Vetiver grass ( Vetiveria zizanioides (L.) Nash) as trap plant for Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and Busseola fusca (Fuller) (Lepidoptera: Noctuidae)

Abstract

The preference of lepidopterous stem borer moths to oviposit on certain wild host plants can be exploited in habitat management systems by using those hosts as trap crops. Vetiver grass (Vetiveria zizanioides (L.) Nash) was evaluated for its attractiveness and suitability to the pyralid Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and the noctuid Busseola fusca (Fuller) (Lepidoptera: Noctuidae). Two choice tests were conducted in the laboratory and in the greenhouse to determine oviposition choice of C. partellus for maize, Vetiver and rice (Oryza sativa L.), and of B. fusca for Vetiver and maize. C. partellus larval survival was evaluated in green house studies. Results indicated that C. partellus chose Vetiver grass over maize though larval survival on Vetiver was extremely low. B. fusca did not show any host preference.

Full text

Ann. soc. entomol. Fr. (n.s.), 2006, 42 (3-4) : 449-454
449
ARTICLE
Vetiver grass (Vetiveria zizanioides (L.) Nash) as trap plant for
Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and Busseola
fusca (Fuller) (Lepidoptera: Noctuidae)
Abstract. The preference of lepidopterous stem borer moths to oviposit on certain wild host plants
can be exploited in habitat management systems by using those hosts as trap crops. Vetiver grass
(Vetiveria zizanioides (L.) Nash) was evaluated for its attractiveness and suitability to the pyralid Chilo
partellus (Swinhoe) (Lepidoptera: P yralidae) and the noctuid Busseola fusca (Fuller) (Lepidoptera:
Noctuidae). Two choice tests were conducted in the laboratory and in the greenhouse to determine
oviposition choice of C. partellus for maize, Vetiver and rice (Oryza sativa L.), and of B. fusca for Vetiver
and maize. C. partellus larval survival was evaluated in green house studies. Results indicated that C.
partellus chose Vetiver grass over maize though larval survival on Vetiver was extremely low. B. fusca
did not show any host preference.
Résumé. L e Vetiver (Ve tiveria zizan ioides (L . ) N as h) , p l an te pi èg e p ou r Chilo pa rtellus (Swinhoe)
(Lepidoptera: Pyralidae) et Busseola fusca (Fuller) (Lepidoptera : Noctuidae). La préférence
des lépidoptères foreurs de graminées à pondre sur certaines gaminées sauvages plutôt que sur
celles cultivées peut être exploitée dans un système de pratique culturale en utilisant ces graminées
sauvages comme plantes-pièges autour des cultures. L’attractivité du Vétiver (Vetiveria zizanioides
(L.) Nash) sur la ponte de Chilo partellus (Swinhoe) de Busseola fusca (Fuller) a été testée, de même
que sa faculté à permettre le développement de ces deux espèces. Des tests de choix binaire ont été
menés en laboratoire et en serre pour déterminer la préférence de ponte de C. partellus pour le maïs, le
Vétiver et le riz (Oryza sativa L.), et celle de B . fusca pour le Vétiver et le maïs. Les taux de survie des
larves de C. partellus ont été évalués en serre. Les résultats ont montré que les femelles de C. partellus
choisissent plutôt le Vétiver au maïs alors que le taux de survie larvaire y est extrêmement faible. B.
fusca n’a montré aucune préférence de ponte vis-à-vis du Vétiver.
Keywords: habitat management, host plant selection, preference-performance hypothesis, rice, trap crops.
J   B
School of Environmental Sciences and Development, North-West University, Private Bag X 6001, Potchefstroom, 2520, South Africa
E-mail: drkjvdb@puk.ac.za
Accepté le 29 juin 2006
The principle of trap cropping rests on the fact that
virtually all insects show a distinct preference for
certain plant species, cultivars or a certain crop stage
(Hokkanen 1991). Preferences of lepidopterous stem
borers for graminaceous plants have been reported for
Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) (van
den Berg et al. 2001; van den Berg 2006; Khan et al.
in press) and for Busseola fusca (Fuller) (Lepidoptera:
Noctuidae) (van den Berg et al. in press). Napier grass
(Pennisetum purpureum L.) and sorghum (Sorghum
spp.) were recommended as trap plants around maize
fi elds in habitat management systems for stem borers
in East and Southern Africa. While the attractiveness
of Napier grass for especially C. partellus have been
shown, its eff ect as trap crop could possibly also be
partly ascribed to its role as a barrier to moths infesting
fi elds outside (van den Berg et al. in press). Reduction
in stem borer infestation in maize fi elds surrounded
by wild grasses have also been reported by Khan et
al. (1997), Ndemah et al. (2002) and Midega et al.
