Download Plantain moth investigation

Plantain moth investigation
FITT FINAL REPORT 13FT13
Year of trial: 2013/14
Group that proposed the trial: Plantain Learner Group
Region: Manawatu
Trial initiator: Alistair Hogg
KEY ‘TAKE HOME’ MESSAGES

A moth/caterpillar complex commonly referred to as “Plantain moth” has emerged as a
significant pest of plantain primarily in the North Island, although damage has also been
reported from Canterbury, and is causing concern amongst farmers. “Plantain moth”
caterpillars are voracious feeders that can reach very high numbers in stands, cause large
production losses and, at times, completely devour crops.

“Plantain moth” is not a single moth species. At least two species, Scopula rubraria
(Doubleday) and Epyaxa rosearia Doubleday appear to be very common in infested stands
and the caterpillars of both feed extensively on plantain foliage. Both species are in the family
Geometridae and the caterpillars are generally known as loopers. Superficially, the moths and
caterpillars appear very similar and the different species have only now been identified.

Very little is known and published about either species.

Given the variation in wing patterning seen within E. rosearia moths, it is possible that a third
looper species, E. lucidata Walker, could also be present.

The relative importance of the two dominant species, or the significance of possible
interaction, is not yet known.

In addition leafroller caterpillars were also observed causing damage to plantain. These were
identified as Merophyas species, possibly M. leucaniana, a native caterpillar, and/or
M. divulsana, an Australian species.

Laboratory rearing of S. rubraria indicated a very short generation time of 7 weeks at 17°C. A
shorter generation time is probable at higher temperatures. Multiple generations per season
being produced by this species are likely with the number of generations attained being
influenced by temperature. Multiple generations per season give rise to very high numbers of
caterpillars.

Anecdotal evidence suggests E. rosearia development may also undergo multiple generations
per season.

Given that multiple generations occur, it is theoretically possible that insecticide intervention in
early summer of second year crops may prevent the build-up of damaging numbers of
caterpillars.
INTRODUCTION – BACKGROUND TO THE PROJECT
“Tonic” plantain has received considerable favourable publicity in recent years as a
consequence of stock performing exceptionally well on it and on plantain/clover mixes (e.g.
Stewart et al. 2014). To many farmers it has been a boon crop, and as a result of the accolades
given to it, has become widespread throughout the North Island and in the last year also within
the South Island. Initially, insect pest damage was minor and could largely be ignored.
Unsurprisingly, this situation has changed. A crop as favourable to grazing animals as “Tonic”
plantain was also likely to be very attractive to grazing insects and this has proved to be the case
with several pests emerging in stands. Looper caterpillars, in particular, have been found to cause
very severe damage and in the last 1-2 years their presence and damage have become more
common. These caterpillars were initially identified as Epyaxa rosearia, the moths of which are
traditionally called carpet moths but which, when found infesting plantain crops, were referred to
as Plantain moths. It is the latter name that has become accepted common usage.
This insect has historically attained very little significance and consequently very little is published or
known about it. It has been recorded as damaging Caucasian clover (Trifolium ambiguum) and
white clover (T. repens), being common on plantain and widely evident in Waikato and Bay of
Plenty (Watson et al. 1996(a) and (b)), Watson et al. also noted that another, very similar species,
E. lucidata was also present. Epyaxa is an Australasian genus and both species found are
considered native although E. rosearia also occurs in Australia.
KEY AIMS – WHAT WAS THE PROJECT TRYING TO ACHIEVE?
In response to the impact caterpillars were having on plantain crops, Beef + Lamb New Zealand
contracted AgResearch to:

Confirm identity of caterpillars causing damage to plantain and provide images to assist
farmers identify plantain moths, caterpillars and damage.

Establish the frequency and extent of plantain moth infestation on site specific locations in the
lower North Island.

Measure caterpillar density in selected plantain stands and correlate density with stand age
and degree of damage.

