Russian-thistle as a summer host plant of the beet leafhopper in the San Joaquin Valley, California, and methods of redu...

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Title:
Russian-thistle as a summer host plant of the beet leafhopper in the San Joaquin Valley, California, and methods of reducing its acreage
Physical Description:
12, 1 p. : ill. ; 27 cm.
Language:
English
Creator:
Lawson, F. R
United States -- Bureau of Entomology and Plant Quarantine
Publisher:
U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine (Washington, D.C )
Place of Publication:
Washington, D.C
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Subjects / Keywords:
Beet leaf-hopper -- Host plants -- California -- San Joaquin Valley   ( lcsh )
Russian thistle -- California -- San Joaquin Valley   ( lcsh )
Beet leafhopper -- Control -- California -- San Joaquin Valley   ( lcsh )
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federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

General Note:
Caption title.
General Note:
"E-573."
General Note:
"July 1942."
Statement of Responsibility:
by F.R. Lawson ... et al..

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University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 030284029
oclc - 779468096
System ID:
AA00023044:00001

Full Text






July 1942 E-573

U.
\DEPARTMENT

AGRICULTURE A LVA'kWK1

LOG
SMaAU Of
WOILGY AM)
PLANT QUARANTIE




RUSSIAN-THISTLE AS A SUMMER HOST PLANT OF
THE BEET LEAFHOPPER IN THE SAN JOAQUIN VALLEY,
CALIFORNIA, AND METHODS OF REDUCING ITS ACREAGE

By F.-R. Lawson, W. C; Cook, and G. T. York, Division of Truclk
Crop and Garden Insect Investigations, Bureau of Entomology and
Plant Quarantine, and R. L. Piemcisel, Division of Sugar Plant
Investigations, Bureau of Plant Industry



INTRODUCTION

The beet leafhopper (Eutettix tenellus (Bak.)) is the only known
vector of the virus of curly top, a disease which causes losses to sugar
beets, tomatoes, beans, and other crops throughout that portion of the
United States west of the Rocky MOuntains.

Russian-thistle (Salsola pestifer A. Nels.) is a very important
host plant of the beet leafhopper in a large portion of the insect's range,
and in the San Joaquin Valley of California it is the most important host
plant of this insect during the summer months from May to October. The
purpose of this circular is to summarize the accumulated information regard-
ing the habits and characteristics of Russian-thistle. its relations to the
beet leafhopper, and possible control measures in California. l/


1/ The information on weed host plants contained in this circular,
aside from the relation of these plants to the beet leafhopper, is largely
abstracted from the manuscript for a technical bulletin of the U. S. Depart-
ment of Agriculture (not yet published) entitled "The Ecology of the Prin-
cipal Summer Hosts of the Beet Leafhopper in the San Joaquin Valley, Cal-
ifornia," by F. R. Lawson and R. L. Piemeisel. In addition; much of the
information presented is based upon the work of Piemeisel and Lawson in
California, "Types of Vegetation in the San Joaquin Valley of California
and Their Relation to the Beet Leafhopper." U. S. Dept. Agr. Tech. Bul.
No. 557, 29 pp., 1937; and on the work of Piemeisel in Idaho, "Changes in
Weedy Plant Cover on Cleared Sagebrush Land and Their Possible Causes."
U. 5. Dept. Agr. Tech. Bul. No. 654,.45 pp., 1938.






-2-


THE BEET LEAFHOPPER AND ITS HOST PLANTS

The life history of the. leet leafhopper in California will be briefly
described before its host-plant relationships are discussed.

For its reproduction this insect depends upon two sets of host plants,
namely, winter annuals, which are the chief constituents of the vegetation
on grazing lands of the western plains and lower foothills, and summer
annuals, which are abundant on idle, intermittently farmed lands in the
valley proper and to some extent also on deteriorated grazing land. In
winter the adult females of the beet leafhopper live on the winter annuals
and toward spring they deposit their eggs. The brood that hatches from
these eggs leaves the ripening winter annuals in April or May and scatters
widely. Many individuals reach the summer host plants and breed on these
throughout the summer, ordinarily producing three generations. When the
summer hosts die the insects are again forced to move, and large num1-ers
again reach the winter annuals -the spring breeding grounds. In the years
when winter annuals have not yet germinated because of inadequate fall rains,
the leafhoppers exist on any green vegetation until germination takes place
and winter annuals are available as food. Thus the annual cycle is com-
pleted.