(2005). Napier grass and forage sorghum are however
not always suitable to farming conditions in low
rainfall areas, or in farming systems where forages are
not important and where free grazing is practised.
Alternative trap crops need to be identifi ed that could
be used in systems where the use of Napier grasses and
forage sorghums is not feasible.
Vetiver grass (Chrysopogon zizanioides (L.) Roberty
=Vetiveria zizanioides (L.) Nash) is a species that is
used globally as soil erosion management tool and in
sustaining agricultural productivity (Grimshaw 2003).
Vetiver grass technology, in its most common form, is
the establishment of a narrow (less than 1 m wide) live,
stiff grass barrier, in the form of a hedge across the slope
of the land (Grimshaw 2003).  e easy availability of
Vetiver grass and its presence on contours between
crop fi elds in many African countries prompted
investigation into the possibility of using this grass as a
trap plant for stem borers in maize production systems.
Stem borers have been reported to damage Vetiver grass
in China where this grass is indigenous (Xinbao 1992).

450
J.   B
Paddy stem borers (Chilo spp.) have been reported to
infest culms and midribs of leaves wherever Vetiver was
planted in Southern China. An interesting observation
was that the levels of mortality amongst stem borer
larvae was high and in the worst case, approximately
39 % of Vetiver stems were damaged by the borer
but no pupae were found (Xinbao 1992), indicating
that larval survival in Vetiver is low. If the oviposition
choice of stem borer moths for Vetiver grass is high,
which seems to be suggested by these observations, the
possibility exists that it could be used as trap plants
around crops on which stem borers are a problem.
 e aim of this study was to determine the oviposition
choice of B. fusca and C. partellus moths for Vetiver
grass and maize and to determine the survival of larvae
on this plant species.
Materials and methods
Laboratory trials: Two-choice bioassays
Oviposition choice tests were done in muslin cloth cages (50
x 55 x 75 cm). Each test was replicated three times. Cages
(replicates) were placed in the centre of a dark room with two
pots placed inside each cage. One pot contained a Vetiver plant
with 4–6 tillers and the other one 4-week old maize plant. Maize
of this age was used because B. fusca was previously observed to
prefer four to fi ve week old plants for oviposition (van Rensburg
et al. 1989) while C. partellus infest plants of any age (Seshu
Reddy et al. 1990; Singh & Sandhu 1978). Five pairs of adult
moths were released into the centre of each cage and allowed to
oviposit overnight before they were removed. Although this is
a high number of moths, not all of them were expected to be
of egg laying age since their mating status was not determined
before collection. Moths were collected in cages on the evening
of emergence from sorghum stalks collected from the fi eld.
Moths were therefore naive and had no oviposition experience.
Since C. partellus is known to attack rice (Seshu Reddy 1990),
a test was also conducted to determine the oviposition choice
of moths when presented with rice and Vetiver plants. One
rice variety (accession number 966) was used in this study.  e
procedure was identical to that described in the two-choice
test with maize.  e number of egg batches per plant, eggs per
batch and mean numbers of eggs per plant on the diff erent
plant species were recorded and subjected to Student t-tests to
determine if there were signifi cant diff erences.
Greenhouse trials: Larval survival on potted plants
In this experiment larval survival of C. partellus was determined
on Napier grass, Vetiver grass and maize plants growing in pots.
 e objective was to compare larval survival on maize with that
on Vetiver and Napier grass, which is currently used in habitat
management for control of maize stem borers (Midega et al.
2005; van den Berg et al. 2001). Pots were kept in a greenhouse.
Each plant species was replicated ten times (10 pots) with one
plant per pot in a completely randomised design. Each plant
was infested with one egg batch in the black head stage when
maize plants were four weeks old. Each egg batch contained 30
eggs. Eggs were produced by wild moths that were collected
in cages in which sorghum stalks, collected from the fi eld,
were kept. Plants were kept in a greenhouse at 25 (±2) °C and
50–60 % relative humidity for 28 days after which they were
dissected to recover surviving larvae.  e number of surviving
larvae was calculated from the number of eggs put on plants.
 e assumption was made that egg hatch was 100% and the
numbers of eggs was then taken to represent the number of
fi rst-instar larvae that there was on each plant after egg hatch.
Larval survival was expressed as a percentage of larvae recovered
after 28 days in relation to the number of fi rst- instar larvae
(number of eggs).
Data on mean percentage survival, calculated in terms of
number of eggs put on each plant were subjected to ANOVA
and means separated by means of the Tukey test.