Gather information on plantain moth biology and development

Provide information on a plantain moth page on PestWeb (www.pestweb.co.nz)

Use the information gathered as a basis for a Sustainable Farming Fund application to develop
management strategies that will allow farmers to optimise the use of plantain as part of their
grazing systems.
METHODOLOGY – WHAT WAS DONE IN THE TRIAL?
This work commenced in late summer 2014.
Identification of caterpillars causing damage
To confirm identification of caterpillars causing damage to plantain, moths and caterpillars were
collected from a plantain stand on a Manawatu farm near Halcombe that had a history of
damage. These were held in an insect rearing laboratory at Invermay Agricultural Centre and
subjected to a constant 17 (+/- 2) oC temperature and a 14 hour light: 10 hour dark photoperiod.
Male and female moths were caged together, in 17 x 19 x 8 cm deep clear acrylic containers
fitted with a mesh lid, to allow mating. Plantain foliage and paper towel pieces were supplied as
substrates for egg laying and cotton wool dental wicks moistened with a 10% honey solution
supplied as food. Caterpillars collected from the field were caged similarly, in groups, and
provided with plantain leaves as food and substrate and allowed to develop to pupae. When
formed, pupae were caged individually in 60 ml specimen containers for moth emergence. The
resultant “lab-reared” male and female moths were again caged together, as above, to allow
mating and further egg laying.
Some of these moths were forwarded to a leading Lepidoptera expert, Brian Patrick, Wildlands
Consultants Ltd, for identification.
Leafroller caterpillars, also found in the Halcombe plantain stand feeding on plantain, were
collected and reared similarly to the plantain caterpillars. Emergent moths were again forwarded
to Brian Patrick for identification.
Establishing the frequency and extent of plantain moth infestation on sitespecific locations in the lower North Island
Six plantain stands near Wanganui, two near Halcombe and one at Palmerston North were visually
inspected for the presence of plantain moths, caterpillars and damage to plants.
In three stands, caterpillar density was measured by counting caterpillars within 44 x 47 mm
diameter rings. Each ring was positioned so that its edge dissected the centre of a plantain plant
crown. The plants were scored for feeding damage on a scale of 0 – 10 where 0 = no damage
and 10 = almost complete destruction of the plant. The caterpillar counts were graphed against
damage score to determine if a relationship between caterpillar density and plant damage could
be established.
Biology and development
Caterpillars hatching from eggs laid by the caged moths (above) were placed individually in 5 ml
vials immediately at egg hatch. Pieces of plantain leaf were supplied as food as required. Twenty
of these caterpillars were weighed at the start of each instar, and every 2 days following the moult
into the last instar. Moulted head capsules were kept for measurement. Resultant pupae were
held in 60 ml specimen containers until new generation moths emerged.
RESULTS
Identification of caterpillars causing damage
Moths reared from eggs laid in the laboratory transpired not to be E. rosearia and were identified
as Scopula rubraria (Fig. 1a). While superficially similar to E. rosearia (Fig. 2a) they are in a different
subfamily (Sterrhinae). Similar to E. rosearia, they are found on exotic plantains in both the North
Island, as evidenced by this investigation, and the South Island (B. Patrick pers. comm.). They are
considered a native species that is shared with Australia although it is possible it is Australian and
was brought to New Zealand accidentally in early European colonial days (B. Patrick pers.
comm.). There is a slight colour difference between northern and southern specimens, which may
be clinal variation (gradual change in a character across the distributional range), or potentially
there could be more than one species in New Zealand. It does not appear to have a common
name.
The leafrollers collected were identified as belonging to the Merophyas genus possibly M.
leucaniana and/or M. divulsana.
The finding that S. rubraria was present at damaging levels at Halcombe does not remove
E. rosearia from the picture. E. rosearia is known to be present in large numbers in plantain stands
and to damage plantain. Caterpillars collected from three damaged Waikato plantain stands in
2014 were reared by P. Gerard, AgResearch, Ruakura Research Centre. Approximately 60% of
those caterpillars were identified as E. rosearia, 25% as very similar to E. rosearia and 15% as S.
rubraria. Specimens were sent to B. Patrick who confirmed both E. rosearia and S. rubraria were
present at two sites while at the third only E. rosearia appeared to be collected (P. Gerard pers.
comm.). Patrick also noted that the variation within E. rosearia was extensive and postulated that
E. lucidata, a New Zealand endemic species may also be present. At the Halcombe site in 2013 a
plantain and clover stand was decimated by “plantain caterpillars”. Initially the plantain was
taken out and subsequently the clover was very badly damaged (A. Hogg pers. comm.).
E. rosearia is known to feed on both plantain and clover (e.g. Watson et al.1996a; Watson et
