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THE SEASONAL ABUNDANCE AND FEEDING DAMAGE OF HYPSIPYLA
GRANDELLA (LEPIDOPTERA: PYRALIDAE) IN SEED CAPSULES OF SWIETENIA
MAHAGONI IN FLORIDA
F. W. Howard and R. M. Giblin-Davis
Abstract
Larvae of Hypsipyla grandella attacked the seed capsules
of West Indies mahoganies, Swietenia mahagoni Jacquin, in spring
(March - April) after the capsules dehisced and the seeds were
exposed, which occurred prior to flushing. One to 5 larvae occurred
per capsule. The seeds apparently were a preferred food source
and 50-96% of the seeds in capsules examined in June were damaged
by larvae. Seed capsules during their period of expansion from
early summer to winter were virtually free of borer attack, and
during this period neither hardened-off shoots nor persistent
capsule cores from previous seasons served as food sources for
more than a few larvae. The hardness of the capsule valves is
apparently a factor in preventing penetration by the larvae. Although
the persistence of seeds in the capsules is transitory, and the
availability of capsules more limited and more variable than that
of shoots, the seed capsule contents appeared to be preferred
as a food source, as higher percentages of dehisced seed capsules
than new shoots were attacked when both were simultaneously available.
The damage by H. grandella to mahogany seeds impacts regeneration
of this tree species.
Resumen
Las larvas de Hypsipylla grandella atacan a las cápsulas
de las semillas de la caoba de las Indias Occidentales, Swietenia
mahagoni Jacquin, en la primavera (marzo-abril) cuando éstas
se abren y las semillas estáu expuestas, lo cual ocurre
antes del brote de nuevas hojas. De una a cinco larvas se encontraron
por cápsula. Las semillas aparentemente fueron la fuente
de alimento preferida. El 50-96% de las semilas en las cápsulas
exminadas en junio estuvo dañado por las larvas. Las cápsulas
durante su período de expansión, a comienzos del
verano, y hasta el invierno estaban virtualmente libres del ataque
de los barrenadores. Durante este período tanto los brotes
endurecidos como los corazones persistentes de las cápsulas
sirvieron como alimento a de unas pocas larvas. La dureza de las
valvas de la cápsula es aparentemente un factor de impedimento
a la penetración por las larvas. A pesar de que la persistencia
de las semillas en las cápsulas es transitoria, y la disponibilidad
de las cápsulas es más limitada y más variable
que la de los brotes, el contenido de la cápsula de las
semillas parece ser preferido como alimento, debido a que más
porcentaje de cápsulas abiertas que de nuevos brotes fueron
atacados cuando ambos estaban simultáneamente disponibles.
El daño causado por H. grandella a las semillas de caoba
afecta la regeneración de este árbol.
Two species of Hypsipyla are important pests of timber
trees of the family Meliaceae. One species, H. grandella (Zeller),
known as the mahogany shoot borer, is considered the major pest
of mahoganies (Swietenia spp.) and cedros (Cedrela spp.) at the
nursery and young plantation stage in the American tropics. The
larvae kill the apical shoot, inducing a secondary shoot that
results in a crooked stem and excessive side branching (Dourojeanni
Ricordi 1963, Grijpma 1974, Howard & Meerow 1994, Lamb 1966,
Yamazaki 1992). Hypsipyla robusta (Moore) plays a similar role
on meliaceous trees in the Eastern Hemisphere tropics (Gray 1972).
Research on the biology and control of Hypsipyla spp. was recently
reviewed by Newton et al. (1993).
Most studies of the feeding habits of these insects
have focused on injury to shoots, but both species also have been
reported to infest seed capsules of meliaceous hosts (Beeson 1961,
Betancourt 1987, Bruner 1936, Monte 1933, Roberts 1966, Solomon
1995, Tillmanns 1964, Wagner et al. 1991). Monte’s (1933) observations
indicated that H. grandella was highly adapted to utilizing seed
capsules: larvae that hatched from eggs laid on green seed capsules
of cedros penetrated into the interior of the fruit and fed on
seeds. Before pupating, the mature larva made a hole in the capsule
valve by which it later exited as an adult. Adults of H. grandella
reared from larvae infesting seed capsules were larger than those
obtained from shoots (Becker 1976), indicating the importance
of fruits in the development of this insect. Hypsipyla robusta
is listed as a pest of fruits of meliaceous timber trees in West
Africa (Wagner et al. 1991). In northern India and Burma, where
H. robusta attacks toon (Toona ciliata H. Roemer), it is known
as the ‘toon fruit and shoot borer’ (Beeson 1961).
