Hexano l : a potent attractant for the black fig fly, Silba adipata

Byron I. Katsoyannos I & Patrick M. Guerin 2 I University of Thessaloniki, Department of Agriculture, Laboratory of Applied Zoology and Parasitology, Thessaloniki, Greece; 2 Swiss Federal Research Station, CH-8820 Wiidenswil, Switzerland

Keywords: Silba adipata, Diptera, Lonchaeidae, black fig fly, fig pest, fig volatiles, hexanol, leaf alcohol, attractant, trapping

Abstract

Hexanol was the only field at tractant for the black fig fly, Silba adipata, among seven generally-occurring plant volatiles tested. This leaf alcohol, dispensed in polyethylene vials inside clear invaginated McPhail traps suspended on fig trees, captured 60 f l ies / t rap/day with a 3:1 ratio of females to males. Hexanol alone was just as effective an attractant as a 2% ammonium sulpate solution but much more selective in attracting S. adipata. However, the combination of hexanol with ammonium sulphate captured three times as many S. adipata as either attractant alone.

Introduction

The black fig fly, Silba adipata McAlpine, fre- quently misidentified as Lonchaea aristella Becker, is a monophagous, multivoltine pest (4-6 genera- t ions/year) occurring in the Mediterranean and Middle-Eastern countries. It oviposits exclusively in the ostiole (eye) of unripe cultivated and wild figs in which the larvae feed (Silvestri, 1917; Anagnos- topoulos, 1939; Talhouk, 1969; Katsoyannos, 1983).

S. adipata inflicts heavy losses on the fig crop in a number of ways. Since wild figs provide a winter refuge for the wasp Blastophagapsenes L. (Chalci- didae), an indispensable agent for the pollination of some widely grown fig varieties, destruction of the overwintering wild figs by S. adipata in autumn results in a marked reduction in the level of pollina- tion in spring. The fly is also indirectly injurious by destroying wild figs of the new crop in spring, which serve as pollen spenders for the cultivated varieties. Finally, direct damage by the feeding larvae results in destruction of a considerable portion of the edible crop. A 50% drop of the immature edible crop and a 95% loss of the overwintering wild figs

Entomol. exp. appl. 35, 71 74 (1984). © Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

has been recorded (Anagnostopoulos, 1939). Since this small black fly (3.5-4.5 ram) is largely

unknown among growers, direct crop losses caused by it are frequently attributed to such factors as non-pollination, physiological deficiencies or even to attack by the Mediterranean fruit fly, Ceratitis capitata Wiedemann (Tephritidae). Consequently, no effective monitoring system for this fly has been developed, and appropriate control measures are seldom taken. Recently, Katsoyannos (1983) re- ported that S. adipata was strongly attracted to McPhail traps baited with 2% ammonium sulphate and established that fresh milky fig-tree sap was very attractive to the flies.

In the course of field tests on the attractiveness of a number of commercially available generally oc- curring plant volatiles for tephritid flies on Chios, Greece in September 1981, we observed that hex- anol was selectively attractive to S. adipata. This report describes more extensive tests designed (1) to confirm this attraction, (2) to compare the attrac- tiveness of McPhail traps baited with either hexan- ol or ammonium sulphate or a combinat ion of the two and (3) to compare the effectiveness of different trap types baited with hexanol.

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M a t e r i a l s a n d m e t h o d s

E x p e r i m e n t 1. The following compounds and com- binations of some of them were tested as supplied by Fluka AG, Buchs, Switzerland: hexanol, hexan- al, hexylacetate, heptanol, octanal, nonanol and nonanal; all were i>97% pure (gas chromatographic analysis). These volatiles were dispensed in cylin- drical 30 mm long 15 mm diameter polyethylene vials with eight 1 mm holes pierced in the top. The release rate of hexanol from this dispenser varied between 50 and 100 mg/day in warm weather (mid- day temperatures 25-30 ° C). The vials were sus- pended inside invaginated clear glass McPhail traps (McPhail, 1937). Flies were caught in the water at the base of the McPhail traps to which a few drops of wetting agent were added.

Exper imen t 2. We compared McPhail traps baited with (1) hexanol, (2) an ammonium sulphate solu- tion, (3) a combination of the two products, i.e. ammonium sulphate in the trap liquid and hexanol dispensed from the hanging vial, and (4) tap water alone as control. In treatments 2 and 3 the water in the McPhail trap was replaced with 300 ml of a 2% aqueous solution of ammonium sulphate.

The tests were conducted during August and September 1982 on the island of Chios, Greece. Traps were suspended on large fig trees (ca. 10 m high and 10-15 m crown width)harbouring S. adi- pata populations. These trees were spaced some 20-200 m apart interspersed with irrigated vegeta- bles and citrus trees. Trap number/tree varied ac- cording to the number of treatments in the experi- ment, with all treatments of a given replicate placed

on the same tree. Traps were suspended 1 m from the outermost foliage and 1-3 m from the ground; treatment positions were randomly assigned with a minimum of 2 m between traps. To minimize error due to the favoured position of a treatment on a tree, the traps were randomized daily (Exp. 1) or rotated daily (Exp. 2). Each replicate lasted 4 days.

