«437 Pineapple Fruit Injuries Caused by Larvae of the Moths Ereunetis flavistriata and Pyroderces rileyi* BY DR. M. B. LINFORD Pineapple Experiment ...»
Pineapple Fruit Injuries Caused by Larvae of the Moths
Ereunetis flavistriata and Pyroderces rileyi*
BY DR. M. B. LINFORD
Pineapple Experiment Station, Honolulu, Hawaii
(Presented by Dr. Walter Carter at the meeting of November 2, 1939)
Among Lepidoptera infesting the pineapple, Ananas comosus
(L.) Merr., in the Territory of Hawaii, the two most abundant spe
cies are Ereunetis flavistriata Walsm., the sugar cane bud moth, and
Pyroderces rileyi (Walsm.), the pink cornworm, locally sometimes called pink bud moth. Although well authenticated reports indicate that both species sometimes attack living parts of certain plants, these insects are generally regarded primarily as scavengers, their larvae feeding most commonly on dead vegetable matter. Both spe cies occur widely on developing pineapple fruits, sometimes in abundance, but the significance of this infestation has been the subject of contradictory reports.
This paper presents evidence, obtained by the writer during two periods of fruit disease investigation, that larvae of both Ereunetis flavistriata and Pyroderces rileyi sometimes attack pineapple fruits, producing characteristic but minor wounds that appear important chiefly as ports of entry for pathogenic microorganisms. The in juries, rather than the insects, were the center of attention in- these studies.
LITERATURE Tryon (5) in a study of fruitlet core rot of pineapple in Queens land, observed (p. 462) that the caterpillar of a diminutive moth, probably one of the Tineinae, frequently occupies the Blossom cup of
the pineapple fruitlet, consuming stamens and style. He states:
"These insects are, however, found in both healthy and diseased fruitlets alike, but usually in those that are quite sound, and, more over, no disease seems to ever attend their presence."
Ereunetis flavistriata, the sugar cane bud moth, so-called because it sometimes eats into young cane buds protected by dry leaf sheaths, was reported by Swezey (4) to feed on the dry leaves of pineapple and sometimes to be numerous among the bracts at the base of the fruit where he considered that they fed merely on dead tissue.
Concerning this same insect on pineapple fruits, Illingworth (2) stated that while the caterpillars are feeding "upon the essential organs of the flower, in the calyx cavity, they frequently gnaw into the surrounding living walls." He also reported these caterpillars * Published with the approval of the Director as Technical Paper No. 122 of the Pineapple Experiment Station, University of Hawaii.
Proc. Haw. Ent. Soc, X, No. 3, August, 1940.
to be common around the base of the fruit where they sometimes eat out small cavities between the eyes, but he neither described nor illustrated the wounds. Illingworth suggested that, during such feeding, these larvae may inoculate fruits with pathogenic micro organisms.
Pyroderces rileyi, an insect known to feed on diverse living plant materials in addition to dead vegetable matter, has been reported in jurious to pineapple by Illingworth (3), who stated: "The tiny pink caterpillars of this species are particularly troublesome on the fruits.
They live in the withered flowers and outer calyx cavities and fre quently gnaw the leaves of the crown. Rot organisms enter through the wounds, causing the breaking down of the fruit." The illustrations cited by Illingworth at this point, however, indicate confusion with yellow spot, a virus disease that frequently causes a breakdown of the fruit following infection and necrosis of the crown.
More recently, Herford (1) investigated both Ereunetis flavistriata and Pyroderces rileyi in relationship to pineapple, chiefly on the island of Oahu. Herford confirmed the abundant association of these insects with pineapple but rieported that fruit infestation with both species is usually confined to young fruits, and that "con trary to certain claims, they appear to do very little damage."
From other statements' it is evident that Herford recognized no pineapple fruit injury attributable to either insect.
During a search for ports of entry of pathogenic microorganisms into ripening pineapples during March 1939, the writer found that larvae of Ereunetis flavistriata are able to feed on fleshy calyx and bract tissues of pineapple fruits in various stages of development, from less than half grown to fully ripe. The relationship of charac teristic feeding scars to this insect was indicated by their close asso ciation with webbing, pellet-covered cocoons, and the gray cater pillars themselves, numerous larvae having been found with their heads in the bottoms of fresh pits and holes.
