Maataloustieteellinen Aikakauskirja
Vol. 62: 33—44, 1990

Infestations and distribution of Rhopalosiphum padi (L.) on different varieties
of barley and oats, effect of nitrogen fertilization and chemical control

SIRPA KURPPA 1 and MARJA SUONPÄÄ2

1 Institute of Plant Protection, Agricultural Research Centre,
SF-31600 Jokioinen

2 Department of Agricultural and Forest Zoology, University of Helsinki,
SF-00710 Helsinki

Abstract. Infestations of Rhopalosiphumpadiwere compared between common varieties
grown at nitrogen levels of 50, 100 and 150 kg/ha in 1986 and 1988, and controlled with
dimethoate in 1988.

Colonization of the aphid was slightly quicker on oats than on barley. The highest peak
inside field cages was 305 aphids and in the open field 74 aphids per plant. On barley, the
number of aphids was highest on var. Pomo and var. Pokko, six-row type varieties with the
longest growth period. On oats the number of aphids remained lower on var. Nasta, an early
maturing variety with strong straw, than on other cultivars. Excessive nitrogen either increased
or decreased the peak number of aphids per plant, depending on the variety and the year.

Over 40 % ofaphids on oats fed on the lower base of the plant at or under the soil sur-
face. On barley, aphids lived slightlyhigher, 30 % in the stem and 30 % on leaves. Dimethoate
spray controlled aphids on the top of the plant but not those on the base and lower leaves.
Its efficacy against aphids on the upper stem and lower leaves on barley was decreased when
an increased amount of nitrogen was used. The percentages of parasitized and diseasedaphids
decreased after the use of dimethoate.

Index words: barley, oat, Rhopalosiphum padi, bird cherry-oat aphid, varieties, nitrogen, distribution, chemical control,
feeding site, dimethoate, parasitized, diseased

Introduction

Differences in Rhopalosiphum padi (L.)
infestation between species of spring cereals
and different varieties have been reported in
Finland by Markkula and Roukka (1972)

2

and Rautapää (1970) and elsewhere eg. Hsu
and Robinson (1963). The order of favour-
ability to R. padi is known to be: barley, oats
and wheat. In practice, wheat is damaged by

33

JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND

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R. padi much less than other spring cereals,
and aphids on wheat seldom need to be con-
trolled in Finland. On the basis of experiments
in cages, Rautapää (1968) has described
changes in yield components on cereals in-
fested with R. padi.

Nitrogen fertilization is known to promote
vegetative growth and the ability of spring
cereals to tiller, thereby creating new sites for
R. padi to feed on. By extending the period
of vegetative development, excessive nitrogen
extends the time for R. padi to multiply. It
has been claimed that excessive nitrogen in-
creases the need for aphid control.

The behaviour of aphids, especially their
choice of feeding site, the stage of plant
growth and biochemical interaction between
aphids and plants have been shown to be im-
portant to the growth of aphid populations
and to yield losses (Wratten 1978, Leather
1982, Leszczynski 1985). Wiktelius (1987)

has presented information about the feeding
site of the bird cherry-oat aphid on spring bar-
ley. R. padi was found generally to favour the
area of the plant that is under the soil surface.

Differences in the abundance and behaviour
of R. padi on different varieties of spring bar-
ley and oats grown at different levels of nitro-
gen fertilization were studied in order to evalu-
ate the effect of variety and nitrogen supply
to the rate of increase of the aphid popula-
tion and to the need for and the efficacy of
chemical control. The experiment was per-
formed in a year with a low expected R. padi
population, 1986, and in a year with an ex-
ceptionally high expected aphid population,
1988, in Finland.