During the spring movements, and to a lesser extent in the late summer
or fall movements, beet leafhoppers temporarily infest practically all
plants in the San Joaquin Valley and establish populations on fa~o -a'_lc
hosts. In most years this infestation extends into tlh. Sacramento Valley
and also into the Salinas Valley, as well as other disti icts along the coast.
Throughout the infested areas those plants that are susceptible to the
disease are infected, and if leafhoppers are sufficiently numerous, serious
losses occur in susceptible crops.

Surveys have shown that the acreages of summer hosts are much smaller
than the acreages of the winter annuals that serve as winter an' sprin
hosts, and as a means of leafhopper control it would .-c casir to reduce ihc
acreage of summer hosts. Although the leafhopper has many summer host
plants, most of these are either very poor hosts or are so restricted in
distribution or abundance that they ari not of practical importance. Only
four are important. Three are weeds, bractscale (Atriplex seranana A,
Nels.), fogweed (Atriplex expansa (S. Wats.)), and Russian-thistle, and one
is a crop plant, the sugar beet. The sugar beets often support high popula-
tions of the beet leafhopper during the summer, but in the San Joaquin
Valley most of the crop is usually harvested before the middle of August,
and insects from these fields must find other summer hosts in order to
survive until the winter hosts germinate.

Bractscale is a very good host and is abundant, but in many areas it
dies early in the season, and the laf"oppers from this source, like those
from beets, must find other summer bests to survive until the winter annuals
come up in the fall.









Fogweed also dies early and, moreover, is usually a very poor host.
Large numbers of leafhoppers may be produced on this plant, but in most years
and in most areas populations are very low if compared with those of other
hosts.

This leaves Russian-thistle as the only summer host which normally
has high populations of the insect and which usually survives until late in
the season. On large areas of this weed the usual fall population is* from
25 to 50 beet leafhoppers per square foot and in some years it is even
higher. For instance, in the fall of 1934 a block of several thousand acres
averaged over 500 leafhoppers per square foot, and one large field had over
1,000 per square foot.

Russian-thistle is also important in another way. Besides being a
very good host, a larger proportion of its acreage occurs on the west side
of the valley and near the spring breeding grounds. Studies of the fall
movements have indicated that only a small percentage of the insects bred.
in the middle and eastern portions of the valley ever reach the spring
breeding grounds, and only those originating near the western foothills have
a fair chance of survival.

ABUNDANCE OF SUMMER HOST PLANTS

The possibility of reducing the acreage of the summer host.plants of
the beet leafhopper depends to a large extent upon the area occupied. The
three important weed hosts, particularly Russian-thistle, are widely dis-
tributed and infest much of the farm land in the San Joaquin Valley. How-
ever, most of this infestation is so light that it is of very little impor-
tance as a source of beet leafhopper populations. For instance, a strip of
weeds 4-1/8 feet wide and 2 miles long by the side of the road is very
conspicuous but actually contains only 1 acre. In estimating the abundance
of weed hosts it is necessary to draw a more or less arbitrary line between
stands which shall be included and those which shall be ignored. In making
the estimates of weed-host acreages that follow, dense patches, if small,
as in barnyards, have been ignored, and so have 'large areas if the stand
was sparse and under 10 percent.

Comparative acreages of the three important weed hosts of the beet
leafhopper for a period of several years are giver in table 1. The figures
do not include San Joaquin, Stanislaus, and Merced Counties, which were
surveyed only 3 years, from 1933 to 1935. These three counties contained
from 2,000 to 8,000 acres of Russian-thistle, 1,000 to 5,000 acres of
bractscale, and 200 to 1,000 acres of fogweed.

A better conception of the magnitude of weed-host areas can be
obtained by a comparison with the acreages of the crops affected. Figures
for sugar beets and tomatoes, which are the two most important crops affected
by the leafhopper in this area, are given in table 2.










Table I.-Comparative acreages of the tAiree importa,.t summer host plants
of the beet leafhopper in the San Joaquin Valley

(These figures are estimates based on partial surveys,
and only large variations are considered significant.)

