Tải bản đầy đủ - 0trang
II. Seed Setting and Production
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
WM. J. WHITE
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.
WM. J . WHITE
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.
WM. J . WHITE
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,
WM. J. WHITE
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
WM. J . WHITE
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
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
ALFALFA 1M PROVEMENT
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) .