Material and methods

The field experiments, carried out at the
Agricultural Research Centre in Jokioinen, in-
cluded ten varieties of barley and oats (Fig. 2)
(subplots) and three levels of nitrogen fertili-
zation (50, 100 and 150 kg nitrogen/ha)
(plots). The soil type was clay. Primary nitro-
gen fertilization was given as a Normal-Y fer-
tilizer containing 10.1 °7o of ammonium-Nand

5.9 % of nitrate-N. The excessive nitrogen to
the levels of 100 kg and 150 kg nitrogen/ha
was given as a salpetre fertilizer, called Oulun
salpietari, containing 13.8 % ammonium-N
and 13.7 % nitrate-N. The number of repli-
cations was three, devided into randomized
plots and randomized subplots, 12.5 m 2 in
size. Sowing dates were May the 19,h in 1986
and May the 16lh in 1988. Seedlings emerged
from ten to fifteen days after sowing. No
aphid control was needed in 1986. In 1988, the
crop was sprayed on the 7th June, a week
from the first peak of migration and the time
when the first migrants were observed in the
crop and 3 days from the second, final peak
of migration. Dimethoate, 0.5 1/ha in 200
litres of water, was sprayed with a normal
farm sprayer. At spraying, most varieties were
at the 2—4 leaf stage. Half of each plot was
sprayed, thus forming subsubplots of 6.25 m 2.

In 1986, at the peak population, aphids
were counted from 25 plants taken randomly
from each subplot. In 1988 the population was
observed more precisely. Migrants and exules
of R. padi were first counted separately five
days after the start of the spring migration,
from a randomly chosen section of 0.5 m
(about 25 plants) of a row, in the centre of
each subsubplot. The second time, aphids
were counted, one week after from the first
count, from five 0.5 m long sections of rows
(one of sections the same as first time). At the
peak of the population (three weeks after first
count), samples from one random row were
taken to the laboratory and deepfrozen quick-
ly, for observation later, in the autumn. In ad-
dition, during the first count, three plants were
chosen from each unsprayed subsubplot, two
with I—s exules and a third with s—lo5 —10 exules
per plant. These plants were covered with an
aerated PVC cage to prevent the aphids from
escaping and to keep their enemies out. The
number of aphids from these plants were
counted again two weeks later, and three
weeks later (the peak of population) the plants
were taken from the soil to the laboratory.
During the last count made from samples of
plant rows or single plants, apterae and

34



nymphs were counted separately from the base
and upper stem from each leaf. The white area
of the stem above the roots was considered to
be the base of the plant. The height of the
plant as well as the length of the flag leaf
of the main shoot and of tillers were also
recorded. During the sampling of the plants
for the last count, number of coccinellids (lar-
vae and adults) and syrphids (larvae) were
counted. The number of parasitized and dis-
eased aphids was recorded in the laboratory
from samples.

Both experimental years were relatively
warm. Daily precipitation and mean temper-
ature are presented in Fig. 1.

Results

Colonization of plants by R. padi

In 1988, the first migrants were observed in
the plants on the Ist1 st of May, three days from
the first peak of the spring migration. (Spring
migration peaked second time on the sth5 th of
May.) The total mean number of spring
migrants that infested oats (2.25 aphids/plant)
was double the number of migrants on bar-
ley (1.0 aphids/plant) (P<0.05). However,
there was no difference between the incidence
of infested plants of barley and oats. No sig-
nificant differences were found between the

Fig. 1. Mean daily temperature (a, c) and daily precipitation (b, d) of the growing season in Jokioinen in 1986
and 1988.

35



number of spring migrants on the different
barley and oats varieties, either. Incidence of
migrants on all barley and oat varieties was
significantly lower at the level of 50 kg nitro-
gen/ha than at the higher levels of nitrogen,
the respective means being 17 + 6 and 26 ± 5 %

of plants, P<0.05.
The number of first exules after the first

days of spring migration was highest with
the medium amount of nitrogen fertilization
on oats (mean at the level of 100 kg/ha 2.5
aphids/plant, total mean 1.76 aphids/plant)
(PcO.Ol). No other differences were found.