East and central portions of the valley __
1931 : 1932 1933 1934 1935 1936 1937

Russian-thistle: 97,000: 55,600 47,600 30,300 6,000 6,500 :
Braotscale :114,000. 44,300 : 36,700 : 16,200 5,700 7,500 :
Fogweed :163,700: 74,000 : 50,000 63,100 :23,800 :39,700 :

West portion of the valley

Russian-thistle: 27,500: 11,300 39,200 :50,400 :30.000 32,400
Fogweed : 1,500 400 : 4,700 2,300 1,800 : 900


Table 2.--Total acreage of sugar beets and tomatoes in the principal

areas affected by leafhoppers from the San Joaquin Valley*

Location 7 1931 : 1932 1933 1934 : 1935 1936 1937

San Joaquin Valley: 7,513: 7,260: 7,931: 14,114: 21,036: 31,816: 31,575
Sacramento and
Salinas Valleys 73,032: 80,413: 85,586: 89,216:101,022:137,942:130,731

Total : 80,545: 87,673: 93,517:103,330:122,058:169,758:162,30G


*Figures obtained from the California Cooperative Crop Reporting Service.

In the San Joaquin Valley the losses from curly top are moderate to
heavy. In the Sacramento and Salinas Valleys losses are light to moderate,
although in some years they have been severe.

Since Russian-thistle is the only summer host found in abundance near
the overwintering areas of the leafhopper, it is reasonable to assume that
if this weed were greatly reduced in area in this district, a proportional
reduction in beet leafhopper populations would be effected (excepting
possibly Kern County. in which the movements of the insect in relation to
the host areas have not been sufficiently studied). This does not mean that
either Russian-thistle or the beet leafhopper would be completely eliminated
as pests, but that the acreages of Russian-thistle, populations 'f the
insect, and crop losses from curly top would be greatly reduced.






-5-


CHARACTERISTICS OF RUSSIAN-THISTLE

It is obvious that if measures designed to reduce the acreage of
Russian-thistle are to be undertaken, careful consideration should be given
-to the characteristics of the plant in order to discover the best methods of
attacking the problem.

Russian-thist],e is strictly an annual growing- from seed each ycar.
It normally germinates in the spring in March or April but grows rather
slowly until the coming of warm weather in May or June. Seed may be set as
early as the first of July, but if sufficient moisture is present in the
soil, growth and seed production will continue until frost. If there is
not sufficient moisture, death may occur any time between germination in the
spring and freezing weather in the fall.

Russian-thistle, like all other wild plants, is often forced to
engage in a struggle for existence with other species. In the San Joaquin
Valley its principal competitors are winter annuals, which germinate in the
fall and grow during the winter. Since Russian-thistle does not germinate
until spring, it is at a serious disadvantage. In this region very little,
if any, rain falls in the summer months from July to September. -Conse-
quently. Russian-thistle on unirrigated land must depend for its greatest
growth on moisture stored in the soil. If winter annuals are present, they
have first chance at the water supply. and if they are sufficiently abundant,
they will use up the soil moisture so that none is left for Russian-thistle.
As a result, Russian-thistle, except underr special circumstances. can grow
only where the winter annual. cover has been destroyed or greatly reduced.
Such conditions are found in the cultivated districts on plowed land. But
where a crop is planted the weed is usually destroyed by tillage, so that
the only places where it can become abundant are on fallow fields, on
abandoned land, or in thin stands of grain. This type of land is common
in the poorer districts where there is excessive alkali or inadequate
irrigation water, and in years when the prices of farm products are low.
many fields lie idle and large acreages of Russian-thistle are produced.
But even here the species leads a precarious existence. Fallow or abandoned,
fields are usually plowed again within 1 or 2 years, or, if they are not,
plowed, the winter annuals come in so quickly that they crowd out Russian-
thistle. Records kept of a number of fields which were not plowed and which
contained heavy stands of the weed showed that only 36 percent survived
2 years in succession. This means that, if Russian-thistle is to hold its
own and maintain a stable acreage in agricultural districts, there must
be a continuous succession of idle lands available, and seed must be carried
to these new areas.

In years when farm prices are high. very little land is permitted to
lie idle, and the only place where Russian-thistle can grow in abundance is
on deteriorated grazing lands. It is on this sort of deteriorated land that
most of the Russian-thistle acreage in the western portion of the valley is
found at present. Throughout this portion there is generally a fairly good
cover of winter annuals, and the struggle for existence is usually severe.