To compare the effectiveness of different trap types baited with hexanol, carboard Tetra traps with glue covered inserts (Arn et al., 1979) were compared with similarly baited McPhail traps.

R e s u l t s a n d d i s c u s s i o n

Exper imen t 1. Hexanol dispensed in polyethylene vials inside McPhail traps was very attractive to S. adipata; none of the other volatiles showed com- parable effects though nonanol was also somewhat attractive (Table 1, column A). Captures were at the rate of 60 f l ies/ trap/day in traps baited with hex- anol. A similar finding was obtained when traps with hexanol were included in a test involving com- binations of some of the other compounds listed (Table I, column B), which were of interest to us as tephritid attractants (Guerin et al., 1983a). The pooled counts for hexanol captures gave a 3:1 ratio of females over males (gQ: 172 + 20.2, ~6': 58 + 11.8; N = 15, P <0.001, t-test). We did not establish the sex ratio ofS. adipata in the natural population but traps baited with ammonium sulphate also cap- tured more females than males (Table 2; Katsoyan- nos, 1983). The higher proportion of other insect species captured in traps baited with hexanol as contrasted with the other compounds tested was

Table 1. Silba adipata captures in McPhail traps baited with some generally-occurring plant volatiles.

A (N = 8) B (N = 7)

Compound Silba Other adipata insects

Compound Silba Other adipata insects

Hexanol 1 986 293 Hexanal 2 8 Hexylacetate 0 14 Heptanal 1 10 Octanal 2 41 Nonanol 45 61 N o nanal 1 12

Hexanol (= A) 1 479 439 Hexanal (= B) 0 9 Hexylacetate (= C) 32 29 Heptanal (= D) 1 18 B+C(I:I) 0 9 C + D (1:1) 12 95 B + C + D (hl:l) 29 127 Water (check) 43 105

Table 2. Silba adipata captures in McPhail traps baited with hexanol, a 2% a m m o n i u m sulphate solution and their combina- tion (N = 9).

Treatments Silba adipata Other insects

~ 9 ~ Total

A m m o n i u m sulphate + hexanol 654 974 1 628 653 A m m o n i u m sulphate 251 332 583 418 Hexanol 209 321 530 70 Water only (check) 0 0 0 2

possibly due to attraction of decomposition prod- ucts from rotting S. adipata in the trap water, since trap captures consisted almost exclusively of the black fig fly during the first days of each test.

Experiment 2. Ammonium sulphate was as effec- tive an attractant as hexanol and a combination of the two products was three times as attractive as either alone (Table 2). The selective action of hex- anol on S. adipata became apparent when the other insect species captured were counted. These made up only 12% of the catch with hexanol but 42% with ammonium sulphate as the lure. Incorporating hexanol with ammonium sulphate in the traps re- duced the latter proportion to 29%. McPhail traps captured 25 times more S. adipata than similarly baited Tetra traps (633 versus 25 insects; N = 1 l). The Mediterranean fruit fly was the other predomi- nant species captured with ammonium sulphate as bait. This species attacks ripening figs which were abundant at the experimental site. Hexanol was not attractive to C. capitata in citrus groves (Guerin et aL, 1983a), nor in our fig trees.

Hexanol occurs widely in the plant kingdom as a constituent of essential oils and natural aromas (Opdyke, 1975) and has also been identified from dried figs (Buttery et al., ! 981). As such, it may play a role in the host plant relationship of S. adipata. Some other six carbon alcohols of the green leaf volatile complex are implicated in the host plant relationship of the Colorado beetle, Leptinotarsa decemlineata Say, (Visser & Av6, 1978) and the related leaf aldehyde, hexanal, is a field attractant for the carrot fly, Psila rosae F. (Guerin et al., 1983b).

While the attraction of ammonium sulphate and other fermentation products to fruit flies is well documented, the attraction orS. adipata to hexanol

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and the super-additive effects of the combination of ammonium sulphate with hexanol are of particular interest. Investigations are in progress to clarify the basis of this attraction and explore its potential for monitoring and controlling the black fig fly.

Acknowledgements

We thank Professor M. E. Tzanakakis for his critical review of the manuscript.

Zusammenfassung

Hexanol." ein wirksamer Lockstofffiir die Sch warze Feigenfliege, Silba adipata

Von sieben verschiedenen, weit verbreiteten, ge- testeten pflanzlichen Duftstoffen war Hexanol der einzige eindeutig wirksame Lockstoff ftir die Schwarze Feigenfliege, Silba adipata McAlpine (Diptera, Lonchaeidae). In Feigenb~iumen aufge- Mingte McPhail Fallen, welche durch Verdunsten dieses Alkohols aus einem in ihrem Innern aufge- h~ingten Polyethylenfl~ischchen gekOdert waren, fingen im Durchschnitt 60 Fliegen/Falle/Tag in einem Verh~iltnis yon ca. 3 Weibchen zu 1 M~inn- chen. Hexanol war gleich wirksam wie eine 2% Ammoniumsulfat-LOsung, jedoch selektiver ftir S. adipata. Die Kombination beider Substanzen fing dreimal mehr S. adipata Fliegen als jede Substanz allein.

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Accepted: September 24, 1983.