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THE MILD winter and a warm, dry May in Britain have nurtured the biggest
explosion in aphid populations for 15 years. But as the greenfly, blackfly
and other pests move into gardens and fields, bringing viruses that threaten
crops, the 25-year-old Rothamsted Insect Survey (RIS) is having to close
down some of its insect traps, cancel the publication of information about
the spread of aphids and cut back on forecasting that is vital to farmers.
One early consequence will be that farmers have to spray more insecticides
as an insurance against the lack of information about pests.

The RIS monitors insects using traps that suck up insects in the air
and collect them for future counting and analysis. The first British suction
trap was set at the Rothamsted Experimental Station in 1964. By 1970, 10
more traps round the country had joined the survey. By last year, 23 traps
– each 12.2 metres tall – were in operation, including six in Scotland.

The RIS is the most intensive network of suction traps in the world.
During 446 ‘site years’, its researchers have counted almost 10 million
aphids from 300 species. Now this invaluable database, which allows scientists
to make accurate predictions about times of infestation for many aphids,
is threatened by government policy on agricultural research that has a commercial
spin-off.

An unpublished government review of agricultural R&D, known as the
Barnes Report, has identified aspects of the RIS, such as its forecasting
work, as ‘near market’. The work can save farmers money, by pinpointing
when they need to apply insecticides to their fields and when they need
not, and so the government believes that it should be paid for by farmers.

Funds for the RIS, which come through the Agricultural and Food Research
Council’s Institute of Arable Crops Research, are being reduced. As a result,
the survey will close some traps down completely while it will monitor others
for only part of the year. The RIS’s weekly publications, the Aphid Bulletin
and Aphid Commentary, which it has distributed by post free of charge to
more than 300 interested people, will not appear this year.

Such a short-sighted view by government threatens an unrivalled database
for entomological and ecological research as well as for farmers. And it
is likely to lead to more profligate spraying of chemicals onto fields by
farmers who will be unaware of the true extent of any likely aphid attack.
A loss of data will also threaten efforts to assess whether environmental
changes, including climatic change, are having long-term effects on insect
populations.

Workers who empty the suction traps look not only for aphids but also
for the aphids’ many predators, such as ladybirds, hoverflies and lacewings,
and for spiders and beetles. All are catalogued and kept.

The damson-hop aphid, Phorodon humili, attacks hops. Insecticides can
prevent it from causing large reductions in the yields of the hop gardens
of Herefordshire and Kent every year. Spraying is so intensive that the
hop aphid has developed resistance to many of the chemicals used. To time
their spraying for maximum effect, hop growers must know when the aphids
migrate from their winter hosts, damsons and sloes, to hops.

Samples collected from suction traps at Wye in Kent and at Hereford
over the past 15 years are combined with weather details to provide a good
database from which to predict the start and finish of the migrations. The
samples reveal that it is possible to use data on temperatures in early
spring, rainfall figures for winter and spring and the amount of sunshine
in summer to predict the migrations.

Since 1983, Wye College and the RIS have each April successfully predicted
the timing of the start of the migration and, by late June, have been able
to predict when it will end. The system has saved farmers money but, perhaps
more importantly, it has reduced pollution by allowing the optimum use of
pesticides.

The black bean aphid spends the winter on spindle trees and migrates
to crops such as spring beans in May and June. Suction traps provide raw
data for an elaborate system that constantly updates forecasts of the size
and timing of the migrations. The system was developed by the RIS with the
government’s Agricultural Development and Advisory Service (ADAS) and Imperial
College, London.

Forecasts are based on the size of the previous autumn’s migration as
measured in suction traps, egg sampling and sampling of aphids on spindle
in the spring and, finally, the early stages of the spring migration. In
1989, researchers expect to see damage to the bean crops in much of eastern
England and the West Midlands. Crop infestations by these aphids have already
occurred some two weeks earlier than usual.

Spring beans remain a relatively minor crop, but they are increasing
in popularity with farmers as a ‘break crop’ in place of the more exotic
but less reliable alternatives, such as lupins and sunflowers.

Aphids can wreak havoc on cereals twice each year. In autumn, they infest
newly sown crops and introduce barley yellow dwarf virus. In summer, the
grain aphid or the rose-grain aphid can directly damage the crop, especially
wheat, by feeding. Both aphids are present this year, and farmers that did
not spray their crops with insecticides last autumn have suffered the largest
populations on their land.

The barley yellow dwarf virus affects all cereals but is particularly
damaging to barley and oats if they are infected at the early stages of
their growth. Infected plants are stunted, so they have a reduced yield.
Barley leaves turn yellow, and oat and wheat leaves turn red.

Rothamsted has developed an infectivity index that can estimate the
potential damage to crops from the barley yellow dwarf virus. This allows
farmers to apply insecticides to crops in late October or early November.
The index integrates the numbers of cereal aphids caught in suction traps
each week with the proportion of the aphids carrying the virus. Survey researchers
identify the virus by testing individual winged aphids caught live in special
low-level suction traps.