-6-


Under these conditions the weed can survive only in those ycars whcn acdi-
tional moisture is received after the winter annuals hNc dic'. This is
provided by late rains or floods, which enable the seedlings that had come
up earlier in the season to survive and continue to grow. In this region
such rains are very irregular in distribution and time of occurrcncz. They
usually are local showers over a comparatively rcstrictcC ar a and maj no-,
occur in the same place two years in succession.

Thus, on grazing land, as in the cultivated dist acs suitable
for the growth of Russian-thistle are continually shifting in location, and
widespread seed dissemination is necessary for the wecd t maintain a stable
or increasing acreage. Such dissemination is dependent on plants that are
well spaced, grow to be rounded in shape, and hac an atunianco of sod,
and not on the crowded plants that become slender in shape and prcucc little
seed. After the round-shaped individuals have died thev 1-rcak loose at the
soil level and roll >before the wind as typical tumblcw7z 3, scattering seed
as they go. However, the slender or small plants cith-r are ntanrled and
are not broken off or, if broken off, do not roll extensively.

Wide distribution of seed is somewhat limited Oy the tendency of
rolling plants to drop the bulk of their seed in the first 2 or 3 milcs,
although the empty bushes may travel long distances. .Iso, c cn the rounic!'
plants are not broken off immediately after death but tend to remain in
place until after the first rains have fallen. Thus there is an interval
when the dry plants can be easily burned before the seed has been widely
scattered.

POSSIBLE METHODS OF REDUCING
THE ABUNDANCE OF RUSSIAN-THISTLE

In the San Joaquin Valley large sums of money haxe bccn spent and
much effort has been expended in attempts to eradicate: Russian-thistle
locally. It should be emphasized that this circular does not d~al primarily
with this type of control. The stands which are ordinc.ily present in farm
land and which cause expense and consiieratle trouble to ".rTrs are usUally
too thin and scattered to be of much importance as lcafLhoppce breeding areas.
Most of the measures which will considered in this paper arc usl:ul Only
for reducing large acreage of heavy stands and preventing increases.

Since the winter annual cover oC much of th, San 'c.uli Valley is o.
such great importance is restricting the area occupie- Ly ussian-thistlc,
any method used in control must not destroy this co-,er, otherwise the
measures applied will do more harm than good. This is true for the permanent
grazing lands only, since in the cultivated district, whrc thv. fields
occupied by the weed are plowed e-,ery year or two anyway, this consi&..ratLon
is a minor one. Here any measure. can I .c used which effectively (estroys the
weed and does not impair the productivity of the soil.





-7 -


Reduction of Russian-thistle Acreage
in Cultivated Land

.Within the cultivated district Russian-thistle acreage can be reduced
by destruction of the growing plants and seeds or by the prevention of seed
dissemination.

Destruction of plants and seeds

Like most annual weeds, Russian-thistle will die if cut off at or
near the surface of the ground below the first branches. The roots are not
extensive, and any cultivating tool which pulls up or cuts off the plants
will destroy them.

The' best time to destroy a stand of Russian-thistle would be before
the plants have produced seed, that is, about July 1. However, on la:ge-
scale experiments in heavily infested sections it would be more practica-le
te'wait until later in the season, even though it is necessary to gaLhcr
an'd'burn the plants to destroy the seeds. Many fields of the weed which
appear to be green and succulent in the spring will die without producing
seed before, the middle of the summer. This premature drying is usually a
result of competition with other species, although very often Russian-
thistle, groWs so thickly that- it dries prematurely. Whether a given, field
Will die before seed is produced cannot be definitely foretold early in
'tIW season, since a late rain may make the difference between survival
Arildeath. In years when rain falls in June, many fields survive that would
tbtrerwise have perished.

S.. Destruction of plants and seeds is continually practiced by farmers
lr .infested -localities to eliminate Russian-thistle as a weed in crops.
-Anewly infested districts county and State authorities have carried out
extensivee campaigns intended to prevent further spread. The sugar-processing
companies that are directly affected by damage from the curly top disease
have destroyed stands of the weed on considerable acreages in strategic
1"cations-where high populations of the beet leafhopper were present.

