certain plants other than alfalfa has been observed by

Knowles (1943), Peck and Bolton (1946), Vansell and Todd (1946), and

Linsley and MacPwain (19474. The desirability and possibility of reducing or eliminating competition has been pointed out by these authors.

Its beneficial effect was demonstrated by Linsley and MacSwain (1947a).

It remains as a possible practical effective means to be more fully explored.

In eliminating or reducing the competitive flora Peck and Bolton

(1946) and Linsley and MacSwain (1947a) have drawn attention t o

the necessity of providing food sources for bees during those seasons of

the year when alfalfa is not in flower. This demands a more thorough

knowledge of the life cycle and nesting habits of many bees than is now


Proximity of nesting sites to fields may be of importance in visitations, as has been shown by Vansell and Todd (1946) in the case of the

alkali bee. I n the case of one field adjacent to nesting sites they



estimated there were 14,520 bees per acre and noted that even the partly

unfolded flowers were being tripped. This raises the question of the

possibility of artificial propagation of wild bees in or adjacent to fields,

and also the effect of culturak and irrigation practice on insect populations. Peck and Bolton (1946) have demonstrated t.he possibility of

attracting certain leaf-cutter bee species to holes drilled in logs and have

cited references on the successful propagation of bumble bees. Bohart

(1947) records that bumble bees can be induced to nest in artificial

domiciles and considers that establishment and transfer should be possible. Crandall and Tate (1947) described the nesting sites used by

Calliopsis sp., and indicated the possibility of encouraging them to nest

in and around fields. Linsley (1946) described tthe nesting sites of many

species he observed, and drew attention to the possible effect of cultivation and irrigation practices. Of the wild bees, all of those so far reported

as trippers, except bumble bees, are solitary and it would seem that

propagation of tIiem would be more difficult than that of the colonial

bumble bee.

Individual alfalfa plants have been noted to differ very markedly in

their attractiveness to wild bees (Knowles, 1943; Vansell and Todd,

1946). The latter authors stated that no plant differences in attractiveness to honey bees had been observed. The reason for the differences in

attractiveness have not been explained. It may involve quality or quantity of pollen or nectar. An intervarietal difference in sugar content of

nectar has been recorded by Vansell (1943). Linsley and MacSwain

(1947a) point out that pollen-collecting bees require nectar to supply

their body needs. Therefore nectar quantity and quality conceivably

could be of flignificance in attractiveness. I n breeding the crop this

characteristic seems to warrant considerat.ion as a possible means of

improving seed yield.

Soil moisture level has been shown to influence the sugar concentration of the nectar and its attractiveness to bees. Vaneell (1943) found

a range in nectar sugar concentration of from 11 to 38.3 per cent in plants

growing on wet and dry soil respectively. Vansell and Todd (1946) also

noted that a wide difference in sugar concentration was associated with

soil moisture level. Their data on honey bees showed that the population

of nectar collectors was positively correlated with degree of succulence,

but that, in the case of pollen collectors, a negative correlation existed.

In cases of production under irrigation, within limits, succulence may be

controlled, and the above cited evidence indicates that it may be of

significance in influencing pollen collection.

Temperature is obviously a dominant governing factor in bee activity

and foraging. There is some evidence that relative humidity is also of



significance. Temperature and possibly humidity affect the ease of

tripping (Hughes, 1943; Tysdal, 1946), and thus exert a dual influence.

In Nebraska Tysdal (1940) noted a marked increase in number of

flowers visited and tripped as the temperature rose from 70 to 100" F.

Tysdal (1946) related maximum temperatures and minimum humidity

to percentage of flowers forming pods. H e concluded that low maximum

temperatures and high minimum humidity during the seed setting period

resulted in a low percentage of flowers forming pods. He noted t h a t

during cool, wet weather insect activity invariably came to a halt.

Knowles (1943) established that a highly significant positive correlation

existed between Fercentage of tripping and temperature, Linsley and

MacSwain (1947a) also established a positive relationship between temperature and low humidity and insect activity, but their observations

showed that above a certain temperature and below a certain relative

humidity further changes in these climatic factors had a depressing effect

on populations of both nectar and pollen collectors.

Certain species of bees are influenced to a greater degree by temperature than others. The leaf-cutter bee has been noted by Tysdal

(1940, 1946) and Peck and Rolton (1946) to cease actsivity at higher

temperatures than bumble bees. Pollen-collecting honey bees have been

shown by Vanseli and Todd (1946) to work a t lower temperatures than

leaf-cutters. While intergeneric differences in this respect exist, humidity

and particularly temperature are nndoubtedly the dominant factors in

the activity of all species.

Competition between species of bee may in certain circumstances

determine the visiting species. Vansell and Todd (1946) have recorded

a case where the Nomia population was so high that honey bees were

not present in the field even although a large apiary was nearby. They

also found bumble bees disappearing as Nomia became abundant. I n

general, however, the poplation of any one species is not sufficiently

abundant to provide severe competition, and various species usually work

the same field and the same plant in apparent harmony.

The relationship between the visitation of bees and the control of

injurious insects by DDT and other insecticides needs further clarification. Vansell and Todd (1946) have shown that tripping was higher than

elsewhere on plots in which lygus and thrips were controlled. Linsley

and MacSwain (1947a) established that dusting when the crop was in

bloom caused an immediate decrease in population, and that 3 or 4 days

were required for the population t o build up to the predusting level. It

is possible that dusting in the prebloom stage would control the injurious

insects without affecting the beneficial species. This topic will be discussed further under Section II-6- (b) .

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II. Seed Setting and Production

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