Planting cotton – have you baited for soil pests first?

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Planting cotton – have you baited
for soil pests first?
OO By Paul Grundy and Adam Quade, DAFF
D
URING the past few seasons there have been increased
reports of establishment issues without an immediately
obvious cause. Closer investigations have shown that
while the responsible factors vary from field to field, an often
overlooked aspect has been the presence of soil pests such as
wireworms and earwigs. The millipede-like pest symphyla has
also been implicated as a causal factor in some fields although
the actual pest status of this organism is unclear.
Conversations with people managing affected fields indicate
FIGURE 1: Comparison between potato and
soaked barley baits, (a) January 2014 and (b)
April 2014. Potato performed well in these
tests and may be more convenient to use than
soaked grain.
uncertainty surrounding the apparent variability between fields
in terms of crop damage and seeming pest presence. This article
will provide a brief overview of type of pests that can cause crop
establishment difficulties and describe how to sample for them. It
is critical that soil pests are detected early as options for remedial
action post planting are extremely limited.
Damage symptoms
Soil pests can reduce plant establishment, row density, and
plant vigour. Damage can occur both pre and post-emergence.
During germination, pests may feed on the seed, tap root, or
emerging cotyledons. Post-emergence, chewing on the hypocotyl
may result in ‘felling’ of new plants or the grazing of root tips can
cause constricted root expansion and symptoms akin to moisture
stress. Affected fields will exhibit patchy emergence and/or
stunted or dying seedlings. Plant mortality may continue until the
plants reach early squaring.
Damage by soil pests is easily confused with symptoms caused
by diseases such as black root rot, allelopathy from incorporated
sorghum stubble, hypocotyl abrasion at the soil surface on crusting
soils in windy conditions, or impeded root development due to
subsurface compaction. Therefore it is essential in situations of poor
establishment to closely examine the affected plants.
A key determinant is chewing or grazing damage on the
hypocotyl or root sections. Obviously another key indicator is
confirming the presence of soil pests with effective sampling.
Sampling procedures
Sampling for soil insects is best conducted using a baiting
technique. Soil digging is potentially useful for confirming the
presence of symphylans, but will not provide an effective measure
of the abundance of insects such as earwigs and wireworms.
Conduct baiting as soon as practicable following planting rain
or pre-irrigation:
OO Soak insecticide-free grain seed (barley, wheat, sorghum) in
water for at least two hours to initiate germination.
OO Bury a dessert spoon-full of the seed under one cm of soil at
each corner of a 5x5 metre square at five widely spaced sites
per 100 hectares.
OO Mark the position carefully as high populations of soil insects
can completely destroy the baits.
OO After five to seven days or upon seedling emergence, dig up
the baits plants and count the insects present.
Tests have shown that the type of seed used makes no
noticeable difference in attracting soil-dwelling insects. Recent
research has shown that small to medium sized potatoes cut
in half and buried in the same manner with the cut side facing
down will produce comparable results to grain baits.
Some people may find the use of potatoes simpler as they
do not require preparation compared to soaked grain and upon
recovery of the baits, damage to the potato is also immediately
obvious. Figures 1a and 1b show comparative results from a
series of replicated fields comparing potato and grain baits for
sampling wireworm and earwigs.
22 — The Australian Cottongrower
August–September 2014
the way forward
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Common soil pests
Cnr Buckland & Yaldwyn Sts
Qld 4350
True and falseToowoomba
wireworms
Phone:
07 4635Agrypnus
9872
Larvae of the true
wireworm
sp. and false wireworm
Fax: 07spp.
4635
6633
species Gonocephalum
and
Pterohelaeus spp. attack
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germinating
Brian Moranseeds as well as the hypocotyl and roots of young
cotton plants. Damage
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Brian on
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The Australian Cottongrower — 23
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The only way to effectively determine wireworm field
populations is through the use of grain or potato baits. There
are currently no specific thresholds for wireworms in cotton, but
densities of one or more larvae per baiting site are considered
damaging for summer grain crops.
Control is typically achieved with the use of in-furrow
insecticide treatment at planting and therefore this pest must be
detected before planting for effective control.
Black field earwigs
An occasional pest of seedling cotton, earwigs feed
predominantly on germinating seed and seedling roots resulting
in poor establishment. Again, only effective bait sampling will
detect this pest.
While there are no specified black field earwig thresholds for
cotton, thresholds for maize and sorghum suggest that control
maybe warranted when more than 50 earwigs are found across
20 baits or two to three earwigs per bait sample.
Stunting of growth (right) associated with symphyla
feeding on seedling root systems.
Black field earwig nymph (7 mm long).