:-The cost of destroying Russian-thistle by tillage depends upon local
W ago ,paid to labor and the price of fuel oil and other materials. A very
eheap and effective method was devised by ranchers in the Carrizo Plain
wheree Russian-thistle was invading dry-land grain fields. A 10-foot weeder
blade was mounted on wheels to run about an inch below the surface of the
14oil. 'An ordinary hay rake was hitched behind each blade, and three of these
:devices were pulled behind a-tractor, so that a 30-foot swath of weeds was
'cut and piled in windrows in one -operation preparatory to burning. The cost
ranged from 25 to 35 cents per acre. This weeder can be used only where the
B'oil-is level and not too compact,

Another relatively cheap method has been used by farmers in the
Mendota section and to some extent by the sugar companies. This consists
in, dragging and piling the weeds with a railroad rail drawn behind a tractor.









This method is not effective except in stands of large, nearly mature plants.
Even in such stands many small plants will be missed by the drag. The method
will prevent the scattering of seed to other fields if the large plants are
burned, but it does not effectively destroy the local supply of seed. It
costs about 75 cents per acre and has Leen used primarily to clear fields for
plowing.

The prevention of seed dissemination

The methods listed under this heading are intended primarily to pre-
vent the infestation of new fields of Russian-thistle and not to reduce
local ones. As pointed out atove, the large acreages of Russian-thistle can
be maintained only if new fields are constantly infested. If extensive seed
dissemination is prevented, very few new fields will be seeded, and existing
infested acreages will be reduced partly by replowing and partly by the
replacement of Russian-thistle by winter annuals.

It has also been noted that Russian-thistle plants do not break
loose and roll immediately after death but remain in place for some time
after the first rains. During this period the plants are dry enough to burn
and can be cheaply destroyed. Trials of burning were made in three counties
of the San Joaquin Valley in 1933 and 1934, relief labor being used. The
work was carried out satisfactorily only in Stanislaus County. Gangs of men
were employed to burn all of the Russian-thistle plants that could be found
in certain portions of this county. In spite of the fact that only hand
labor was used and the work could not be completed until after the rainy
season had begun, there was a marked reduction in Russian-thistle acreage
as a result of the program. It was found that heavy stands could be burned
in place early in the season by careful tending and resetting of fires.
Where there were thin stands, piling the plants was necessary, and later
in the season during wet weather piling was necessary in all kinds of stands.
Piles of the weed would burn readily even when wet if the fire was started
with a hot flame, such as that of a blow torch.

In 1933 Russian-thistle was destroyed within a 41-square-mile area
in Stanislaus County. In this year there were 1,736 acres of the weed in
fields of 5 acres or more and in stands of 10 percent or greater density.
The following year there were only 556 acres in such stands, a decrease of
68 percent. In an adjacent area of 73 square miles. partly in Stanislaus
and partly in Merced County, where no burning was done, there were 2,222
acres in 1933 and 3,410 acres in 1934, an increase of 53 percent. The
experiment was repeated in the fall of 1934 with the inclusion of some
additional territory, and there was a reduction of 75 percent, from 931
acres in 1934 to 234 acres in 1935. In the check section there was a
reduction of 63 percent, from 3,035 acres to 1,129 acres. Taking the changes
in the checks into consideration, there was a reduction in the experimental
section of 80 percent from 1933 to 1934 and 32 percent from 1934 to 1935.

When dry plants are handled as they were in this experiment, an ade-
quate supply of seed is dropped locally, so the benefits derived do not
result from the reduction of infestations already present, but are due
largely to the destruction of seed available for the infestation of new areas.





-9-


Another method of preventing seed dissemination is by the erection of
fences to catch wind-blown plants. This method was used in the Carrizo
Plain infestation to supplement more direct measures. It is necessary to
patrol the fence at intervals and burn the collected plants. Otherwise,
when the infestation is heavy, weeds will pile up against the fence in such
numbers that they mash it down or they may be rolled over the top of the
pile. The fence should be at least 5 or 6 feet high, and burning can be
more conveniently done if the fence is made of metal posts and wire.

Any of the above methods can be applied to Russian-thistle growing
in cultivated sections. In large-scale programs probably a combination of
methods would be most effective. On permanently abandoned land the most
effective measure is the prevention of further destruction of the developing
winter annual cover. If unimpeded, winter annuals will replace Russian-
thistle in 1 or 2 years, but cultivation, excessive grazing, or fire may
retara or reverse the process.