Population growth on caged plants and in
the open field

The number of R. padi in caged plants in
1988 on barley and oats in the highest level
of infestation increased very quickly, in three
weeks to a maximum of 305 aphids per plant
on var. Pokko (Fig. 2). There was a signifi-
cant difference(P< 0.01) in number of aphids

between two groups of varieties. The group
of varieties having the highest number of
aphids included: Pomo (1), Pokko (3), J01439
(5), Eero (7), Puhti (11), Virma (Hja 75430)
(14) and Titus (Tiitus) (16). The second group
of varieties, which had the lowest number of
aphids, included: Agneta (4), Arra (6), HlOl4
(9), Veli (12), Pol (13), Stil (Karhu) (19) and
Nasta (20). No differencewas found between
the pooled results for barley and oats or be-
tween the pooled results for different levels of
nitrogen fertilization. The peak number of
aphids was, on average, 2.5 times higher in
cages than in the surrounding open field.

In the open field in 1988, significant differ-
ences were found in aphid numbers per plant
between varieties Pomo, Agneta and Ida at the
lowest level of nitrogen fertilization (Fig. 3).
But at higher amounts of nitrogen fertiliza-
tion, the differences were not as clear. (Porno
and Agneta are six-rowed and Ida two-rowed
varieties). On var. Ida only, the number of
aphids increased continually with increasing

Fig. 2. Mean peak number of R. padi in plants grown in PVC cages. Primary infestation s—lo5 —10 aphids/plant.
Dots indicate varieties taken for further observations in the open field. Barley cultivars: 1) Porno, 2) Kalle,
3) Pokko, 4) Agneta, 5) J01439, 6) Arra, 7) Eero, B) Pohto, 9) Norwegian HlOl4, 10) Ida. Oat cultivars:
11) Puhti, 12) Veli, 13) Pol, 14) Virma (Hja 75430), 15) WW 17593, 16) Titus (Tiitus), 17) J01057, 18) Vou-
ti, 19) Stil (Karhu), 20) Nasta. Letters a and b indicate the two groups of varieties differing significantly
(P<0.01) from each other.

36



amounts of nitrogen (Fig. 3). With the same
variety, the number of (subsidiary) tillers more
than doubled, from 0.5 to 1.2 tillers/plant, as
the nitrogen level was increased from 50 kg/ha
to 150 kg/ha (P<0.05). On var. Porno, the
number of tillers first increased from a mean
of 1.3 to 1.8, but it then decreased to 0.9 as
the amount of nitrogen was increased from
50 kg/ha to 100 kg/ha and 150 kg/ha. On var.
Agneta, however, the number of tillers re-
mained unaffected. When the aphid numbers
per shoot (main shoot + tillers) were com-
pared only the number of aphids on cv. Porno
at the lowest nitrogen level (32.5) was sig-
nificantly higher than the numbers of aphids
on other varieties (mean 20.75 respectively)
(PCO.01).

On oats, the number of aphids on var.
Puhti tended to be higher than on var. Nasta
(Fig. 3). The number of aphids per plant re-
mained almost unaffected as the amount of
nitrogen was increased, but the number of
aphids per shoot (main shoot + tillers) de-

creased slightly. With var. Puhti, the decrease
was from 34.3 to 20.2 aphids/shoot between
50 and 100 kg nitrogen/ha, and with var. Titus
(Tiitus) from 27.4 to 15.8 aphids/shoot be-
tween 50 and 150 kg nitrogen/ha (P<0.05).
With Nasta, the number of aphids per shoot
(mean 22.4) remained unaffected.