Experimental confirmation was obtained when larvae collected from pineapple fruits were confined to limited areas on ripe or nearly ripe fruits that initially were entirely free from anything that might be confused with caterpillar injury. Cylinders of transparent sheet celluloid, approximately 2\ inches in diameter by 4 inches tall, were attached to the fruit with dental plaster. Larvae of Ereunetis flavistriata were then introduced and the top of the cylinder closed with a piece of closely woven white cloth, secured in place with rub ber bands. Six such experimental cultures were established March 9, with first ratoon fruits and with larvae chiefly half grown or larger, both fruits and larvae collected from the Palawai Basin, Lanai. When the cages were removed for final fruit examination March 17, caterpillar injury was found on each of the 6 fruits.
Some of the insects had pupated and some had escaped, with the result that larvae recovered were always fewer than introduced.
No insect of any other species, however, was found in any cage. An indication of the extent of injury is provided by the numbers of feed ing scars, shown in Table 1. Fruits and larvae from Wahiawa, Oahu, handled similarly have given comparable results. Adults of E, flavistriata were reared from both Oahu and Lanai larvae.
Table 1. Ereunetis flavistriata Injury to Experimentally Infested Fruits
Feeding occurs either directly in a crevice between fruitlets (fig.
3) or, more commonly, low on the sides of the crevice, that is, on the shoulders of calyx ^nd bracts (fig. 2). The wounds vary from less than 1 mm. in diameter to over 2 mm. wide and, fn extreme cases, 10 mm. long; and, although usually very shallow, sometimes are rounded cavities as much as 4 mm. deep. Such deep wounds, if made before the fruit ripens, may fill with soft pale gum that gradu ally hardens and darkens to deep amber (fig. 6).
On a fruit that is not yet ripe, these Ereunetis flavistriata wounds are at first clean and white (figs. 1, 2) but they rapidly turn light brown with the formation of wound cork and, more slowly, develop narrow, dark brown borders (fig. 5). Wounds made early, while the fruit is still enlarging, are gradually lifted, by growth of the bracts and calyx from their bases, until they may lie as much as 5 mm. from the bottom of the crevice. Early wounds, if particularly deep, may result, at maturity, in continuous corky surfaces from one fruitlet to another (fig. 4).
During ripening, a pineapple fruit loses its capacity to heal wounds by cork formation, with the consequence that the wounds on ripe fruits either dry out, if the fruit is of a relatively dry, opaqueflesh type, or promptly exude juice if highly translucent. Espe cially the juicy wounds are attractive to insects, including Carpophilus spp., Atherigona excisa (Wied.), Ohymomyza procnemis (Will.), and Drosophila spp. '• Bud moth injury is most frequent on surfaces that have been pro tected from sun and wind. On a fruit)that has stood erect during its development, these are chiefly near/the base. In ratoon fields, Figs. 1 to 6.—Ereunetis flavistriata injuries on pineapple fruits. x2.
1. Fresh, white, elongate wound in bottom of crevice between fruitlets.
2. Two relatively fresh wounds on calyx shoulder; also pellets of excrement distributed in a sparse web.
3. Deep holes extending directly through bottom of crevice.
4. Older corky wounds on opposite sides of crevice, with narrow band of cork across crevice. Note dark margins of wounds.
5. Old corky wounds with dark margins, situated high on calyx shoulder in consequence of growth after feeding occurred.
6. Two drops of hardened gum in a pair of deep holes within the area of a larger shallow wound.
however, where fruits frequently lie almost horizontally, with one side closely pressed against leaves, bud moth injury may extend far towards the crown along the protected side. Where fruiting is irregular, injury is more frequently observed and, characteristically, is more extensive, on fruits that have been crowded and shaded by tall surrounding plants.
Although Herford (1) found infestation chiefly limited to young fruits, the present observations indicate that injury occurs chiefly after fruits are approximately 1/3 grown. Prior to that time, the calyxes and bracts of individual florets tend to form conical projec tions with very narrow crevices between. Later growth, however, is accompanied by a flattening of the fruitlet surface and a widening of the crevices between fruitlets, and it is in these crevices that this insect injury occurs.