Greenhouse trials: Two-choice bioassay
A two-choice experiment that involved more plants and larger
cages was subsequently conducted in a commercial green house
where plants were growing in the soil.  e aim was to determine
C. partellus oviposition choice and subsequent larval survival on
plants. Two rows of maize were planted on one side of a row of
established Vetiver grasswhile one row was planted on the other
side.  e inter-row spacing was 0.75 m with 30 cm between
plants and one plant per hill.  e Vetiver grass row was one
year old and had a dense stand of tillers. Six muslin cloth cages
(3.0 x 1.5 m x 1.5 m) were placed transversely over the three
rows of maize and one row of Vetiver grass when the maize
plants were fi ve weeks old.  e maize and Vetiver plants were
approximately 0.7 m and 1.0 m high respectively. Each cage
formed a replicate and enclosed within it an average of 14 maize
plants and a dense 1.2 m long row of Vetiver grass.  e surface
area inside the cage planted to maize and Vetiver was 2.25 and
0.60 m2 respectively. Twenty male and twenty female moths
were released into each cage and allowed to oviposit on plants
for two nights after which cages were removed.  e number of
egg batches and eggs per batch on maize was determined by
carefully inspecting the foliage of each maize plant. One half of
the Vetiver row in each cage was removed from the soil in order
to inspect each leaf for egg batches.  is was done to facilitate
fi nding of eggs which is diffi cult in the dense foliage of the
grass.  ere were approximately 54 tillers (splits) of Vetiver in
the half-row in each replicate. Leaves were removed from tillers
and checked for eggs.  e assumption was made that there
would be a similar number of eggs on the section of the row
that was not removed from the cage. All the maize plants were
left in each replicate. Data on mean number of egg batches and
mean numbers of eggs on maize and Vetiver was standardized
to numbers/m2 to facilitate comparison between the two plant
species. Data on mean numbers of egg batches, eggs per batch,
total number of eggs and larval survival were subjected to
Student t-tests to determine if diff erences were signifi cant.
Results
Moth oviposition preference and larval survival
Laboratory two-choice bioassays
Results from the two-choice experiment with B.
fusca indicated that there was no signifi cant diff erence
(t = 0.152; P = 0.886) between the number of egg

Vetiver grass as trap plant for Chilo partellus
451
batches per plant with 3.3 (S.E. ± 0.66) and 3.0 (S.E.
± 2.08) batches per plant recorded from maize and
Vetiver respectively. However, the number of eggs per
batch recovered from Vetiver were signifi cantly lower
than those on maize (t = 4.176; P = 0.013). Of the total
number of B. fusca eggs recorded in this experiment
92% was from maize. Results from the fi rst two-choice
experiment with C. partellus indicated that there were
signifi cantly more egg batches per plant on Vetiver (t =
–3.528; P = 0.024) (fi g.1) while egg batch size did not
diff er between maize and Vetiver plants (t = –0.231; P
= 0.828).  e average number of eggs per batch was 33
(S.E. ± 2.2) on maize and 34 (S.E. ± 3.2) on Vetiver.  e
numbers of eggs per plant was therefore signifi cantly
higher on Vetiver (t = 4.294; P = 0.012) with only 18
% of the eggs recorded on maize (fi g. 2).  e number of
C. partellus egg batches on rice was signifi cantly higher
(t = 3.283; P = 0.030) than on Vetiver. Rice plants
received an average of 16 (S.E. ± 3.7) egg batches per
plant while maize plants received only 4 (S.E. ± 0.6).
 e total number of eggs per plant was also signifi cantly
lower on Vetiver (t = 3.752; P = 0.019) with 690 (S.E.
± 101.8) and 264 (S.E. ± 50.1) eggs per plant on rice
and Vetiver, respectively. However, egg batches were
signifi cantly larger on Vetiver (t = 2.893; P = 0.044)
with 65 (S.E. ± 3.5) and 44 (S.E. ± 6.1) eggs per batch
on Vetiver and rice, respectively.  e percentage larval
survival of C. partellus on potted plants 28 days after
infestation (DAI) diff ered signifi cantly (F = 572.1; P =
0.00001) among plant species (fi g. 3). On average 63.0
% and 2.8 % of larvae survived on maize and Napier
grass, respectively. No larvae survived on Vetiver grass.
Greenhouse bioassay: Oviposition data
Signifi cantly higher numbers of egg batches (t =
3.932; P = 0.002) were again recorded on Vetiver grass.