Some observations have been made on the seasonal
occurrence of feeding on seed capsules by Hypsipyla spp. Hochmut
& Manso (1975) reported that in Cuba H. grandella attacks
seed capsules of meliaceous trees during the dry period when young
shoots of these hosts are not available. In northern India, the
first generation of the growing season of H. robusta feeds on
flowers of T. ciliata, the second on seeds in the green fruit
capsule, and the remaining generations (third through fifth) in
shoots of the current year (Beeson 1961). In Nigeria, Hypsipyla
sp. feeds on flowers of African mahogany, Khaya ivorensis A. Chev.,
from September to November, on seed capsules from November to
February, and on shoots during the remainder of the growing season
(Roberts 1966).
In southern Florida, the host of H. grandella is
the West Indies mahogany, (Swietenia mahagoni [Linnaeus] Jacquin),
which is native to Florida and is the only representative of Meliaceae
native to the continental United States (Harlow & Harrar 1968,Pennington
1981). Although the insect attacks exotic species of Meliaceae,
few of these are planted in southern Florida. Attacks on shoots
of West Indies mahogany by H. grandella peak in May of each year,
coinciding with the spring flush (production of new leaves and
shoots) at the beginning or just before the advent of the wet
season (June - September). During the remainder of the wet season,
shoot production is sporadic and attack by H. grandella diminishes
greatly, and is virtually nil during the dry season (October -
May) (Howard 1991). This paper elucidates the seasonal abundance
and feeding damage of Hypsipyla grandella (Lepidoptera: Pyralidae)
in seed capsules of Swietenia mahagoni in southern Florida.
Methods and Materials
The study was conducted during the period March 1995-January
1996. We identified the immature stages of H. grandella by diagnostic
characters illustrated by Ramirez Sanchez (1964). We collected
20 larvae from different seed capsules and reared them to the
adult stage on excised mahogany shoots with their bases in water
or on mahogany seeds. We compared the specimens of adults with
illustrations in Becker 1976, Grijpma 1974, Ramirez Sanchez 1964,
and Roovers 1971. For confirmation of the identification, 4 of
the specimens of adults were examined and identified as H. grandella
by J. B. Heppner (Florida State Collection of Arthropods, Gainesville).
All trees in this study were 5-20 year old West Indies
mahoganies planted on the Fort Lauderdale Research & Education
Center. A total of 338 trees of this species are planted on this
site.
Physical characteristics of the seed capsules and
seeds that might influence larval feeding were observed. The length
of time that the winged seeds persisted on the capsule and thus
remained available as food for larvae was determined. Capsules
were marked with the date that they opened and observed on April
17 and 24 for the numbers of seeds persisting and the numbers
that had been shed as indicated by placental scars.
The thickness of capsule valves were measured at
their midpoints with calipers in April and in October. The diameters
and lengths of capsules were measured in July, October and January.
In April and October, the resistance to penetration of seed capsules
and their contents was determined with a Model RP-3T Missouri
Type Rind Penetrometer (Alan Machine Co., Ames, Iowa 50010). Points
of penetration included seeds, cores, and valves at midpoint and
seams between valves. A common bottle cork (processed bark of
Quercus suber L.) was used as a standard.
The first field observations in this study were made
on March 29, 1995, prior to the spring flush. Twenty dehisced
capsules, all of which showed larval damage (gallery plus frass
typical of this species), were removed from trees and dissected
and examined to confirm the presence of larvae of H. grandella
and to determine where the larvae had fed.
Between April 4 - May 8, three trees, all about 6
m in height, were selected arbitrarily among those bearing unopened
seed capsules. These trees were observed every 1-3 days to determine
the dates of flushing, dehiscence of the capsule, and initial
damage to shoots and/or seed capsules. The dates that the capsules
dehisced and that borer damage was first evident were written
on 2 cm × 9 cm aluminum tags fixed to the peduncles. On May
3 and May 8 the percentage of marked seed capsules and of 50 randomly
selected new shoots with borer damage was determined and all marked
seed capsules (n = 26) were dissected and examined for larvae.
From May 1995 - January 1996, West Indies mahogany
trees on the Research Center continued to be examined frequently
for evidence of damage to seed capsules or shoots. In January
1996, a total of 400 capsules of the current season were clipped
from trees and examined in the laboratory for evidence of borer
damage. One hundred of these were examined with a 10 × hand
lens for eggs of H. grandella. We had previously observed that
cores of a large portion of the seed capsules of West Indies mahogany
persist after valves and seeds have been shed and may remain on
the trees for up to 2 seasons. To determine whether these served
as food sources for H. grandella larvae in winter, 78 cores of
the past summer’s and 22 of the previous year’s seed capsule which
remained on trees were examined. In addition, the stems and branches
of 75 trees < 2 m in height were examined closely for larvae
or damage by them.