A similar tendency in the results for aphids
on various varieties and with different levels of
nitrogen was found in 1986, the year with a
lower aphid population. On Pomo, the number
of aphids (9.3/plant) was significantly higher
than on other varieties at the lowest level of
nitrogen (total mean 4.7/plant) (P<0.05).
At the level of 100kg/ha varieties Porno, Pok-
ko and Eero had significantly higher numbers
of aphids (10.6, 9.2 and 8.7 aphids/plant)
than other varieties: Kalle, Agneta and Arra
(mean 3.3 aphids/plant) (PC0.05). The num-
ber of aphids on var. Ida was 7.6, on average.
Already in 1986, the absolute maximum (14.2
aphids/plant), significantly higher than that
of any other variety (P<0.05), was found on

Fig. 3. Mean number of aphids per plant on different varieties of barley (Pomo, Agneta and Ida) and oats (Puh-
ti, Titus and Nasta) fertilized with different levels of nitrogen in the field. Significant differences (PC0.05)
between varieties at each level of nitrogen indicated by capitals, and significant differences (PC0.05) between
levels of nitrogen within each variety indicated by small letters.

37



Pokko at the highest level of nitrogen.
On oats in 1986, the mean number of

aphids on var. Nasta at the two highest level
of nitrogen (2.9 aphids/plant) and on var. Pol
at the medium level of nitrogen (2.2 aphids/
plant) was significantly lower than on other
varieties (mean 6.5 aphids/plant at nitrogen
level of 100 kg/ha and 5.9 aphids/plant at
nitrogen level of 150 kg/ha) (P<0.05).

Distribution of R. padi on barley and oat
plants

On oats, 44 % of aphids fed on the base
of plant and 29 % fed on the upper stem. On
barley, 19 % fed on the base, 32 % on the

stem and up to 34 % on the lowest leaves
(Table 1) (Fig. 4). There were only slight
differences in the distribution between levels
of nitrogen (Table 1) and varieties. The rela-
tive number of aphids on the strong and
short straw of Ida (27—41 %) was signifi-
cantly higher than that of Porno (22 —32 %)

(P<0.05)Leaves of var. Porno were broader
and attracted relatively higher number of
aphids.

In the comparison between the main shoot
and tillers, significantly more aphids (P<0.05)
were found on the upper stem of the main
shoot than on the stem of tillers, and in tillers
the number of aphids on leaves was signifi-
cantly higher.

Table 1. Horisontal distribution of R. padi (% of total population) on barley and oat plants treated with dif-
ferent levels of nitrogen fertilization.

Barley: main stem/tillers Oat: main stem/tillers
50 N 100 N 150 NSO N 100 N 150 N

Leaves 5—7 1/0 2/1 2/1 9/0 9/1 9/1
Leaves 3—4 11/17 15/12 14/13 6/27 7/18 7/17
Leaves I—2 38/39 31/38 33/32 15/14 8/13 9/12
Stem 35/21 34/24 26/34 29/22 24/24 33/25
Base 15/23 18/25 25/20 41/37 52/44 42/45

38



Parasitized and diseased aphids

In the area without chemial control, the
mean number of parasitized aphids was 0.36/
plant (1.6 %) on barley and 0.16/plant (0.6 %)

on oats. On barley the mean number of dis-
eased aphids was 4.2/plant (16.4 %). On oats,
the respective figure was 0.45/plant (1.5 %)

for var. Puhti, 2.38/plant (4.6 °/o) for var.
Titus (Tiitus) and 3.73/plant (14.3 %) for var.
Nasta. The difference between the varieties of
oat was significant (P<0.05).

In dimethoate sprayed areas, the mean
number of parasitized aphids on barley was
0.06 (0.4 %), and the mean number diseased
aphids was 0.76 (4.6 %). The respective num-
bers on oats were 0.06 (0.3 °/o) and 0.7/plant
(2.8 %).

Efficacy of dimethoate application

The distribution of R. padi on plants greatly
affected the efficacy of dimethoate. The ef-

-39

Fig. 4. Number of R. padi on different parts of barley (Pomo, Agneta and Ida) and oats (Puhti, Titus and Nas-
ta), a) 7th upmost leaf, b) 6th leaf, c) sth leaf, d) 4th leaf, e) 3th leaf, 0 2nd leaf, g) Ist leaf, h) upper stem,
i) base of plant, at different levels of nitrogen of each variety.



