Insects on Flowers

1

1 Offprints from: Edi:cd by:

Published by:

E~ologic:-rl Entomology

Carl

n. Huffak~r and

Ho

en L.

Rabb

John Wiley & Sons, Lnc. Copyright, 1984

Chapter 20

Insects on Flowers PETER G . KEVAN and HERBERT G. BAKER

20.1 20.2 20.3 20.4 20.5

Flower-visiting insects

608

Floral attractants and insect senses

614

The rewards of visiting flowers

616

Foraging, physiology, and behavior

620

Physical environment

624

20.6

Before angiosperms 625

20.7

Community ecology 625

20.8

Conservation

627

References

628

607

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The harmonics between insectan and floral coadmS between anthophilc> may heighten the cffc:ctheness of the basic mutualism in assuring the reproductive success of l>oth plants and po llimttor>.

and elongation of mouthparts, uptilling of the head, a nd elongation of the protho rax. Curculionidae are known to be associated with man)' palm inflorescences. and recently Elaeidobius kamemnicus has been introduced from Africa to Malaysia to pollinate the oil palm Elaeis guiueensis, also of African origin (Syed et al. 1982). The resu lts have been startlingly successfu l and were evaluated at (USA) S 11 5 X 106 per annu m in increased o il crops in 1982. 20.1.2

Diptera

Diptera have also been suggested as early pollinators. The 1'\ematocera are the most primitive. ln most famili es the proboscis is short, although variable in form . Sciaridac have bt:t:n recorded from t he flowers of Drimys (Win-

610

P. C. Kevan a nd H. C . Baker

tcraceae). a primitive Oowering tree of the tropics. Its Oowers, like those visited by other Nematocera, have readily acccs:;ible nectar. Most Oowe rs visited by nies have nectar which is exposed or partially exposed in shon tubes (e.g., Achillea, Senecio, Polygonum, various Cruciferae, and Umbelliferae) or even hidden (e.g .. Salix). Most 'emalocera are small (t\fycctophilidae, Cccidomyidae, Simuliidae, Chironomidac, Ceratopogonidae, etc.). For the most pan, these insects seck nectar. although some feed on pollen [e.g., Bibiu, Scatopse, Sciam, a nd AtrichojJogon (Downes 1971)). The larger Tipulidac arc restricted to the same sons of Oow~::rs, as they too have sho n mouth parts. T he ematocera with longer proboscides (e.g., Culicidae and Bibionidae) also visit such flowers, but includ ed arc some with deeper tubular corollas (e.g., Composit' may be pollinato rs. T cuigoniids, especially Couocephalus, may be freque nt a nd destructive flower visitors. The peculia r Australia n Zaprochilinac a re adapted for a ntho phily, h aving na rrowly prognathou s head s (Rentz & Clyne 1983). Acrididae a re freque mlr e ncountered on flowers, but discounted as incide ntal visitors. Ea rwigs hid e in flowers where they a re generally destructive. Thrips are notorious flower visitors, and some have mouthparts especially adapted for pie rcing and sucking out pollen gra ins (Lewis 1973). T heir role in pollina tion has been investigated in European Ericaceae (Haste rud 1974) and in Malaysia n Diptc rocarpaceae (Appana h & C han 198 1). Hete ro pte ra a re conspicuous a nd common a nthophiles. Nabidae, Miridae, Lygaeidae, Corcidae, and Pe ntmomidae a re the most freque ntly found a nthophilous famili es; they frequ e nt flowers with easily accessible nectar (e.g., Compositae and Umbelliferae). Some Phymatidae usc flowers as places to prey upon o the r insects (Baldu f 1941 ). There is liule information on

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P. G. Kevan and H. G. Baker

614

the importance of the anthophilous habits of these insects to either the insects or the plants. There are occasional records of Neuroptera, Mecoptera, and Trichoptera as flower visitors feeding on nectar, or pollen, or both (Porsch 1958).

20.2

FLORAL ATTRACTANTS AND INSECT SENSES

The appreciation insects have of their environment can be understood through an ecological view of physiological adaptation. Anthoph ilous insects have finely attuned senses of vision, olfaction and taste, mechanoreception, and time, which extend especially to their appreciation of floral attractants. 20.2.1