al.1996b). In the Invermay rearing investigation, some S. rubraria were given only white clover as
food on which they refused to feed and no records of this caterpillar feeding on clover have been
found by the authors. Although far from definitive evidence, this suggests that S. rubraria was not,
at least solely, responsible for the 2013 damage in Manawatu and it is at least plausible that some
may have been caused by E. rosearia.
The issue of caterpillars causing damage to plantain stands is not as simple as it initially seemed
when it was assumed damage was being caused by a single emergent pest. Two major pests,
E.rosearia and S.rubraria, now appear to be responsible for the significant damage being
observed. How these species interact is not known and likewise their relative importance as pests is
currently unknown. Superficially at least they appear very similar at both the moth (Figures 1 and 2)
and caterpillar stages but it is possible, for instance, that there are differences in fecundity and
generation time which makes one pest more damaging than the other. It is unlikely, however,
that many farmers or industry field representatives will be able to tell them apart. E. rosearia moths
are slightly larger, with a wing length of approximately 12 mm, than S. rubraria with a wing length
of 9-10 mm. Although variable within species the patterns on the wings can also be used to
differentiate between Epyaxa and Scopula. E. rosearia caterpillars are fatter than those of S.
rubraria (Fig. 4). Currently we do not have a confirmed photograph of an E. rosearia caterpillar.
There are also large variations in pattern within confirmed specimens of S. rubraria (Fig 4.
Given insecticidal control is currently the only feasible method of reducing the impact of these
pests, the presence of these two species (and other caterpillars) may be unimportant. The
exception could be if the use of an insect growth regulator is contemplated and iIn such situations
differences between species could become important in determining optimal application time.
Figure 1: S. rubraria moth on plantain. Inset photo shows a S. rubraria moth with a different pattern.
Figure 2: Three colour and pattern variations of E. rosearia moths. (photos T. James, AgResearch)
Figure 3: Young S. rubraria caterpillars - indicated by arrows.
Figure 4: Large “plantain moth” caterpillars. Despite the various patterns and
colourations,these are all likely to be S. rubraria (photo T. James AgResearch Ltd)
Figure 5. Caterpillars can reach very high numbers. Notice the clover is relatively untouched
(photo T. James, AgResearch)
Figure 6. An area of plantain damaged by “plantain caterpillars” in an otherwise healthy stand
Establishing the frequency and extent of plantain moth infestation on site
specific locations in the lower North Island
“Plantain moths” were observed in all of the nine plantain stands visited. However, at the time of
visitation it had not yet become apparent that more than a single moth species was involved.
Consequently identifications of the moths observed were relatively cursory. It was subsequently
possible to ascertain through photographs taken at one Wanganui site, the two Halcombe sites
and the Palmerston North site that S. rubraria was the dominant species present. This does not
imply E. rosearia was absent from these sites. As a general observation 1-year-old stands were
infested only lightly with “plantain moths”, whereas in 2 and 3 year old stands caterpillars were
very numerous while in a single older stand moths and caterpillars were present but at low
numbers. Also it appeared that monocultures supported more moths and caterpillars than mixed
swards. Given species identification is suspect and topography and climate varied considerably
between sites the only conclusions that can be reached with confidence are that “plantain moths
are distributed very widely and established within most plantain stands. This is supported by
observations made by industry representatives (particularly Alistair. Moorhead, Dereck Ferguson
and Glen Judson, all Agricom) that suggest “plantain moths are present wherever plantain is
grown throughout the North Island. One report has indicated damage in a mid-Canterbury stand
(Scott Hardwick AgResearch Lincoln).
At the three sites (a, b and c) where caterpillar density was assessed it can be assumed with some
certainty (by examination of photographs of moths taken at the sites and identification of
caterpillars from the sites raised in the laboratory) that S. rubraria were the most common moths
and caterpillars present. The size of caterpillars present ranged from 3 mm long and the width of a
hair to 25 mm long and 2 mm wide. Clearly the smallest caterpillars were difficult to find and this
was more so where litter and vegetation were dense. The densities measured ranged from 0 to
10,300/m2. The mean densities of caterpillars per site were 1,861/m2, 873/m2 and 5,207/m2 at sites a,
b and c respectively.
The severity of plant damage varied considerably within the stands and very badly affected
plants were often found in close proximity to relatively undamaged plants. Caterpillar density did
not appear to be directly related to plant damage at any of the sites (Fig 7 a,b,c) or over all sites
(Fig 7 d). Caterpillars were as often as not found on plant litter between plants suggesting that