Only 4.3 % of the total number of egg batches was
laid on maize. In this experiment an average of 1.92
(S.E. ± 0.58) egg batches/m2 was laid on maize while
155.7 (S.E. ± 37.8) egg batches/m2 was laid on Vetiver.
 e size of egg batches recovered on Vetiver was also
signifi cantly larger than those on maize (t = 3.449; P =
0.006), with 34 (S.E. ± 3.8) and 16 (S.E. ± 3.6) eggs
per batch for Vetiver and maize respectively. Of the
total number of eggs recorded in this experiment 96.3
% were laid on Vetiver which was signifi cantly higher
than that on maize plants (t = 2.600; P = 0.026) with
2716.8/m2 (S.E. ± 903.6) and 101.3/m2 (S.E. ±10.6)
being recorded on Vetiver and maize, respectively.
Greenhouse bioassay: Larval survival
 e percentage of larval recovery from maize in
the greenhouse was signifi cantly higher (t = 6.098;
P = 0.0001) than on Vetiver.  e number of larvae
recovered 28 days after oviposition was 132 % (S.E. ±
21.6) on maize versus 0.56 % (S.E. ± 0.08) on Vetiver.
 e recovery rate on maize was therefore 32 % higher
than the number of eggs recorded on maize plants.
Figure 3
Mean numbers of larvae recovered from maize and vetiver plants in a
greenhouse, 28 days after oviposition by Chilo partellus. (Bars indicate
Standard Error).
Figure 1
Mean number of egg batches per pot laid by Chilo partellus moths on maize
and vetiver in two-choice tests. (Bars indicate Standard Error).
Figure 2
Total number of eggs per pot laid by Chilo partellus moths on maize and
vetiver in two-choice tests. (Bars indicate Standard Error).

452
J.   B
Discussion
Busseola fusca moths chose to lay their eggs on maize
rather than on Vetiver.  e poor status of Vetiver as
ovipositional host is ascribed to the physical properties
of the plant and B. fusca oviposition behaviour. B. fusca
moths position egg batches between leaf sheaths and the
stem of host plants. Since Vetiver “stems” are composed
of a large number of tight-fi tting leaves with no real
stem, no suitable oviposition sites exist. Vetiver grass only
produce fl ower-bearing stems in more tropical areas.
Due to the low numbers of eggs recovered on Vetiver no
further studies were conducted with B. fusca. Both the
laboratory and green house experiments showed that C.
partellus moths chose to lay eggs on Vetiver grass and not
on maize.  e numbers of larvae recovered from Vetiver
plants in all the experiments were low. In the greenhouse
experiment more larvae were recovered from maize than
the number of eggs actually laid on plants. On maize
132 % of larvae was recovered and on Vetiver grass 0.56
%.  e high larval numbers on maize can be ascribed
to the unsuitability of Vetiver for fi rst instar larvae and
subsequent emigration of larvae from these plants to
maize.  is is probably due to high levels of larval spin-
off and mortality on this plant. After egg hatching many
fi rst-instar larvae were observed hanging on silk threads
from leaves.  e total number of larvae that could have
been recovered collectively from maize and Vetiver in
the experiment, if 100 % of larvae survived emigration
off Vetiver to maize, would have been 5778, but a total
of only 986 larvae were recovered from maize.  is
indicated that in spite of having a suitable host plant
(maize) next to the Vetiver plants, larval mortality was
still very high with 83 % of larvae not accounted for
at the end of the experiment.  e high preference and
low larval survival of C. partellus on Vetiver grass cannot
easily be explained.  ese observations do not support
the “preference-performance hypothesis” which states
that oviposition preference should correlate with host
suitability for off spring development, because females
get maximum fi tness by ovipositing on the optimal host
(Jaenike 1978). It has also been hypothesized that C.
partellus selects oviposition sites most suitable for egg
survival since larvae are mobile and could fi nd a suitable
host plant if eggs were laid in close proximity to host
plants (van den Berg & van der Westhuizen 1997).