The level of protection afforded by neonicotinoid cotton seed
dressings for this pest particularly at higher densities is unknown.
Given that earwigs need to feed on seedlings to acquire a lethal
dose, it is likely under high pest densities that damage will occur.
The application of insecticide treated grain baits at or prior to
sowing may offer additional protection, as in-furrow insecticide
treatments have been found to be generally ineffective for
earwigs in grains where dense populations are present.
Symphyla
Symphyla are white, soft-bodied ‘millipede-like’ arthropods,
three to seven mm long with 12 pairs of legs. Sensitive to light,
they are very active when exposed. Symphyla are relatively
common in most soils and generally feed on decomposing
organic matter. Establishment problems in cotton due to
symphyla have been previously isolated to some fields within the
Theodore irrigation district, although they are recently suspected
of contributing to damage in fields west of Moree and Dalby.
Research into the pest status of this organism is underway.
Damage is likely to manifest as random patches of seedlings
exhibiting apparent moisture stress and reduced vigour. Over time
these symptom become more pronounced in affected areas, and
in severe cases afflicted plants can senesce even though subsoil
moisture is adequate.
Symphyla can feed on rootlets and root hairs with the result
of continuous surface grazing causing either a ‘witches broom’
root system or a lack of lateral root expansion. Feeding generally
appears to occur where the soil is moist and as the profile
dries out, the continuous pruning of the roots can leave plants
stranded in the top 10–15 cm of soil upon an otherwise full
profile. Symphyla are very active and will move up and down in
the soil profile to reported depths of below 0.8 metres.
24 — The Australian Cottongrower
Symphyla damage to root systems. Feeding on the root tips
(left) has resulted in a significant reduction in root area
development.
Symphyla sampling
The detection of symphyla prior to planting is difficult as
distribution within a field is generally patchy. Although recent
results with a specific sampling technique using potato and a
‘net’ pot has shown excellent success for attracting symphyla,
the relationship between bait captured symphyla densities and
potential for crop damage is unknown, and will be the focus of
research during the 2014–15 season.
Where plants are showing symptoms of moisture stress or
poor vigour, a basic soil survey will adequately confirm the
presence of symphyla. Insert a shovel to full depth at the plant
line on the hill and carefully lever the soil out so that it can be
inspected more closely. Starting with the wettest soil, carefully
break soil clods apart, keeping a close eye on the inner surfaces
for movement. Symphyla are fast moving and will rapidly shift to
avoid sunlight.
It is important not to confuse symphyla with other soil
organisms such as diplurans or collembolans (spring­tails).
Diplurans closely resemble symphyla but are smaller, more rapid,
and only have legs on the upper body. Symphyla have legs along
the entire body and lack the long appendages at the end of the
August–September 2014
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Theodore by responding to tell-tale symptoms of moisture
stress with timely irrigation and the avoidance of cool planting
conditions.
Cultural aspects
A symphylan (left) is very similar in appearance to a
dipluran (right), but has legs all along its body like a
millipede and lacks the dipluran’s long rear appendages.
dipluran’s abdomen. Collembolans have a curved body and the
capacity to jump when disturbed.
There are currently no registered control options for symphyla.
Where damage is suspected, the use of irrigation may prove
beneficial in assisting root pruned plants to establish. If damage
is serious enough to contemplate replanting, wait for warmer
conditions that will aid more rapid plant establishment or
consider planting a fibrous root crop that is less likely to be
affected. The impacts from symphyla have been managed at
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A regular question asked by people is whether or not changes
to farming practices such as the greater retention of stubble
is influencing overall soil pest abundance. In a general sense
changes to tillage, soil applied pesticide usage, farm hygiene and
stubble retention will affect the composition and abundance of
soil pest species. For example, weedy fallows and generally poor
levels of farm hygiene can encourage soil pests whereas clean
fallows generally cause pest insect numbers to decline due to a
lack of food.
The influence of field stubble in particular is contentious.
Several studies have shown that while high stubble loads within
fields will promote the abundance of soil pests, stubble can also
provide a diversionary food source as well as increase the diversity
of soil fauna such as predatory beetles (carabidae), centipedes,
and earthworms. The incorporation of grain stubble prior to
planting cotton may increase black field earwig populations
whereas false wireworms are commonly found under a range
of cultivation and stubble retention regimes. To simply associate
stubble retention with increased soil pest activity would seem
to be a too simplistic proposition at this stage and that perhaps
the most prudent approach would be to pay closer attention to
sampling soil insects where stubble is present prior to planting.
Our take home message is that as you consider planting
in the months ahead, take the time to lay some soaked
grain or potato baits as it is nearly always too late to solve
soil insect pest problems once field symptoms occur.
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