Cycad Pollination

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Introduction:

For quite a long time it was believed that cycads were pollinated through the action of wind. While this may be true in certain cases more recent research indicates that specific insects are more likely to be the vectors responsible for pollination in cycads. Much research still needs to be done in this field especially in connection with Australian cycads. At this stage I can offer some personal observations confirming the very limited role of air-borne dispersal of cycad pollen.

Just recently a customer at my nursery told me he had observed some ripe seeds in a cone of a Zamia furfuracea plant in his garden. I asked whether he had done any hand pollination and he replied that he had not. I decided to have a look and was able to confirm his observation. The plants concerned were growing in a very exposed position near the summit of a hill overlooking much of the surrounding countryside. A male plant with some dried-up cones was growing about two metres away slightly downhill so that prevailing seabreezes would blow pollen towards the female plant which now had some ripe fully formed seeds in its cone. What particularly interested me was the fact that the seeds were all in a ring around the top of the cone. There were only 6 seeds in a cone which could easily have produced more than a hundred seeds indicating that the rate of pollination was very poor indeed. The location of the seeds around the cone near the top corresponded to the way the cone opens when receptive to pollen. It was apparent that pollination was only effectire where the pollen was able to enter direcfiy through the opening in the cone but the pollen was definitely not distributed through the remainder of the cone. This points to the fact that, where insect pollinators are absent, wind pollination is only accidental and relatively ineffective.

The problem with insect pollinators is that they appear to be very specific so that a certain species of insect is associated very closely with one or more species of cycads. In the absence of the correct pollinator it appears that other insects will not do the job. I have observed bees in great numbers frequenting the male cones of Cycas revoluta. European bees visited the cones but were unable to enter as they were too large but the much smaller native bees were observed carrying away quantities of pollen. The problem was that they were only flcetingly interested in the females "cones". Once they realized there was no pollen in them they confined their interest to the male cones and of course were of no help at all in the pollination process. Of more interest in the case of Cycas revoluta was a small unidentified beetle found in reasonable numbers in both male and female cones. What their interest in the cones is and what benefit they obtain from being there is not known. It was observed that they were mating in the cones but whether they were actually reproducing in the cones is also not known. Although the fact that these insects were found in both male and female cones might suggest that they could be a factor in pollination, this is by no means certain as it would depend on the insects moving back and forth between male and female plants. At present it has not been possible to find anyone who could identify these insects so it is not known whether they are native or introduced. The only cycads native to this area are Macrozamia miquelii and M. lucida but I have not observed the same insects in cones of M. lucida in my nursery. At present I need all the Cycas revoluta seeds I can get so I have hand pollinated cones to maximize seed production. As a result I do not know whether they might be effective in pollinating in the absence of hand pollination. I would be interested in the observations of any readers who have cycads in their gardens particularly as to the presence of insects and whether pollination of cones has taken place without human assistance.

Another observation I have made is with M. lucida. A friend had a number of mature plants on his dairy farm. He did not want these plants and was slashing the leaves regularly and told me I was welcome to remove them. I re-established these plants in my garden and soon they produced both male and female cones. I noted that quite a considerable number of small weevils were present in the cones and presumably playing a role in pollinating. What was interesting was that year after year the number of weevils decreased until they are now completely absent. It would seem to me that unless there is a reasonably large number of plants in a locality the insect population cannot be maintained.

There are a number of implications for people interested in cycads. From the growers' point of view it is apparent that very little pollination will take place unless this is done artificially. This is done quite easily by collecting pollen from the male cones and, using a camera lens brush with a blower bulb on the end, picking up the pollen and blowing it into the receptive female cones. Most cones open when receptive and pollen can be blown through the openings. I have found this quite effective in most cases so that often there is dose to 100% pollination. From the point of view of conservation there are very great implications. In cases where plants are lost from the natural environment it is often possible to take cultivated plants and replace them into the habitat they once occupied. With cycads this may not be feasible. The insect pollinators appear to be very specific in their association with cycads, often living in a symbiotic relationship with them. When the population of cycads of a given species in a locality falls to very low levels the insect may become extinct. When cultivated cycads are reintroduced into the area to replenish the population they may do well and it may appear that the procedure has been very successful. The problem is that without the insect pollinators there will be no natural regeneration and in the long run it will all be a waste of time unless there is human intervention on a continual basis. Another consequence is that in cases where the population of cycads in an area becomes too low and the insect pollinator cannot survive there is not much point in trying to protect the remainder of the plants and thereby hope that the population will recover. Without the pollinator this will never happen.

All the above probably may not apply to at least some species in the genus Cycas It is well known that Cycas rumphii, C. circinalis and C. thouarsii have seeds which contain spongy tissue so that they readily float in water. It is thought that by this means they have spread from island to island throughout the range that they presently occupy. It would be most unlikely that the insect pollinator would accompany the seeds yet these cycads are reproducing freely over a vast range, on islands separated by great expanses of water. It appears that not a great deal is known as to how these cycads are pollinated and what insects (which would presumably have been present on remote locations before the cycads seeds arrived) may be responsible.

Perhaps it is possible that a pollinator insect may be found somewhere that will pollinate cycads indiscriminately. If this was possible it would be a significant help in saving natural cycad populations from total extinction. It is certain that the subject of cycad pollinators is a vital one for those who wish to conserve these fascinating plants. It is to be hoped that more research can be done on this subject.


Contributed by:

Will Kraa (from Palms & Cycads No 40, Oct - Dec 1993).

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