there may be significant movement between plants. It is quite likely that abiotic factors stressing
the plantain contribute markedly to the impact of caterpillars but this may not always be so. Figure
6 shows an area of plantain severely damaged by caterpillar feeding in an otherwise vigorous stand.
12
10
8
caterpillars/17.3 6
cm2
4
2
0
0
1
site a
2
3
4
5
6
damage score
7
8
9
10
0
1
2
3
4
7
8
9
10
7
8
9
10
20
15
caterpillars/17.3 10
cm2
5
0
5
6
damage score
site c
20
15
caterpillars/17.3 10
cm2
5
0
0
1
all sites
2
3
4
5
6
damage score
Figure 7. Caterpillar numbers associated with plant damage and sites a, b and c and all sites
combined.
Figure 8. Indicative scores (numbers at bottom left of photographs) allocated to damaged plants
(only plants representative of scores 0, 2, 4, 6, 8 and 10 are presented).
Biology and development
Of the 20 S. rubraria reared from eggs 19 successfully developed to adult, there were consistently
five larval instars (developmental size classes) for caterpillars of both sexes. Mean instar durations
and larval weights are shown in Table 1 and Figure 8. By far the most larval growth, as measured
by weight gain, occurred in the fifth instar although the rates of growth achieved by 1st and 2nd
instar larvae were greater. When at maximum size the average weight of the caterpillars was 36
mg (± 1.5). The development of caterpillars was rapid with each stage lasting only 3.5 to 5.5 days
with the total time spent as larvae averaging 28.0 days (± 0.4). Fully grown fifth instar larvae
constructed pupal cells from paper towel, leaf material, frass and silk. The pupal phase averaged
11.8 days (± 0.3). The newly emerged adults laid eggs after 1-2 days and these hatched in 7 days.
The total length of the life cycle at 17°C + 1°C was approximately 49 days.
Table 1. Mean duration (days) (SEM) and weight (mg) (SEM) of immature S. rubraria development
stages when reared at 17oC.
Duration of
stage
egg
1st instar
2nd instar
3rd instar
4th instar
5th instar
pupa
7
5.5
4.8
3.5
4.2
10.1
11.8
(0.1)
(0.1)
(0.2)
(0.2)
(0.1)
(0.3)
0.04
0.32
1.64
4.6
9.95
21.8
(0.00)
(0.05)
(0.10)
(0.25)
(0.59)
(1.47)
Weight at
start of stage
0.04
5th instar (end)
0.035
0.03
pupa
Weight (mg)
0.025
0.02
0.015
0.01
5th instar (start)
0.005
4th instar
2nd instar
3rd instar
0
0
2
4
6
8
10
12
14
16
18
20
22
Time (days)
Figure 4. Growth and development of S. rubraria when reared at
24
26
28
17oC.
The mean ((tmax+tmin)/2) daily temperature (NIWA Virtual Climate Station (VCS) data) at
Halcombe from 1 October 2013 to 14 April 2014 was 16oC. Even given that the laboratory raised
caterpillars and moths were held at 17oC it is possible that up to 5 generations of S. rubraria could
have occurred in the field at Halcombe over this period. The number of eggs laid by female moths
was not determined, but even without that information, 5 generations/season indicate the
potential for very rapid population build up.
With multiple generations resulting in high populations of this caterpillar , insecticides should be
targeted against this pest early in the season so that the population build-up is interrupted and
damage prevented from occurring. It is also likely that because the caterpillars will moult several
times within the expected activity period of the insect growth regulator, diflubenzuron, (2-4 weeks)
that this relatively benign insecticide could also be used but this has yet to be tested.
Although not measured in this investigation, E. rosearia is also likely to have several generations per
year as it is recorded as flying at most times of the year (Watson et al. 1996a). Understanding the
life history and generation time of E. rosearia would add considerably to determining if the two
insect species can be considered together in control programmes.
CONCLUSION
“Plantain moth” has been shown to be not one, but at least two species of moths and caterpillars
which co-exist within plantain stands. In addition, other caterpillars have also been found feeding
on plantain. In the course of this project the life history, generation time and potential for rapid
population increase have been elucidated for one of the plantain moth species - S. rubraria. To
more completely understand the situation with regard to caterpillar damage to plantain, similar
information is needed for E. rosearia. The relative importance of these two species needs to be
determined and also how they interact with regard to damage to plantain, with and without
clover. The information gathered for S. rubraria and anecdotal evidence for E. rosearia suggest
that damage could be prevented by targeted insecticide use but this needs further investigation.
There are no registered insecticides for control of plantain moth caterpillars but broad spectrum
insecticides with good activity against other caterpillars may also provide control of plantain
moth. It would be wise to consult with agrichemical professionals before using these insecticides
for off label use.
HOW WILL THE GROUP APPLY THE PROJECT RESULTS TO THEIR
AGRI-BUSINESSES?
The final report will be advertised in e-diary and loaded on the Beef + Lamb NZ website.
Identification aids have been loaded on Pestweb (www.pestweb.co.nz). Discuss as a topic item at
BRIG, AgInnovation, other B+LNZ extension field days and via the Massey Farmer Learning Group.
MORE INFORMATION
Acknowledgements
The authors acknowledge:

The co-operation of all the landowners (A. Hogg, R. Campbell, D. McIntosh and AgResearch
Palmerston North) whose properties were sampled as part of this investigation and who were
enthusiastic supporters of the project.

Beef and Lamb New Zealand for initiating and being primary funders of the project.

Agricom for supporting the project financially and contributing valuable insights to “plantain
moth” and plantain.

Other AgResearch researchers, particularly Dr P. Gerard for reviewing this report, providing
valuable information about and photographs of E. rosearia and Dr T. James for photographs of
S. rubraria.

Mr B. Patrick for identifying the Lepidoptera discussed in this report and providing valuable
unpublished information about inter-species variation, distribution and “history” of the looper
species.
References

Stewart A, Kerr G, Lissaman W and Rowarth J. 2014. Pasture and Forage plants for New
Zealand. New Zealand Grassland Association. Grassland Research and Practice Series No. 8.
Fourth edition. January 2014. p 64-65

Watson RN, Neville FJ and Bell NL. 1996a. Insect pests associated with white and Caucasian
clover in a Bay of Plenty dairy pasture .Proc. 49th N.Z. Plant Protection Conf. 234-238

Watson RN, Neville FJ, Bell NL and Harris SL. 1996b. Caucasian clover as a pasture legume for
dryland dairying in the coastal Bay of Plenty. Proceedings of the New Zealand Grassland
Association 58: 183–188.
This publication is made possible by sheep and beef farmer investment in the industry. Beef + Lamb New Zealand is
not liable for any damage suffered as a result of reliance on the information contained in this document. Any
reproduction is welcome as long as you acknowledge Beef + Lamb New Zealand as the source.