Results
Seed capsules of West Indies mahogany (Fig. 1) develop
from small (5 mm diam) flowers that bloom in June-July in Florida
(Howard et al. 1995), expanding rapidly in summer. By the end
of July the capsules were about half their mature size of about
65 mm × 75 mm, which they reached in January when they began
to dihisc. Hypsipyla grandella rarely penetrated the capsules
until they dehisced, after which they readily attacked them. The
relatively thick, hard capsule valves are no doubt a factor in
preventing most H. grandella larvae from penetrating them to reach
the food-rich seeds. Newly dehisced valves were a mean of 8.9
mm thick (range 7-15 mm, n = 20) measured at midpoint and 5 -10
× harder than the cores, as measured by penetrometer readings.
The resistance to penetration of the cores was similar to that
of common bottle cork, while that of the seeds was lower (Table
1).
In both October and April, sites along the seams
at the juncture of valves were generally 1/3 to half as resistant
as the valves at midpoint. However, in April about 30% of the
sites along the seams had about the same resistance as seeds,
i. e., about 10% the resistance of tissue at valve midpoints,
indicating that the seams were weakening as the capsules dehisced.
However, with the exception noted below, larvae did not utilize
this potentially easy entry point.
On March 29, 1995, the first day of field observations,
about 30 trees had seed capsules. Only a small portion of the
total capsules had dihisced and all these had H. grandella damage.
Of the 20 of these that were sampled, 11 were infested with larvae
and 2 contained cocoons of this species. There was never more
than one late-instar larva per capsule, but earlier instars occurred
up to 5 per capsule. The 9 capsules that did not contain H. grandella
immature stages showed evidence of their damage, but the larvae
had left.
One predehisced mature capsule observed in April
had a hole along a seam. The capsule was dissected and found to
contain a larva of H. grandella. In August, a young capsule 42
× 44 mm had a hole interior to the margin of the valve about
3.6 mm in diam with frass identifiable as that of H. grandella.
The hole was filled with the gummy exudate characteristic of wounds
in mahoganies. Dissection of the capsule revealed a late instar
of H. grandella.
West Indies mahogany seeds persisting in capsules
apparently were a preferred food source for H. grandella larvae.
They usually bored through the layers of seed wings and, upon
reaching the core, attached themselves between the core and the
seeds, hollowing out the seeds from their proximal sides so that
from the outside of the capsule the larvae were not visible and
the seeds appeared sound. Early and late instars were observed
feeding on seeds, but only late instars were observed in cores,
suggesting that they fed on seeds first and then bored into cores.
The seeds are susceptible to attack by H. grandella
only as long as they persist in capsules, and this parameter is
highly variable. Five of the marked capsules shed most of their
50-80 seeds in a few days. Others shed seeds more gradually. In
3 capsules observed, all of the seeds persisted 18 days after
they had dehisced, in spite of winds on one day estimated at up
to 25 kph that buffeted the tree branches and caused the seeds
to flutter. Once larvae began to attack seeds, they enveloped
them in webbing, which prevented them from falling from the capsules.
Most seed capsules on the 3 sample trees dehisced
on different days between March 31 - April 13 and were attacked
by H. grandella larvae prior to flushing of these trees. Flush
occurred during the period April 14-19. Hypsipyla grandella larvae
apparently entered 2 of the capsules the day that these split
open as evidenced by frass issuing from between the seeds. In
the other 24 capsules, frass was first seen 7 to 30 days after
the capsules opened.
Higher percentages of dehisced seed capsules than
new shoots were attacked when both were simultaneously available.
When counts were made on May 3, 70 - 100% of the dehisced capsules
on the 3 sample trees (n = 26), compared to 14 -22% of the new
shoots (n = 150), had been attacked. A week later, there was a
slight increase in the percentage of dehisced capsules attacked
but no discernible increase in shoots attacked.
Table 1. Mean penetrometer readings (kg pressure)
on parts of seed capsules and seeds of West Indies mahogany and
on common cork for comparison.
Capsules (n = 22) dissected on June 5 revealed that
larvae of H. grandella had fed on seeds and excavated galleries
in the cores of almost all capsules and were still present in
81.8% of them. Six to 37 seeds persisted per capsule, but 50-96%
of the seeds in different capsules were damaged by larvae, either
with large holes or completely hollowed out from the inside. A
maximum of 5 larvae were found in one capsule, these being of
early instars. Most capsules had 1 or 2 larvae. A single mature
(fifth instar) larva was found in each of nine capsules. Two capsules
had pupae. Capsules (n = 4) dissected on July 31 were similarly
damaged, and one contained a single pupa of H. grandella.
Of the 400 capsules examined in January, a total
of 5 capsules had dehisced. Larvae of H. grandella were in 4 of
the dehisced capsules and in one predehisced capsule, distributed
as follows: Two capsules had apparently been dehisced at least
since December and each harbored a fifth instar H. grandella larva.