Table 2. Efficacy of a dimethoate spray (Vo) in decreasing the numbers of aphids on different horisontal areas
of barley and oat plants at different levels of nitrogen application (50, 100 and 150 kg N/ha).

Efficacy of dimethoate "!n
Barley Oat

50 N 100 N 150 N 50 N 100 N 150 N

Leaves 5—7 94 95 94 93 94 94
Leaves 3—4 69 45 42 71 33 66
Leaves I—2 84 30 0 30 40 30
Stem 68 56 37 20 10 20
Base 0 0 0 0 0 0
Total 50 24 5 23 18 20

Table 3. Horisontal distribution of R, padi (% of total population) on barley and oat plants in the unsprayed
and sprayed field.

Barley Oat

Unsprayed Sprayed Unsprayed Sprayed

Main Til- Main Til- Main Til- Main Til-
Shoot lers Shoot lers Shoot lers Shoot lers

Leaves 5—7 2±2 I±l 2±2 I±l 9±5 I±l 6±4 I±l
Leaves 3—4 13±7 14±3 10±8 12±5 7±2 21 ±9 4±2 11±7
Leaves I—2 34 ± 10 36±4 25±7 29±11 10±7 13 ±8 10±5 13 ±6
Stem 32 ±8 26±7 20±10 16 + 9 29± 10 24±6 33 ±ll 20±9
Base 19±9 22±5 43±14 45 ±ll 44± 11 42±12 47±9 55±9

ficacy was excellent against aphids at the top
of the plant, but nonexistent against the
aphids feeding on the base (Table 2). This re-
sult was clearly reflected in the total efficacy
of dimethoate spray, which on barley, at the
lowest level of nitrogen fertilization, was
reasonable but on oats was unacceptable. In-
creased nitrogen affected the efficacy of
dimethoate on barley (Table 2), which was due
to the decreased efficacy against aphids on the
upper stem and lower leaves. On sprayed
plants, aphids concentrated more on the base
and the lowest leaves (Table 3). On one
sprayed variety, Porno, up to 71 % of aphids
remained on the base of plant.

Correlations between R. padi population
growth and plant growth

The number of tillers of oats was decreased
by the aphids feeding the base of plant, regres-
sion equation: number of tillers = 2.16

0.017 ± 0.007 x number of aphids in thebase,
R 2 = 0.26, Pc0.05. The significant differences
(PcO.Ol) between varieties and positive effect
of excessive nitrogen in the number of tillers
became clear in sprayed subsubplots, only.
This was mainly due to an unusual result ob-
tainedwith var. Puhti, where spraying caused
a decrease in the mean number of tillers/plant;
at the two lowest levels of nitrogen the de-
crease was from 1.0 and 0.88 to 0.30 and 0.41
(secondary) tillers/plant (PcO.Ol). On barley
aphids had no effect on number of tillers.
Spraying did not affect significantly the num-
ber of tillers of barley, either.

In the cages, a slight positive correlation
was found between adults and nymphs on
the base and upper parts of tillers and the
height of the main shoot (r = 0.17, P<0.01)
and the length of the flag leaf in centimetres
(r= 0.175, PcO.Ol). In the field a positive
correlation was found between the number
of aphids on the base of tillers, or the num-

40



ber of aphids on the stem, and the height of
the tiller (aphids on base r = 0.06, aphids on
stem r = 0.105, PcO.001). Similarly, a slight
positive correlation was found between the
number of aphids on the base or the stem of
oats (r = 0.09, PcO.001) or aphids in the
whole plant of barley (r = 0.16, PcO.Ol) and
the length of flag leaf. A negative correlation
was found between the number of aphids on
the second to the fourth leaf and the height of
the tiller of barley and oats (barley r= —0.096,
Pc0.05, oats r =—0.176, PcO.001) and
the length of flag leaf of oats (r = —0.066,
P C 0.05). The tiller height of both cereal spe-
cies, mean 33.9 cm for barley and 31.4 cm for
oats, was not affected by excessive nitrogen.
The length of the flag leaf of barley, mean
13.9cm, was increased (from 13 to 15 cm) by
excessive nitrogen, but the mean length of the
flag leaf of oats, 16.8 cm, was not affected.
Spraying had no significant effect on these
parameters.