Renswick & Chew (1994), in a review of oviposition
behaviour in Lepidoptera however observed that larvae
are relatively immobile and that the judicious choice
of a food plant and the oviposition step is of particular
importance in the Lepidoptera. Host specifi city may
however also play a role in these observed insect/
plant interactions.  e nature of oviposition choice
in Lepidoptera has been shown to be highly adaptive
with females choosing hosts on which larval survival is
superior to that on hosts rejected by females ( ompson
1988). Observations on C. partellus host selection
and off spring performance on sorghum lines with
diff erent levels of antibiosis resistance do not support
this (van den Berg & van der Westhuizen 1997).  e
latter authors reported C. partellus moth preference for
sorghum varieties with high levels of larval antibiosis. If
the host plants used in this study for C. partellus were
put in a preference hierarchy for oviposition it would be
rice << Vetiver << maize. Larval survival on rice was not
determined but survival rates on Vetiver approached
what is referred to by  ompson (1988) as one of the
extreme relationships between oviposition preference
and off spring performance where females oviposit on a
host that is fatal to the immatures.  e causes of good
or poor performance on a plant species in natural or
managed communities does not always result directly
from interactions between an insect and plant but
could be due to interactions with, amongst others,
abiotic diff erences in microhabitats in which the host
plant species grow ( ompson 1988).  e relationship
between preference and performance can vary under
diff erent ecological conditions and is infl uenced by
geographic variation in host use ( ompson 1988).  e
most feasible explanation for C. partellus behaviour on
Vetiver in this study is the eff ect of geography on the
growth pattern of the host plant which seldom fl owers
outside tropical areas. Vetiver produces fl owering stems
in its area of origin in which Chilo sp. is able to survive
in very low levels once they enter these stems.  is
aspect was however not addressed in this study. Low
levels of survival of Chilo spp. have been reported to
infest culms and midribs of leaves wherever Vetiver
was planted in Southern China (Xinbao 1992). Field
observations made by Shangwen (1999) and Zisong
(1991) in China showed high numbers of infested
Vetiver plants but low larval recovery.  e occurrence
of fully grown larvae of another Chilo sp., C. polychysus
(Meyrick), although in low numbers was reported on
Vetiver in Vietnam (Truong 2005a & b). Chilo partellus
is an alien invasive species in Africa. Chilo partellus is
from the Old World tropics, from where it dispersed
to East and Southern Africa during the fi rst half of the
twentieth century (Maes 1998). Similarly, Vetiver grass
originated in South Asia. India was most probably the
primary centre from where it may have dispersed to
other areas (Lavania 2000).  e strong preference of
C. partellus for Vetiver grass could therefore possibly be
ascribed to an old association between this insect and
Vetiver during the period before its current primary host
plants (maize, sorghum and rice) were domesticated.

Vetiver grass as trap plant for Chilo partellus
453
Although this study did not show that C. partellus
prefers Vetiver to rice the possibility exists that it and
also other Chilo spp. may choose Vetiver grass to other
rice varieties. Many Chilo spp., including the notorious
C. partellus attack rice in many parts of the world
(Seshu Reddy 1990). If Chilo spp. prefer Vetiver grass
it could have a potential as a trap crop around paddy
rice fi elds where it is used as fi eld boundaries and as
a soil conservation measure to protect rice from fl ood
damage during the rainy season (Huq 2000).
Vetiver grass has the most important characteristics
of a trap crop for C. partellus, i.e. it is highly attractive to
the target pest (Hokkanen 1991). Once oviposition has
taken place, the suitability of the host plant for larval
feeding and development is one of the most important
aspects of a plant’s potential as a trap crop (Hokkanen
1991). Poor larval survival and/or development are also
essential for a successful trap crop (Shelton & Nault
2004). If the trap crop has the additional quality that it
allows no or very low larval survival, it could be termed a
dead-end trap crop (Shelton & Nault 2004).  e overall
acceptability of the trap plant for feeding is therefore
also important (Potting et al. 2005) since it prevents the
possibility of larval emigration to neighbouring plants,
which may be the target food crop, as was observed in
the greenhouse study. Infestation levels in this study
were however abnormally high and natural enemies
that could have a huge impact on survival of migrating
larvae were absent from the study.
Conclusions
 is study showed that the preference of
Lepidopterous stem borer moths to oviposit on
certain wild host plants can be exploited in habitat
management systems by using these preferred hosts
as trap crops in integrated management strategies for
stem borers. It also highlighted the phenomenon of
lack of correlation between moth preference for host
plants and performance of off spring that is sometimes
observed in the Lepidoptera.  e eff ect of Vetiver grass
as trap crop should be evaluated further and in fi eld
experiments under diff erent agro-ecological zones.
Furthermore, future studies should be conducted to
determined the potential of Vetiver as trap crop for
other Chilo spp., especially those that attack rice in
areas where Vetiver is indigenous.
Acknowledgements.  e Gatsby Charitable Foundation
provided funding for this study.  e technical assistance of
Charles Midega is highly appreciated. Hanalene du Plessis and
Ursula du Plessis participated in conducting experiments and
data analyses.  e National Plant Genetic Resources Centre
of the National Department of Agriculture, Pretoria, provided
seed of rice.
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