| Determining the diet of
insects present in cotton should be among the first steps in any applied
research program. Omnivorous species are particularly important as they
have the potential to suppress pest populations through prey feeding,
but also to cause economic damage to the crop through plant feeding.
The apple dimpling bug (Camplyomma liebknechti) is a good example of
a species with a mixed diet. The Pacific damsel bug (Nabis kinbergii)
is normally considered a strict carnivore and important predator of
cotton pests such as aphids and heliothis.
But there is recent circumstantial evidence that Pacific damsel bugs
may also feed on squares and bolls. We caged Pacific damsel bugs on
cotton branches in the glasshouse and watched individual bugs in the
field and glasshouse to determine if plant feeding occurs.
What was done?
Feeding cage studies
We caged Pacific damsel bugs on cotton branches at squaring and boll
filling growth stages for three days to determine the importance of
plant and prey feeding.
The cages contained bugs or were bug free, with and without access to
plant parts, and with and without access to prey.
We predicted that if bugs fed on cotton then treatments with access
to plants would have improved survival and lowered prey consumption.
Plant feeding may reduce the total number of fruit and increase the
number of damaged fruit depending on whether damaged fruit are aborted
or retained.
The presence of prey may reduce plant feeding, so plant feeding may
increase in treatments with prey absent.
Observations on behaviour
We observed the feeding behaviour of Pacific damsel bugs in the field
and glasshouse. In the field study we observed 330 nymph and adult Pacific
damsel bugs in commercial cotton fields.
Observations on each bug lasted 10–15 minutes and were conducted
during the day. We tallied the number of nymph and adult Pacific damsel
bugs observed at least once in one of the following feeding activities:
• Plant tissue;
• Plant extrafloral nectar;
• Insect prey; or,
• Did not feed.
We combined the data for the nymph and adult life stages, as feeding
activity was consistent between the stages. We predicted that Pacific
damsel bugs would mainly feed on prey, followed by extrafloral nectar,
and then plant tissue, if at all.
In the glasshouse we observed adult Pacific damsel bugs on potted cotton
plants in 12 large cages. Plants were infested with heliothis eggs and
larvae as prey.
Observations were conducted at hourly intervals night and day. We expected
that bugs would feed more often during day than night sessions.
What was found?
Feeding cage studies
The survival of Pacific damsel bugs at all assessment dates in the square
and boll feeding cage studies was significantly improved if bugs had
access to cotton. The provision of prey improved bug survival at two
and three days after release (DAR) in the square feeding study, but
made no difference in the boll feeding study. Provision of prey appeared
more important for survival of bugs without access to plants than for
bugs with access to plants (Figure 1, boll feeding study only).
The presence of Pacific damsel bugs significantly increased mortality
of heliothis eggs in the square and boll feeding studies. Male bugs
consumed slightly more eggs per day than female bugs in the square feeding
study, but this difference was not significant.
Prey (egg) mortality was not significantly affected by access to plants
in the square and boll feeding studies.
The total number of fruit was not significantly influenced by the presence
of bugs or prey in the square and boll feeding studies (Figure 2, boll
feeding study only).
Observations on behaviour
Our observations of Pacific damsel bugs in the field and glasshouse
showed that bugs readily fed on prey (eggs) and cotton extrafloral nectar,
but not on intact plant tissue.
No bugs fed on plant tissue, 7.3 percent fed on extrafloral nectar,
3.0 percent fed on prey and the remaining 89.7 percent did not feed
when observed. Individual bugs that fed did so on either extrafloral
nectar or prey — never both — within the 15 minute observation
period.
Bug feeding activity was significantly greater during day than night
sessions in the glasshouse study. No bugs fed on plant tissue during
day or night sessions (Table 1).
What does this mean?
The results from the square and boll feeding cage studies do not clearly
indicate whether bugs feed on intact plant tissue (see Figure 1). Improved
survival of Pacific damsel bugs with access to plants suggests they
gain essential moisture or nutrients from feeding on intact plant tissue
and/or plant extrafloral nectar. This is not certain because bugs with
access to plants may benefit from favourable microclimatic conditions.
Egg mortality was not affected by access to plants in the square and
boll feeding studies.
The total number of fruit and number of damaged fruit were not affected
by the presence of Pacific damsel bugs, which indicates that bugs did
not feed on intact plant tissue, although it is conceivable that feeding
occurred but did not cause damage symptoms (Figure 2).
Observations of feeding behaviour in the field and glasshouse provide
the necessary detail to clarify the omnivory status of Pacific damsel
bugs. The bugs mostly did not feed at all, although they readily fed
on extrafloral nectar and prey, but never on intact plant tissue (Table
1).
So increased survival of Pacific damsel bugs with access to plants in
the feeding cage studies is attributed to either extrafloral feeding
and/or favourable microclimatic conditions, but not to plant tissue
feeding. Our study leads us to dismiss the suggestion that Pacific damsel
bugs are plant tissue feeders — but they do feed on extrafloral
nectar.
Cotton extrafloral nectar is secreted from nectaries located on the
largest midribs under the leaves and at the bases of bracts. This nectar
is a recognised food source for many omnivorous arthropods, including
predaceous bugs such as big-eyed bugs (Geocoris spp.) and pirate bugs
(Orius spp.).
Extrafloral nectar contains various amino acids, sugars and water that
are necessary for insect growth and development. It does not constitute
a complete source of nutrition as some essential amino acids are missing.
This may explain why access to extrafloral nectar increases survival
but does not support development and reproduction in big-eyed bugs (Georcoris
pallens), lady beetles (Coleomegilla maculata) and lacewings (Chrysoperla
plorabunda).
Our feeding cage studies suggest that extrafloral nectar may sustain
Pacific damsel bugs during brief periods of prey absence. When prey
is more abundant, the importance of extrafloral nectar may be reduced.
Apart from the first day of the boll feeding study, prey (egg) mortality
was not influenced by access to plant food. So any feeding on extrafloral
nectar does not interfere with prey consumption. This is important for
biological control strategies.
Further studies on the effects of extrafloral nectar consumption on
bug survival, development and reproduction, and the relative consumption
of prey and plant resources as prey density changes are warranted to
extend our understanding of plant feeding by Pacific damsel bugs.
This knowledge will be useful in the development of artificial food
supplements and understanding the potential effects from plant feeding
on transgenic pest resistant cotton cultivars.
Key points
Our studies have shown that Pacific damsel bugs feed on cotton extrafloral
nectar, but not on intact plant tissue. This means that farmers and
crop consultants should consider these bugs as a beneficial but not
a pest species of cotton when making pest management decisions.
The results indicate that extrafloral nectar is important in the diet
of Pacific damsel bugs. Consumption of extrafloral nectar may sustain
bugs during brief periods of low prey availability, and should cause
minimal disruption to prey feeding when prey are in high abundance.
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