In the unsprayed subsubplots, there was a
negative correlation between the yield of bar-
ley and oat varieties (kg/ha) and the number
of aphids on the lower base of plants (barley
r= —0.33, P<0.05, regression equation: yield
= 4774—16±5Xn0 of aphids on the base,
oats r = —0.24, PcO.Ol, regression equation:
yield = 4091—32± 11 xno of aphids on the
base). The mean yield of barley varieties
Pomo, Agneta and Ida was 4445 kg/ha (un-
sprayed), and increased significantly with the
increase of nitrogen fertilization from 50 kg/
ha to 100 kg/ha (PcO.Ol). The mean yield of
oat varieties was 3320 kg/ha (unsprayed), and
it also increased with an increased amount of
nitrogen (P<0.05).

On barley, a variable yield increase 9 %

on Pomo and 4 °/o on Agneta and 3 % on Ida
was obtained by a dimethoate spray at the

lowest level of nitrogen, but at highest level
of nitrogen, dimethoate caused a greater
yield loss, with Porno 9.4 % and Ida 6.6 %.

On oats, the mean yield increase due to
dimethoate spraying was 11.6 %, but the ef-
fect of dimethoate was highly variable (from
slight negative to a positive effect of 50 %).

Discussion

A higher incidence of migrants at the higher
levels of nitrogen referred to the effect of an
intense green colour of an excessively fertilized
crop. But, the observation that a higher num-
ber of spring migrants landed on oats than on
barley was a surprise, on the basis of infor-
mation about the poor ability of R. padi to
select host plant presented by Ahman et ai.
(1985). However, this result agrees with the
findings reported by Weibull (1987). The
higher numberof exule nymphs at the medium
level of nitrogen was partly resulted from in-
cidence of migrants, but might also indicate
some minor changes in the biochemical bal-
ance of free and structural amino acids in
plants (Niraz et al. 1985).

The unstricted population growth of R.
padi in cages was comparable to the results
presented by Rautapää (1968). Great differ-
ence between population growth in cages and
the open field may partially result from caging
out facultative predators such as carabid
beetles, and of the specific enemies coccinel-
lids, the number of which increased explosive-
ly until the end of June in the open field. The
numbers of syrphids and hymenopterid para-
sites, on the contrary, were very low. Anoth-
er possible reason for the excessive popula-
tion growth could be a great improvement in
conditions inside cages, such as increased hu-
midity.

The differences in the number of R. padi
between barley varieties were in agreement in
the two experimental years, as were those for
cages and the open field. The differences were
obvious in a year of lower aphid population,
but during higher pressure resulting from a
larger population, the differences were not so
clear. During an aphid outbreak, there is no
way chemical control can be avoided by the
selection of a specific commercial variety. On
oats, which is favoured more by R. padi, the
differences between cultivars seem to be mi-
nor in the field, even at a low level of aphid
infestation.

The decrease in the number of aphids on

41



oats at the higher levels of nitrogen is in agree-
ment with the results of Weibull (1987). He
reported that extra nitrogen fertilization
resulted in a decrease in the total content of
free amino acids in oats at the beginning of
stem elongation. Weibull increased the nitro-
gen level from 50 to 100 kg/ha and had one
variety of barley and oats. Differences in the
reactions of different varieties are thus possi-
ble. Another way nitrogen could have an ef-
fect on the aphid population would be by
causing changes in the number of tillers. Bar-
ley seemed to be sensitive to this. In our ex-
periment, unfortunately, it was not evaluated
how much nitrogen was actually used by
plants in the prevailing dry conditions.