Bud moth injury to pineapple fruits was widespread during March and April 1939. On the island of Lanai, where the initial observations were made, the injury occurred in every field examined and on almost every fruit in some fields. It has since been found on eastern Kauai and in the Wahiawa and Kunia districts on Oahu.
Variations from field to field include not only differences in per centages of fruits infested and in extent of injury to these, but differ ences in stage of fruit development at which injury occurred.
Because of its shallowness and limited extent, Ereunetis flavistriata injury appears important only insofar as it opens ports of entry for pathogenic microorganisms. Wounds in unripe fruits, because of their prompt healing, appear unimportant even in this way, except in that their corky surfaces may crack during ripening.
Of greater importance are the wounds made during ripening that, being attractive to miscellaneous insect vectors of yeasts, bacteria and fungi, appear to contribute to early deterioration of the ripened fruit.
These are not the only wounds on ripening pineapples that serve as ports of entry, others being, apparently, growth cracks resulting from stresses set up in the expanding fruit. Deep growth cracks usually occur between fruitlets, especially between contiguous mar gins of floral bracts, but some occur through the middle of a bract, particularly where the phyllotaxy is irregular. The relative impor tance of growth cracks and bud moth injury in allowing the entry of insect vectors of microorganisms is yet to be determined.
During these studies, very few Pyroderces rileyi larvae were seen on fruit surfaces, and these did not appear to be associated with any feeding injury.
PYRODERCES RILEYIWhile studying internal characteristics and diseases of pineapple fruits, chiefly in the Makawao district, Maui, during August and September 1933, the writer found the pink larvae of Pyroderces rileyi eating into ripening fruits, thereby allowing the entry of fungi that promptly initiated a firm dark-brown rot of limited extent.
The fruits dealt with came chiefly from 2 fields. Smaller numbers examined from other fields in this area indicated that the injury occurred generally here, but limited observation during the same period on west Maui, Lanai, and Oahu indicated a much more spar ing occurrence than near Makawao. Since that time no comparable study has been undertaken in the Makawao district, and fruits from other localities have shown very little of this injury.
The feeding wounds of Pyroderces rileyi larvae are of different nature and occur in different locations on the fruit than those of Ereunetis flavistriata. Habits of the insects themselves on pine apples are different. Unlike the sugar cane bud moth, which is found between sterile bracts at the fruit base or, on the fruit surface, hiding under its pellet-strewn web in crevices between fruitlets or under margins of fruitlet bracts, P. rileyi larvae were found, during these studies, predominantly within blossom cups, enclosed by the fleshy sepals. Here they feed chiefly on the remains of petals, sta mens and style,' and, only occasionally, during ripening of the fruit, in the flesh itself..
Wounds made by this insect vary from shallow pits to irregular tunnels of variable diameter, length, and direction, extending inward from the lining of the blossom cup (figs. 7, 8, 9). This lining is hard and smooth except at the bases of petals, stamens and style, and it is chiefly through either the base of the style, in the very bottom of the blossom cup, or, less often, through bases of petals or stamens at the side of the cup, that larvae eat their way into the fruit. Those floral parts normally wither and die soon after blossoming. In some fruitlets, however, some or all remain fleshy for an indefinite period and, by their bulk, prevent the usual tight closure of the blos som cup by the-sepals. Pyroderces rileyi injury appeared more abundant where these parts had remained fleshy than in normal fruitlets.
The withered floral parts within blossom cups are usually infested by various fungi, especially species of Penicillium. Usually these fungi are unable to enter the flesh but, when the lining of the blos som cup is injured, as with a needle, they enter promptly and; pro duce a firm dark-brown rot of limited extent. It was not surprising therefore to find a rot of this type beginning promptly at points of entry of Pyroderces rileyi larvae. Commonly, the entire feeding tunnel was enclosed in a rounded mass of firm, dark-brown rotted tissue (figs. 7, 8, 9) from which the only microorganisms that could be isolated consistently were 3 species of Penictilium. That such rot may develop while the larva is still feeding is indicated by tun nels that were involved in the characteristic rot-near the blossom cup but that, at their inner extremity, were fresh and clean.