Two dehisced capsules each harbored 1 early instar of H. grandella.
There were no entrance holes in the valves, indicating that the
larvae had entered after dehiscence of the capsule. An early instar
larva was on the outside of a predehisced capsule. In the laboratory,
this larva bored into the valve during the next 4 days, but eventually
died. There were 2 capsules with superficial feeding scars that
may have been caused by H. grandella larvae, but larvae were not
present. Two capsules had initial entrance holes with pitch tubes,
which may have expelled attacking larvae.
With the exception noted, neither eggs nor borings
by larvae were observed in the predehisced capsules in January.
At this time, the previous summer’s leaves persisted on the West
Indies mahogany trees and no new leaves or shoots were produced.
There was no evidence of shoot or stem attack by H. grandella.
Also in January 1996, 69.2% of the past summer’s
and 90.9% of the previous year’s cores had borer damage. Many
of the cores of the capsules of the current year contained deteriorated
silk and frass that had persisted since the spring and one had
an empty pupal case of H. grandella. However, no live immature
H. grandella were found in the cores. The galleries of many of
them were occupied by other arthropods, mostly predators, including
Pseudomyrma sp. (Hymenoptera: Formicinae) workers with brood,
other ant species, wasps, and spiders. The persistent cores were
dry and deteriorated and of doubtful value as a food source for
H. grandella, which is adapted to feed on living plant tissue.
None of <2 m trees (n=75) closely inspected in
January had H. grandella larvae or damage to stems.
Discussion
Our observations indicate that H. grandella larvae
attack seed capsules in spring with dehiscence of the capsules
and exposure of the seeds, which occurs prior to flushing. About
70-100% of the open capsules on different trees are attacked.
The larvae hollow out seeds and penetrate the core. This insect’s
apparent preference for seed capsules is consistent with Becker’s
(1976) observation that larvae reared from capsules are larger
than those reared from shoots. When the trees flush, < 25%
of the shoots on the same trees are attacked. Larvae rarely penetrate
the capsule valves, probably because of the thickness and hardness
relative to that of the seeds and capsule cores. Seed capsules
of the current year are virtually free of borer attack during
their period of expansion from spring to early winter, as are
persistent capsule cores from previous seasons. We have occasionally
found larvae in shoots in late summer, but they apparently are
very scarce to absent from this host plant from midsummer to the
next spring flush. The question of where and under what conditions
H. grandella passes this period in Florida remains a gap in our
knowledge of their life history. Gravid females are presumably
present and either abundant or efficient enough to find the few
dehisced seed capsules present during January, more than 3 months
before an abundant supply of dehisced capsules and new shoots
are available in April. Meliaceous trees other than S. mahagoni
are ruled out as alternate hosts, because they are extremely rare
in southern Florida.
Both excised shoots and seeds of West Indies mahogany
support the development of larvae to maturity in the laboratory.
Although in the field a higher percentage of capsules than shoots
were attacked and apparently are a richer food source, their availability
is less certain than shoots. Young trees less than 5 years old
do not produce seed capsules, and mature trees produce many more
shoots than capsules. Annual production of seed capsules is highly
variable. We have routinely observed that mature West Indies mahoganies
produce from 0 to about 50 seed capsules compared to many hundreds
of shoots. A maximum of about 300 capsules was observed on a tree
in 1995.
The results of this study indicate that H. grandella
may severely restrict the regeneration of West Indies mahogany
in Florida. Where seed production is important, West Indies mahogany
capsules should be protected against H. grandella attack, but
methods have not been investigated.
Acknowledgments
We thank J. V. DeFilippis and Martha Howard for field
assistance, Omelio Sosa, Jr., for lending us the penetrometer,
and Kimberly Klock and Thomas Weissling for reviewing the manuscript.
This is Florida Agricultural Experiment Station Journal Series
No. R-05083.
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Fig. 1. Parts of seed capsules of Swietenia mahagoni;
A) Dehisced capsule with damage and frass of H. grandella larvae;
B) Dehisced insect-free capsule (s, seeds, which are born on a
central core; v, valves); C) Capsule in initial stage of dehiscence;
D) Core after seeds have been shed, with galleries made by H.
grandella larvae.
University of Florida
Fort Lauderdale Research & Education Center
3205 College Avenue
Fort Lauderdale, FL 33314, USA
Plant Part
N
Mean ± SD
Common cork
10
3.7 ± 0.1
April, dehisced capsules:
Valves, midpoints
25
16.4 ± 3.7
Valves, seams
15
9.3 ± 4.5
Cores
20
3.9 ± 0.6
Seeds
10
1.6 ± 0.3
October, predehisced capsules:
Valves, midpoints
11
13.5 ± 2.9
Valves, seams
11
9.7 ± 4.3