The feeding area of R. padi was shown to
be an important factor in the efficacy of pest
control application; this finding partly ex-
plains many of the complaints about the ef-
ficacy of dimethoate presented in 1988. The
phytotoxic effect of dimethoate on barley be-
came obvious, as the efficacy at highest level
of nitrogen was minimal. A slightly earlier ap-
plication, better directed at migrating aphids,
might have given better efficacy. The de-
creased tillering of var. Puhti in the area
where aphids became controlled on the upper
parts of plant indicates a change in apical
dominance. Feeding on the upper area of the
main shoot possibly resulted in a decrease of
apical dominance there, and tillering was
promoted. When controled, feeding stopped
right at the beginning, apical dominance re-
mained effective. This issue has been discussed
by Harris (1974).

The percentage of parasitized aphids was
low, and the percentage of diseased aphids
fairly high compared to counts made in
Sweden in 1980—83 (Wiktelius and Ekbom
1985). The decrease in parasitized aphids due
to dimethoate spraying was expected, but the
decrease in the percentage of diseased aphids
was more surprising. However, the effect of
insecticides in eliminating sensitivity to disease
in the rape blossom beetle, Meligethes aeneus
F., has been discussed by Hokkanen et ai.

(1988). The highest proportion of diseased
aphids was found in oats on a variety that
generally supported the lowest populations of
R. padi. This might be an indication of
weakened condition and induced sensitivity of
aphids on that variety. Differences in disease
abundance may also indicate differences in
humidity within the crop, crop structure af-
fecting the penetration of fungus spores or mi-
nor differences in facultative microflora on
the plant surface.

Vigorous plants had the highest number of
R. padi, especially in cages, as also reported
by Honek (1985). However, there was a slight
negative correlation between the height of
plants and the length of the flag leaf of oats
and the number of aphids on leaves in the
middle part of plant; similar results were
reported for winter wheat by Havlickova
(1984). The correlations referred to the be-
haviour of aphids in the plant when the popu-
lation peak is approaching, but for practice
the results had just a value of curiosity.
Aphids on the base of plants seemed to be im-
portant in connection to yield loss, the mech-
anism most possibly being a decrease in the
numberof grains. As the plant becomes older
and the base becomes dry, R. padi typically
moves to higher parts of plant. There R. padi
could be expected to affect grain quality, grain
weight and protein content, as reported with
Metopolophium dirhodum Wlk. (Wratten
1978). These changes were demonstrated by
Rautapää (1968) in caged plants, but at a
very high population level. After moving to
upper parts of plants, R. padi in the field very
quickly started the summer migration. Inju-
ries on plants in this stage last a few days only,
and are certainly of minor importance.

The effects attained by changing between
available Finnish barley or oat varieties or
decreasing the amount of nitrogen fertilizer
proved to be very complex. The differences
discussed here are valid for explaining varia-
bility in population growth and the results of
pest control after the fact, but difficult to
incorporate as part of practical pest control.

42



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Hokkanen, H., Husbero, G.-B. & Söderblom, M. 1988.
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Ms received August 19, 1989

SELOSTUS

Typpilannoituksen ja kemiallisen torjunnan
vaikutukset luomikirvojen esiintymiseen ja
tuhoihin eri ohra- ja kauralajikkeissa

Sirpa Kurppa and Marja Suonpää

Tuomikirvan esiintymistä verrattiin tavallisimpien Suo-
messa viljeltyjen kaura- ja ohralajikkeiden ja muutamien
jalostuslinjojenvälillä vuosina 1986 ja 1988. Samalla tut-
kittiin typpilannoituksenvaikutusta kirvoihin. Typpimää-
rät olivat kylvön yhteydessä annettuina 50 kg, 100kg ja
150 kg/ha puhdasta typpeä.

Keväällä muuttavatkirvat levittäytyivät hieman tasai-
semmin runsaasti typpilannoitetuissakaura- ja ohrakas-

vustoissa jajäivätrunsaslukuisempina kauraan kuin oh-
raan.

