Pieris Brassicae L. Established in Chile; Another Palearctic Pest Crosses the Atlantic (Pieridae)
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VOLUME 28, NUMBEH 3 269 PIERIS BRASSICAE L. ESTABLISHED IN CHILE; ANOTHER PALEARCTIC PEST CROSSES THE ATLANTIC (PIERIDAE) BHlAN O. C. GAHDINEH A.R.C. Unit of Invertebrate Chemistry & Physiology, Department of Zoology, Downing Street, Cambridge, England In about 1860 Pieris rapae L. (the imported cabbage worm) was recorded from Canada (Seitz, 1924). It spread rapidly and already by 1870 was causing great damage to cruciferous truck crops from Montreal to New York and within a surprisingly short time had spread throughout the Union (Chittenden, 1905). In 1972 I received for identification some white butterflies from the region of Santiago, Chile. They are undoubtedly Pieris brassicae L., the large cabbage white butterfly, to give it its English vernacular name. The specimens were forwarded to me by fellow member J. H. Robert who had received them from Sf. Luis E. Pena who reports that they are now (1972) "flying around gardens in the vicinity of Santiago" which means the species is clearly established and was doubtless introduced some years ago. In view of the enormously rapid rate of spread of which Pieris species are so clearly capable it would seem desirable to give some details of it so that it can be immediately recognised and dealt with, if that be possible. Already grave concern is being expressed about an Mrican honeybee, Apis mellifera adansonii, which is sixteen years has spread virtually throughout the whole of South America and is heading fast toward the U.S. (Orsak, 1973). It would seem quite possible for P. brassicae to follow the same course, it is a noted migrant and just as fond of Cruciferae as is P. rapae; indeed its larvae will feed on plants of any family containing mustard oil glucosides. However, in the Canary Islands it is not a pest, the larvae feeding only on Tropaeoleum (Fernandez, 1955). The probability that it will now spread through South America appears to be a very real one, as Sf. Pena informed me in June 1973 that it is already widespread in all the Province of Valparaiso and adults are already flying in other provinces and the Cruciferae are being destroyed. It would seem desirable therefore to take the opportunity to give some account of the species, so that it can be looked out for; and at the same time to correct certain errors concerning it in the literature and put on record some new observations. P. brassicae differs 270 JOURNAL OF THE LEPIDOPTERISTS' SOCIETY Fig. l. Typical eggbatch of P. hrassicae. quite markedly from other European members of the genus. There are some quite good reasons for considering that it should be separated off into another genus, and it is only the great confusion that this would cause that seems to have prevented this step from being irrevocably taken. Egg. Fig. 1 shows a batch of eggs and Fig. 2 eggs in situ on cabbage. They are laid in more or less regularly arranged batches which vary in size from a few eggs to a hundred or more, the number varying according to the age of the butterfly, with an average around 40-50. When first laid the eggs are a very pale straw color; within twenty four hours this has darkened to yellow and in at least one subspecies (P. h. cheiranthi Hueb) they are bright orange. Eggs from butterflies whose larvae have been reared on semi-synthetic diets not containing cabbage leaf powder, remain a very pale straw color, indeed may be almost white. A female is capable of producing 750 eggs during a full lifespan (David & Gardiner, 1962) but it is doubtful if the full number is ever produced under feral conditions. In very warm weather the eggs will hatch in 4-6 days but may well take 2-3 weeks in cold weather. A few hours before hatching the eggs turn black and the form of the larva can be seen through the shell. The first larvae to hatch turn round and often commence to eat the tops of the shells of the other VOLUME 28, NUMBER 3 271 larvae. In this fashion the hatch of a batch of eggs will take place over about 30 minutes. The young larvae consume the eggshells and then, en mass, spin a silken pad on which they rest when not feeding. Larva. Fullgrown larvae are shown in Fig. 3A. The larvae are gregarious throughout their life, unlike P. rapae and other "small white species" which are not only solitary, but cannabalistic, both to smaller brethren and, in particular, to their own eggs, which they eat and kill (unlike brassicae which merely eat the top of the shell and release the contained larva). They invariably have five instars and, depending on the temperature, the larval