STUDIES ON THE CEREAL ECOSYSTEM
133
Barley yellow dwarf virus (BYDV) is transmitted by aphids (e.g.
Doodson, 1967) and other Homoptera (Slykhuis and Watson, 1958)
and is widespread throughout the world. It was first recorded in
Britain in cereals in 1954 (Watson and Mulligan, 1957) and since
then has been recorded throughout the country. The leaves of infected
plants may be discoloured yellow (barley), red to yellow (oats), or
bronze red (wheat). Severely affected plants may be stunted and grain
yields reduced. Early infections of field crops can cause yield losses of
over 90% but in most years losses do not exceed 10% (Doodson and
Saunders, 1970). Even without the virus an aphid infestation can
reduce yields by 25% (Kolbe, 1969).
Cereal aphids were noticed in the study area several times prior to the
present study. Most infestations before 1968 were slight and concerned
Rhopalosiphum padi (L.). In 1968 17 spring barley fields in a sample of
59 contained notable numbers of aphids (mainly Sitobion avenue (F.))
on the ears; in ten of these fields the numbers were given as “few”,
in seven as “many”. I n July several heavy infestations of S. avenae
were found in winter wheat; they clearly reduced the yield.
A more widespread occurrence of aphids occurred in 1969, although
the numbers on the ears were very much reduced. The immigration to
the cereals was late: the peak for Sitobion avenae was 10 June, the peak
for S. fragariae (Walker) another five days later. The peak of apterae
was reached in the b s t week of July and in spring barley averaged
100/m2, the dominant species being S. fragariue, S. avenue, Rhopalosiphum padi, Metopolophium dirhodum (Walker) and R. maidis (Fitch),
some Sipha sp. being present. Later-sown barleys had greater numbers
of aphids but the maximum recorded density was only 1112/m2.
Sitobion avenue was more numerous than S. fragariae on winter oats
and wheat, but relatively small acreages of these crops were sown in
the study area in 1969.
Many 8. fragariue eggs were found on bramble (Rubus fructicosus)
in the winter 1969-1970 (average 0.25 eggs per leaf rosette, 561 rosettes
examined); although the bramble is exceptionally common in the area,
few of the aphids reached the cereal crops in 1970. Many eggs were
found punctured and drained, probably by Anthocoris nemorum (L.),
which was active on bramble throughout the winter, and on some
rosettes where 31 eggs had hatched, by I7 March only nine live aphids
could be found.
On the other hand, Metopolophium dirhodum immigrants were
numerous and apterae of this species built up rapidly throughout
June. This species peaked at an average density variously and nonfiducially estimated at about 1300/m2 in spring barley, at which point
it far outnumbered all other arthropods in the field layer.
133
Barley yellow dwarf virus (BYDV) is transmitted by aphids (e.g.
Doodson, 1967) and other Homoptera (Slykhuis and Watson, 1958)
and is widespread throughout the world. It was first recorded in
Britain in cereals in 1954 (Watson and Mulligan, 1957) and since
then has been recorded throughout the country. The leaves of infected
plants may be discoloured yellow (barley), red to yellow (oats), or
bronze red (wheat). Severely affected plants may be stunted and grain
yields reduced. Early infections of field crops can cause yield losses of
over 90% but in most years losses do not exceed 10% (Doodson and
Saunders, 1970). Even without the virus an aphid infestation can
reduce yields by 25% (Kolbe, 1969).
Cereal aphids were noticed in the study area several times prior to the
present study. Most infestations before 1968 were slight and concerned
Rhopalosiphum padi (L.). In 1968 17 spring barley fields in a sample of
59 contained notable numbers of aphids (mainly Sitobion avenue (F.))
on the ears; in ten of these fields the numbers were given as “few”,
in seven as “many”. I n July several heavy infestations of S. avenae
were found in winter wheat; they clearly reduced the yield.
A more widespread occurrence of aphids occurred in 1969, although
the numbers on the ears were very much reduced. The immigration to
the cereals was late: the peak for Sitobion avenae was 10 June, the peak
for S. fragariae (Walker) another five days later. The peak of apterae
was reached in the b s t week of July and in spring barley averaged
100/m2, the dominant species being S. fragariue, S. avenue, Rhopalosiphum padi, Metopolophium dirhodum (Walker) and R. maidis (Fitch),
some Sipha sp. being present. Later-sown barleys had greater numbers
of aphids but the maximum recorded density was only 1112/m2.
Sitobion avenue was more numerous than S. fragariae on winter oats
and wheat, but relatively small acreages of these crops were sown in
the study area in 1969.
Many 8. fragariue eggs were found on bramble (Rubus fructicosus)
in the winter 1969-1970 (average 0.25 eggs per leaf rosette, 561 rosettes
examined); although the bramble is exceptionally common in the area,
few of the aphids reached the cereal crops in 1970. Many eggs were
found punctured and drained, probably by Anthocoris nemorum (L.),
which was active on bramble throughout the winter, and on some
rosettes where 31 eggs had hatched, by I7 March only nine live aphids
could be found.
On the other hand, Metopolophium dirhodum immigrants were
numerous and apterae of this species built up rapidly throughout
June. This species peaked at an average density variously and nonfiducially estimated at about 1300/m2 in spring barley, at which point
it far outnumbered all other arthropods in the field layer.