Color and Color Vision

The visual spectrum of insecu; is shifted approximately I 00 nm to the shorter wavelengths of the spectrum as compared \,rith humans: their vision extends from about 300 (UV) tO 650 nrn (yellO\,·-orange). Goldsmith and Bernard ( 1974) show that most insects so far rested have peaks of sensitivity in UV, blue-green, and yellow. In Apis and Bombus, color vision has been shown to be trichromatic, that is, using those three primary colors. Some flies appear to be deuteranopic (color blind, analogous to n;d-green color blindness in humans) and confuse blue through )'ellow but distinguish UV. Some insects may have onl}' tonal, or black and white, vision. Kevan ( 1978, 1983) has placed insect color vision, as represented by Apis, into an ecological context, especially in anthecology, by considering the properties of daylight and the spectral reflectance of flowers across the insect visual spectrum. From this he has devised a method of colorimetry and color naming by adapting techniques used in the trichromatic color-naming scheme used for colorimetry in the human visual spectrum. Although this technique may have shortcomings, it provides a method whereby humans may start to have an appreciation of the diversity of color patterns in the insect world. These studies have stressed that UV is no more important to insects than their other primary colors and that all wavebands of concern to insects must be considered when attempting to understand floral colors as insects may see them. Kevan (1983) examined whole floras of particular habitats-the Canadian high arctic and Canadian weeds-and showed that the colors of the flowers are more diverse and more discrete to insects than to humans. Furthermore, color patterns within flowers are more diverse and contrasting when looked at in the insect visual spectrum. These color patterns, or nectar guides, assist insects in obtaining rewards on complex or large flowers. Some of these are bulls-eye patterns as in Myosotis with its blue coloration and yellow center, or in many Compositae with yellow centers (insect-red) and yellow + UV peripheries (insect-purple). Others are patterns of stripes

'

Insects on Flowers

615

and spots such as can be seen on Viola, Digitalis. manr lilies. :md so on. In general, butterfly flowers show the highest incidence (83%) or nectar guides, fo llowed by Zygomorphic flowers and then capitulate ones. Even about half of the bowl-shaped flowers examined have nectar guides. These patterns may change with age, telling the informed visitor the state of the flowers. In Aesculus the orange-spot nectar guides turn red as the flowers age and cease nectar production; t hey are then ignored br bumblebees. Senecio heads become brown in the center as they age, cease producing nectar and pollen. and a re then ignored by hoverflies. Numerous legumes change the colors of their banner petals as they age (e.g., Luphms, Lotus. O>..;•trojJis, Caesalpinia, Parkinsonia). They often also change their shape, some by \\•ilting, after pollination. Postpollination changes are often rapid in onset in orchids (cf. Gori 1983). From the foregoing, it is obvious that fl ower color is imponaru to anthophiles in their recognition of plant species and the potential for reward offered by the flowers. Other visual attractants also play a pan in attraction. The size of flowers , inflorescences, or the corporate image of floral groups have been shown to be positively related to attractiveness over distance. Flicker fusion. t hat is. the speed at which flicket-ing images blur wgether and appear to cease to flicker, is very much fas ter in insects than in humans. Thus, floral movement and the outlines of flowers \,·here they contrast against the background . do not blur-out as the insect moves towards and about the flowers and the ommatidia of' the compound eye are repeatedly and sequentially stim u lated. Flowers with broke n outlines or moving pans are generally more auract ive, but these phenomena have been litLle stud ied. Some generalizations on the color preferences of insect groups for flowers can be cautiously made. Flowers reflecting blue are frequented by bees. but these flowers are often structurally adapted to bee pollination (e.g. , Legum inosae, Scrophulariaceae, Boraginaceae, Labiatae). Nocturnally pollinated fl owers are pale, as are flowers of the deep forest, and contrast against dark or ill-lit backgrounds. Yellow fl owers attract an almost unlimited diversity of visitors. Some unspecialized Coleoptera, Diptera, and Lepidoptera seem to show preference for yellow. Red flowers are mostly associated with bird pollination, but others have butterfly pollinators. Some bunertlies have been shown to have red-sensiti,·e vision. There are almost no UV flowers: Papaver rhoeas is one, being red (invisible lO most insects) and UV. The UV reflective patterns on OjJI11ys flowers pollinated by pseudocopulation by male Corytes wasps offer a "supernormal" visual image in mimicking the female wasp; the flowers have more UV insect reflectance than the model. 20.2.2

Odor

It is more difficult to generalize about floral odors. Odors arc difficult to a nalyze and insects' powers of olfaction a rc more diverse than their pO\vcrs of vision. Many floral odors have no counterparts outside blossoms: we

616

P. G. Kc•·an and H. G. Ba ker

associate the seems " 'ith flowers. In diurnal fl owers. it seems that fl oral odors act as a close-in attractalll lO e ntice landing after long-distance attraction by general coloration a nd at intermed iate distances by color patterns. However, in oranges and other plants, the corporate scent of large stands may act over long distances. I n nocturnally blooming pitar. /Jot. b tl . 104: 11 5- 164. Powell. J. A. a nd R. A. Mackie. 1966. Uuiv. Cnlif. l'ubl. E utomol. 42: 1- ·16. Priesner, E. 1973. 7.oou. S uppl. 1: 4 3- 5