Ohrista Pomon jaPokon kirvamäärät nousivat kaik-
kein suurimmiksi. Kun kirvoja oli yleisesti vähän, kirva-
määrät lisääntyivät merkitsevästi typpiannosta lisättäes-
sä. Vuonna 1986, 50 kg -100 kg/ha typpiannoksen lisäyk-
sen seurauksena lisääntyivät myös Eeron kirvamäärät jyr-
kästi. Kallen, Agnetan ja Arran kirvamäärät olivat alim-

43



millä typpilannoitustasoilla (50 kg—loo kg/ha) muita
merkitsevästi alhaisemmat. Tuomikirvan massaesiintymä-
vuonna, 1988, kirvamäärät ylittivät alimmillakin typpi-
lannoitustasoilla selvästi torjunnan kynnysarvot, ja kir-
vamäärät lisääntyivät typpilannoituksen lisäyksen myö-
tä tasaisesti ainoastaan Ida-lajikkeessa. Muiden lajikkei-
den kirvamäärät jopa laskivat typpilannoitusta lisättäessä.

Kauralajikkeiden väliset kirvamäärien erot kasvustos-
sa olivat hyvin pienet. Ainoastaan Nastan kirvamäärät
jäivät, varsinkin runsaimpia typpiannoksia käytettäessä,
muita alhaisemmiksi. Versoakohden laskettu kirvamää-
rä aleni yleisesti typpilannoitusmäärää lisättäessä.

Kaurassa yli 40 % tuomikirvoista oli asettunut tyven
valkeaan osaan, siis maanpinnanrajaan tai päällimmäis-
ten maakokkareiden tasalle. Ohran tuomikirvoista 25 %

oli asettunut tyveen, noin 30 % korteen ja noin 30 % alim-
mille lehdille. Myös lajikkeiden välillä, erityisesti ohris-
sa, havaittiin lieviä eroja.

Kirvojen sijoittuminen viljakasvissa vaikutti ratkaise-
vasti dimetoaatin tehoon. Tehoa tutkittiin kuivana ja läm-
pimänä kasvukautena 1988.Kasvustossa dimetoaatilla ei
havaittu lainkaan tehoa kasvien tyvessä oleviin kirvoihin.

Torjunnankokonaisteho jäiohrissa parhaimmillaan noin
50 %:iin ja kauroissa vain noin 25 %:iin. Teho ohrissa
heikkeni ennestään kun typpiannosta lisättiin, 100kg/ha
typpiannostakäytettäessä teho oli 30 % ja 150kg/ha typ-
piannosta käytettäessä vain 5 Vo. Kauran ruiskutusten te-
hoon typpilannoituksella ei ollut vaikutusta, mutta kir-
vatorjunta vähensi lajikkeen Puhti versontaa alimpia typ-
pilannoitusmääriä käytettäessä.

Dimetoaatin havaittiin vähentäneen kirvojen loisia ja
myös sienitauteihin siarastuneiden kirvojen määriä. Sie-
nitautisten kirvojen yleisyys vaihteli myös kauralajikkeit-
tain ollen runsain kirvoille ilmeisen epäedullisimmassa
lajikkeessa, Nastassa.

Lajikkeiden ja typpimäärän merkitys osoittautui ko-
vin monitahoiseksi ja vaihteli vuosittain. Näyttää siltä,
että viljan typpilannoituksen tehokkuudella ja nykyisin
viljelyssä olevien lajikkeiden välisellä valinnalla on käy-
tännössä hyvin vaikea suunnitellusti vaikuttaa kirvator-
junnantarpeeseen ja kirvatorjunnan tehoon.Kirvapopu-
laatioiden kasvun ja torjuntatulosten vaihteluita lajike-
valinnalla ja typpimäärän vaihtelulla voidaan kylläkin
jälkikäteen selittää.

44