Hawk moth pollinators in papaya
David Astridge & Vern Hansen, Department of Primary Industries and Fisheries.
Anna Morrisen, Central Queensland University.
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Larva of the grapevine hawk moth (Hippotion celerio ).
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Moth of the grapevine hawk moth |
General
Contrary to all previous opinions, a recent study of pollination in papaya indicated that the only significant method of pollination of papaya was by hawk moths (family Sphingidae). Neither native, or European honeybees, nor wind, were significant in the pollination of dioecious papaya.
Seasonally occurring periods of low fruit set and seed set were due to the a number of factors including the availability and viability of pollen, the absence of hawk moth pollinators and the stagnation of tree growth including a decrease in open flower numbers in winter. Some papaya lines (particularly Hybrid 29) showed better adaptation to heat and cold stress in respect to pollen quantity and viability. Seed set, irrespective of viability status, increased fruit size by an average of 0.89 g/seed. Some apomictic (without fertilisation) seed production was recorded.
The author concluded that research should be carried out to examine the use of hawk moth host plantings in or near papaya orchards. Improvements could be made in pollen production by papaya breeding work and investigating the potential of apomictic seed production.
Hawk Moth description
Adult
Seven species of hawk moths have been identified as pollinators of papaya and a further four species are suspected of a similar role. Adult pollinators are striking medium sized moths of similar body shape, irrespective of species. All have a large head and thorax and a tapering torpedo-shaped abdomen. The wings are narrow and held in a swept back position at rest, somewhat similar in appearance to a jet fighter. Adults make a whirring sound when flying and are sometimes mistaken for carpenter bees. Species pollinating papaya vary in size and may be 60 to 100 mm across the outspread wings. All eleven species of hawk moths (including those potentially pollinating papaya) have a proboscis (sucking mouthpart) between 2.5 and 3.6 cm long.
Eight of the hawk moth species are active during the night and the other three are day flying. All are attracted to male, female and bisexual papaya flowers by their similar scent emissions and the ultra violet light reflecting properties of the petals. In addition, male and female flowers, irrespective of nectar production, bear nectar guides that provide hawk moths with tactile and contact chemosensory stimuli while feeding.
Immature stages
Fully developed larval instars are about 60-80 mm long and quite variable in colour and markings. Larval colouration from any one species can vary between green, black or brown, depending on host plant. Most larvae bear stripes and eye like spots on their abdominal segments. All hawk moth larvae are characterised by their large tail-like straight or backwards curved darkish spine-like structure protruding from the last abdominal segment. The pupa is approximately 50-60 mm long.
Life history
Hawk moths showed marked patterns of seasonal occurrence. They were absent in Central Queensland from the middle of June until the middle of August. This absence was related to minimum weekly temperatures below 10.4oC. Hawk moths overwinter in the pupal stage. The larvae pupate in leaf litter or in soil pupation chambers. The pupal stage lasts 2 to 3 weeks in summer and increases with decreasing temperatures.
Distribution
Hawk moths are found in all papaya growing areas in Australia.
Host range
Some potential host plants may be planted close to orchards to help attract hawk moths and increase the chances of better pollination of papaya.
Native vegetation close to orchards including hostplants such as Canthium odoratum, Carissa ovata, Morinda jasminoides, Pavetta australis, Psychotria loniceroides, Cissus oblonga and equally plantings of grape vine (Vitis vinifera) and sweet potato (Ipomea batatas ) help to attract hawkmoths and increase chances pollination of papaya.
Hand pollination
Hand pollination is not normally required in commercial papaya crops to obtain adequate fruit set. To be effective, hand pollination needs to done twice a week. It is labour intensive and is unlikely to be justified on economic grounds.
However, it may be an advantage when weather conditions are unsuitable for the pollinators or cause poor pollen viability. Under these conditions hand pollination will give increased fruit set and larger fruit.
For hand pollination collect newly opened male flowers or flowers that are about to open. Gently remove the flower petals. Pollen should be visible as masses of yellow grains adhered to the split anthers (10 yellow structures around the mouth of the corolla tube). Gently brush the visible pollen from the loaded anthers over the stigma of the female flowers just after flower opening. Repeat the process to ensure adequate pollen transfer. The stigma is the exposed deeply lobed yellowish structure in the centre of the female flower. Repeat process to ensure adequate pollen transfer. Any available male papaya plants can be utilised as pollen donors as pollen does not need to be from the same variety as plants to be pollinated. After pollination mark female flowers by removing one or two petals.
Further information
- Common, I.F.B (1990). Moths of Australia. Melbourne University Press, Carlton, Victoria.
- Morrisen, A. nee Garrett, A. (1995) The pollination biology of papaw (Carica papaya L.) in Central Queensland. PhD thesis accepted by the Department of Biology, Central Queensland University
- Moulds, M.S. (1981). Larval food plants of hawk moths (Lepidoptera: Sphingidae) affecting commercial crops in Australia. General Applied Entomology, 13: 69-80)
- Moulds, M.S. (1984). Larval food plants of hawk moths (Lepidoptera: Sphingidae) affecting garden ornamentals in Australia. General Applied Entomology, 16: 57-64)
- DPI&F Call Centre on 13 25 23 - local call 8 am to 6 pm Monday to Friday (non-Queensland residents phone 07 3404 6999). E-mail: callweb@dpi.qld.gov.au
Current contact address: Dr Anna Morrisen, Australian Quarantine and Inspection Service (AQIS), Far North Queensland, Tel: (07) 4030 7822. Fax: (07) 4034 8090 email: anna.morrisen@aqis.gov.au
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 4 May 2005
