| Central Queensland (CQ)
currently enjoys an ascochyta blight free status, but the use of chickpeas
as a winter trap crop has created a potential risk for the introduction
of the disease. To address this problem we have been evaluating alternative
winter active legumes (popani vetch, namoi vetch, faba beans and field
peas) for their suitability to be substituted for chickpeas as a spring
trap crop in central Queensland.
Of the legumes tested, field peas have stood out from the rest and have
been found to be highly attractive to egg laying heliothis moths. And
the eggs laid on field pea suffer from very high mortality compared
to chickpea — making it an excellent trap crop alternative. So
far results are suggesting that field peas may be a viable trap crop
alternative for chickpeas that would avert potential problems associated
with ascochyta blight in the CQ region.
Background
A strategic trap cropping program targeting heliothis on cotton has
been implemented in central Queensland since the beginning of winter
in 1997. Growers typically plant one to two per cent of their cropping
area to a trap crop of chickpea in winter and pigeon pea in summer.
Similar trap cropping strategies have been implemented in southern regions
where chickpeas are utilised as a spring trap crop to capture Helicoverpa
armigera populations that emerge from diapause at that time. But a potential
problem with the use of chickpeas is their susceptibility to ascochyta
blight, a seed-borne fungal disease.
At present CQ is free of ascochyta blight. But representatives from
the pulse industry have expressed concern that the use of chickpea as
a spring trap crop in CQ may inadvertently lead to the introduction
of ascochyta.
So we decided to evaluate a number of winter active legumes for their
potential to be substituted as a winter trap crop under CQ conditions
and thus alleviate any cross-industry conflict. The major focus was
to observe the number of eggs laid versus larvae numbers on each trap
crop, as it was suspected that the high larvae numbers so often observed
in chickpeas may be due to a lack of beneficial heliothis egg-eating
insects, allowing a higher proportion of the total eggs laid to survive.
Comparisons Made
Replicated experiments over the past two seasons in CQ have aimed to
compare chickpea with vetch, faba beans and field peas. On each occasion
the experiments have been conducted in the centre of fields of wheat
with replicated treatments arranged in a randomised design with each
plot being surrounded by a buffer of wheat on each side.
Heliothis were abundant during the experiments. Counts of heliothis
eggs were made on destructive samples taken from each treatment throughout
the season.
The fate of heliothis eggs laid on the chickpeas and field peas was
also investigated during the experiments. This involved tagging newly
laid eggs in each treatment and then re-visiting the eggs every day
to determine if they had hatched or disappeared (presumed eaten or fallen
off the plants).
Results
As we expected, chickpeas carried significantly more larvae than the
other legume treatments. This suggests that chickpeas attract significantly
higher numbers of heliothis than the other legume species.
But the number of eggs recorded suggest otherwise, with field peas attracting
far more egg-laying activity than chickpeas over the past two seasons.
The field peas carried on average 50–70 and 30–50 eggs per
metre of row for most of September in 2001 and 2002 respectively. The
month of September is the period for which a trap crop needs to be most
attractive for CQ conditions.
The interesting phenomena was the fate of the eggs laid on field peas.
It could be expected that the highest egg densities in field peas would
have also produced the highest larvae numbers.
But only a third of the eggs laid on field peas were observed to survive
as opposed to over two thirds survival on chickpeas (Figure 3). The
losses observed in the field peas may have been due in part to predation
by ladybirds and lacewings that were abundant in the field pea treatments.
The waxy surface of the field pea leaves may have also contributed significantly
to the observed losses by causing eggs or newly hatched larvae to fall
off plants particularly during windy conditions. In contrast chickpeas
had neither the beneficial insects or waxy leaf surfaces which may have
contributed to greater egg survival to larvae in this treatment.
As a trap crop is intended to attract and divert egg laying heliothis
moths, the field peas outperformed chickpeas in these experiments by
attracting more eggs. As an added bonus, in contrast to chickpeas, field
peas support populations of predators such as lady beetles and lacewings.
Robust under CQ conditions, the use of field peas would overcome the
ascochyta disease risk associated with chickpeas.
Conclusions
While at this stage field peas may appear to be a viable alternative
to chickpeas, we will be first substituting field peas for chickpeas
on several growers’ properties at Theodore and Emerald to gain
a measure of how successful this alternative trap crop may be in a commercial
situation. By the end of spring this year we will be in a better position
to make an informed decision as to whether or not field peas should
be substituted for chickpea in CQ.
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