The index works so far for East Anglia, the country’s major grain-growing
area, but it needs refinement for other areas. Mild winters, such as those
of the past two years, can increase the levels of infection in aphid populations
as larger numbers survive. The work is going on in conjunction with ADAS
and the Long Ashton Research Station outside Bristol, Rothamsted’s sister
station within the Institute of Arable Crops Research.

A reliable forecasting system is urgently needed because recent research
has shown that synthetic pyrethroids, the pesticides most frequently sprayed
by farmers to control aphids in autumn, may be harmful to predators of the
aphid, such as some species of spider.

The situation during the other danger period, in summer, is also complicated.
Migrations in May and June by small numbers of cereal aphids never result
in outbreaks of aphids. But large migrations sometimes do. Suction traps
can pinpoint some years when there will be no outbreak of aphids, so reducing
the temptation for farmers to spray as an insurance policy. At the moment,
researchers are finding more grain aphids and rose-grain aphids in suction
traps than is normal for the time of year. The findings indicate potential
problems in June and July.

Both ICI and the European Commission are helping to finance a five-year
research project to find out why migrant aphids do not always result in
outbreaks of aphids. The early signs are that natural enemies, such as hoverflies,
ladybirds, ground and rose beetles and parasitoids and spiders, are an important
factor influencing the development of aphid numbers. The mild winter has
also resulted in large numbers of natural enemies surviving in crops. These
may prevent the aphid populations from reaching the level of an outbreak.

The potato virus Y is a particular threat to farmers who save their
own seed. Infected tubers produce plants with a greatly reduced yield. The
virus is spread by many aphids and, again, the suction trap is helping to
forecast its spread.

Research at Rothamsted has shown that two species, the peach potato
aphid, Myzus persicae, and the leaf curling plum aphid, Brachycaudus helichrysi,
are numerous at the critical time for infection of potatoes by the virus
and that they carry the virus, even though the leaf curling plum aphid neither
feeds nor reproduces on potatoes. Forecasts of the spread of the virus,
based on counting the aphids, mean that farmers now know by early August
the risk of planting their own seeds as potatoes for the next year. Already
in 1989, large numbers of vector species have been caught. They herald a
year of serious spreading of the virus.

The database provided by the RIS is a valuable source for ecological
research. Roy Taylor, who conceived the idea of the RIS, has used the data
on aphids and on moths collected in light-traps to develop theories about
the distribution and spatial dynamics of populations of insects. His work
has ranged from the study of individual species such as the hop aphid to
the formulation of the ‘power law’ which relates the average number of animals
to their spatial variability.

The hop aphid proved especially interesting because its summer host
is geographically limited to hop gardens, which are found almost exclusively
in Herefordshire and Kent. Using suction traps, Taylor established how far
the hop aphid would travel during its autumn migration. Such parameters
are unknown for most insects.

Tony Dixon of the University of East Anglia has studied the biology
of the sycamore aphid. He says that the RIS database has been vital in extrapolating
data from his local studies to populations throughout the country.

An understanding of population biology is essential for the development
of strategies for the conservation of species. The mobility of species,
for instance, determines the size of refuges that are necessary for their
survival. The RIS is uniquely able to measure distribution patterns of aphids
and many other insects.

The database will also help to explain the rise and fall in abundance
of insects’ predators. One study that will begin soon will relate the regional
differences in the breeding success of birds that eat insects with the distribution
of the insects that they eat.

A second aspect of the Rothamsted Insect Survey covers moths, which
are caught by light traps that attract nocturnal insects. There are some
80 traps monitored by the survey in a wide range of habitats throughout
Britain. The database goes back to the 1960s. The set of data, collected
at Rothamsted itself, goes back to 1933. The main database contains details
of more than 6 million moths of 650 different species, built up from more
than 1800 ‘site years’. Unlike the aphid part of the survey, the value of
the work on moths has been realised. Currently, this part of the survey
is not threatened by funding cuts since it is not identified as being ‘near-market’
research.

Long-term data, such as that now provided by the RIS, will prove valuable
in determining trends in aphid population biology. Such trends may be linked
to changes in land use or climate. This role for the survey has not so far
attracted much attention, but it may turn out to be its most important as
concern about changing climate gathers ground.

Unfortunately, the long-term monitoring that is essential to such work
is often the poor relation of science because it is ‘non-experimental’ and
needs no sophisticated equipment. Thus the Natural Environment Research
Council gives a low priority to its Biological Records Centre. The problem
is that long-term monitoring is often incompatible with short-term decisions
about funding. Once a continuous sequence of data is broken it can never
be recovered.

Nick Carter is a senior scientist at the Rothamsted Insect Survey with
a special responsibility for cereal aphids.