A further possibility of reducing tne area of Russian-thistle on
cultivated land would be through the application of land-improvement measures
designed to eliminate intermittent farming. The maintenance of large
acreages of the weed is dependent upon the temporary abandonment year after
year or considerable areas of land. Any measure which decreases the extent
of this practice, such as the improvement of the water supply or the per-
manent abandonment of the poorer sections, would result in a proportional-
reduction in the acreage'of leafhopper host plants..

Reduction of Russian-thistle Acreage in Grazing Land

On grazing land the possible methods of reducing the abundance o-f
Russian-thistle are limited by the necessity of considering the cover of.
winter annual forage plants. In order to show more clearly how the present
vegetational cover affects Russian-thi3tle the diagram in figure 1 has been
included.. There are two things which must be present for Russian-thistle
to grow. These are seed and soil moisture during the growing season. Seed-
supply is dependent upon the amount and distribution of the production of
the preceding year. Since these lands are not irrigated, soil moisture comes
directly from rain or from floods. Most of the rain falls during the
winter. Winter rains are general throughout the valley and are a reliable
source of soil moisture, but spring rains are uncertain, local in character,
an-d may-be intense, so as to cause flash floods. The outer large circle in
the diagram (fig. 1) reprcscnts the general winter conditions in the valley,
but, since on grazing land a good cover of winter annuals is ordinarily
present, these plants use up. the moisture from the winter rains before
Russian-thistle can get a good-start. As a result it is normally restricted-
to bare soil or -to those areas where the cover of winter annuals has been..
greatly reduced. But' if- rain, falls late in the spring after the winter
arnuals have died, soil moisture is available even where a good cover of -
these plants is present, and Russian-thistle can continue to grow and produce
seed-. -The interaction of these factors is illustrated by the overlapping
of the small circles in the diagram;.





- 10 -


The undependable nature of the late spring rains and the restricted
area they usually cover has bc mentioned. Thus, if the western plains
of the valley were evenly covered by a stand of winter annuals so that
Russian-thistle would be entirely dependent on spring rains as a source of
soil moisture, it could not grow in most years. Since most of the seed in
the soil is germinated the first year by the winter rains, one or two un-
favorable seasons without spring :ain would so deplete the seed supply that
the establishment of heavy stands would be impossible oven in the most
favorable years. Under present conditions, however, where the winter annual
cover is broken by bare or nearly bare spots, Russian-thistle on these
spots is able to produce a seed supply sufficient for a rapid increase
when conditions become favorable.

There are two possible methods of reducing Russian-thistle acreage
on grazing land. The most obvious is a direct attack on the seed supply
by the destruction of growing plants and seed or by the prevention of seed
dissemination. The second mptho' in-olv-s an indirect attack by improver:.t
of the present winter annual cover to the point where Russian-thistle cannot
compete successfully.

Direct methods

Destructive methods such as cultivation or burning, which remove the
natural cover of the soil, make a more favorable situation for Russian-
thistle the next year, and, although if rigorously applied they might
reduce the area occupied by weeds, it would be necessary to continue these
measures indefinitely unless complete eradication could be effected. This
seems a remote possibility in xiew of the large area infested. Such a pro-
gram, even if effective. would be ,ery expensive, not only in funds actually
expended in control measures, but also in the loss of forage and in injury
to livestock ranges.

There are some direct or mechanical means of Russian-thistle reduc-
tion that might be applicable to grazing land. For instance, in some sec-
tions fencing could be used to stop wind-blown plants. However, if the piles
along the fences were destro ac by burning, precautions would need to be
taken to prevent the fire from spreading and burning off the forage on the
adjacent range.

It is probable that other direct means of reducing Russian-thistle
areas on these plains can be found. It seems likely that some machine could
be used to collect the plants of the weed without seriously disturbing the
cover of winter annuals. This would prevent seed dissemination and, if
carried on for several years, might greatly reduce the present infestations.
Research and experiment might show that methods such as burning or disking,
if applied under specific conditions, would reduce the acreage of the weed
without seriously injuring the future growth of the winter annual cover.

It should Le pointed out, however, that while some direct measures
may not injure the cover, neither are they likely to improve it. The present






- 11 -


areas of Russian.thistle might te reduced, but i f nothing is done to 1mp -.-re
the present stan of wnter annuals anJ thrcr!y decrcase the aree 'herc tb
weed can grow, constant vigilance will be necessary to prevent a rein-
festation,

Indirect methods

On the basis of present information the most effective imEans of
reducing Russian-thistle areas on these plains would be through an imp1-ov -
ment of the present cover of winter annuals.