stage lasts from two to eight weeks. It has been erroneously stated by both Klots (1958) and more recently again by Wigglesworth (1972) that five ins tars only occur under cold conditions and that the number of ins tars falls to four and finally only three as the temperature of rearing increases. These statements are based on an observation of Klein (1932). Frohawk (1934), a careful recorder, who had the experience of rearing every species of British butterfly, considered only five instars, and David & Gardiner ( 1962a) proved conclusively that the number of instars is constant at five over the very wide range of environmental conditions at which rearing is possible, and further extensive rearing by the present author with various stocks and races of brassicae has subsequently confirmed this. The color of the larvae is virtually the same in all instars; blue grey or yellowish ground color, a yellow dorsal stripe and irregular and intricate black markings which are more intense the lower the temperature of development. The yellowish ground color is recessive to the blue-grey (David & Gardiner, 1962a) but appears to be so common in the wild that there must be some advantage in it. The larvae prefer to feed openly on the outside of the leaves. Fig. 3B shows an aggregate of mainly fourth instar larvae and Fig. 4 the remains of a garden cabbage plot. Chrysalis. These are formed in a similar fashion to those of P. rapae, that is to say suspended by a cremaster and a silken girdle. Also as in P. rapae similar situations are sought by the larvae in which to pupate. The color of the chrysalis is either a pale straw or a shade of green, with variable black markings, and in general the color is lighter or darker according to the background. It has been stated (Babers & Pratt, 1952) that the color is influenced by the illumination of the larva before pupation. As a result of numerous experiments and the rearing of more than one million larvae I have never found any evidence of this. I have, however, found conclusive evidence that diapausing chrysalids are much more inclined to be green in color than summer 272 JOURNAL OF THE LEPIDOPTERISTS' SOCIETY Figs. 2-4. P. brassicae: 2, eggs in situ on cabbage leaf; 3A, fullgrown larvae on same plant; 3B, gregarious cluster of mainly fourth ins tar larvae; 4, devastated crop of cabbages in a garden plot. brood ones (Gardiner, in prep.). This was strikingly born out by the chrysalids I received from Chile, the straw-colored ones eclosed a few days after receipt; the green-colored ones are still unchanged after several weeks and therefore clearly in diapause. The pupal stage of summer brood specimens lasts 10 days in warm weather, but may be as long as 60 days if the weather is cold. If the pupa has entered diapause then this stage will last for 6-8 months. Adult. In general appearance the adults of P. brassicae are similar to those of the imported cabbage worm P. rapae, but are quite distinctive and sexually dimorphic. In particular the black markings have a sharp cut-off from the white instead of the gradual fade-out from one to the other as in P. rapae and P. napi. Both sexes are white with a black apical spot. The female only, has two black distal spots and a black discal streak along the inner margin. Both sexes have two black dis cal spots on the underside. The underside of the hind wing tends to be very variable and may be yellow to orange (race cheiranthi); or pale straw, greenish, and at times almost black. Greenish and blackish forms are an over-all effect VOL UME 28, NUMBER 3 273 produced by a light to heavy sprinkling of black scales. Unlike other Pieris species the veins of the wings in brassicae are never heavily marked to give a rayed or chequered effect. In size brassicae is larger than all other United States Pieris with a wingspan of from 55-65 mm in certain bred examples (David & Gardiner, 1961), up to 63-76 mm in wild caught specimens (Frohawk, 1934). All other United States Pieris have a wingspan of under 50 mm (Chang, 1963). P. rapae crucivora from Japan are exceptionally large. Esaki & Yokoyama (1955) give the wingspan as 55 mm and I have bred specimens up to 60 mm. Fig. 5 shows one such bred specimen for comparison with brassicae. Since Esaki & Yokoyama use a different basis for their wingspan measurement than Chang (which gives a lesser figure), the actual size of P. r. crucivora comes out as the mean of the P. brassicae bred by David & Gardiner (1961). As in other Pieris the black coloring of the spring brood is much paler than in the summer broods. The Chilean examples have the typical upperside facies, but the hindwing underside is of the dark green form.