This can be done most effectively by a decrease in the prt.sean rate
of removal of forage. Any area of grazing land produces each year a certain
amount of forage. Part of this can '-, removed without injury to the pro-
ductive capacity of the rang-. But if all or nearly all the more palatable
plants are removed, those that remain are not able to compete with less
palatable species and eventually die out. This decline continues through
a series of less and less palatable plants until only a few scattered weeds
remain on the soil. Daring this process of deterioration the p'o-'uctive
capacity of the land steadily decreases, but if the rate of rcrro-.al of
vegetation is decreased to a point :elow the immediate producti,'c capacity,
the process is reversed and the' capaeil.v of the ranp-c increases. Am; p-ncy
which destroys vegetation, whether it is livestock, rodents, fire. or plow-
ing, will cause deterioration if the sum total of its effect added to that
of the others is greater than the current productive capacity of the lanii'.
In a series of wet years temporary improvement may occur if produce '.e-:
capacity increases faster than the range load, but suej, gains are usually
wiped out very quickly when a return to a cycle of normal or below--normal
rainfall occurs.

The grazing lands of the San Joaquin Valley over a long pJ.od of
years have been gradually deteriorating as a result of excssieC us-. There
are numerous indications of this, Lhc most obvious of which include "he
presence of unpalatable range ,'ecs, the absence of the formerly alunce:
perennial forage plants, and the presence of tare spots. Excessive gra- >*g
by livestock is p-otably the chief cause of deterioration, although in some
areas rodents may be an important factor.

The unfavorable results of e. ergrazing, such as excessive, soil
erosion, the loss of Tango capacity, the silting of reservoirs, and the
destruction of wilAif1 have been Cealt with by numerous range investigators.
Here it should be emphasized that unless overgrazing is checked in the
San Joaquin Valley, Russian-thist I- w411 in the future become much more
abundant than it is at present, with a consequent increase in damaging
numbers of the beet leafhopper.

A program of range rehabilitation would not only reduce Russian-
thistle acreages but over a period of years would pay for itself by increas-
ing the forage value of the ranges.








- 12 -


SUMMARY

Russian-thistle, because of its suitability as food and its alundance
in areas immediately adjacent to -he spring breeding gi.ounCs, is h most
important host plant of the bect lealfhopper in the San _'oaquin Valiy Curing
the summer months from May to October.

Estimates of the abundance of this weed from 193. to 133 7 a.: g cn
For beet leafhoppir populations the most important ar.as o' th.c w(.cd ar-,
found at present in grazing land on the west side of the valley.

Russian-thistle is a summer annual incapable of compsting success-
fully with the typical winter annual vegetation of this rc.4ior. In con-
sequence. important sian(s ar, restricted Io grainfi1s n 1.ow or
abandoned land in the cultivated district, and to deteriorated grazing land.

Russian-thistle is a typical tumbleweed in its habits of seed dis-
tribution.

On cultivated land it can be reduced in area by methods which d estroy
growing plants and seed or which prevent seed dissemination.

On grazing land such direct methods of reduction o3r limited by the
necessity o.0 pre-serving th .. winter annual cover of %egotaLion. which at
present limits the acreage and distribution of the weed.

On the basis of present information the most effective method of
reducing Russian-thistle acreag, in grazing land would h;c through an improvc-
ment in the present cover of winter annuals. This can be done by a reduc-
tion in the present rate of forage removal.












~and wi th
UnifrmlyDense Cover of V7inter Amnuals
except
within Circle 2
3 2


















i~,ure whreRussian-thist~le Wlo- etiner iul rs n aue
Seed Creere Sparse or
epe t r "Absent









Lat Spring Rains






Figure~~~~~~~~ l.-iar of th efet friflse upy.cmeiin
and~~~~~~~~~~~ th ineato ofteefcoso h rr fRsintite
lirea where Ru~sian-thist-lo alor etinl 1,l g n mtue
B, are wher "",/-hstewl ro u ~lldepe,,Iu yi
th tzld sdes; oru. iCer luscnti lewl o srv e
except iniisihr aes-rn r- n r is;lyhay




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