Araceae · Reproductive Biology
THE
DUMB CANE
One of the best-measured pollination systems in the family — and the same beetle is a partner at low numbers and a pest at high ones
The dumb cane is on more windowsills than any other aroid except the pothos, and almost nobody who owns one has seen it flower. In the wild it runs one of the most thoroughly measured pollination systems in the family — over a thousand marked plants, nearly five thousand marked beetles, and three field seasons.
Dieffenbachia is a Central and South American genus of soft-stemmed forest-floor herbs. It sits at the base of a tribe that is otherwise made up of geophytes, and it is the only member of that tribe to reach Central America, Mexico and the West Indies. What people grow indoors is mostly one species, Dieffenbachia seguine, in a great many variegated selections.
How thin the evidence base actually is
The genus has 57 described species and perhaps 140 in reality. Pollination or reproduction data exist for five of them, across six populations.
Everything on this page comes from that handful, and most of it from a single Costa Rican forest.
What is known is known well
Every studied species is pollinated by cyclocephaline scarab beetles, and in the one study that checked, only the scarabs carried any pollen at all. Flies, true bugs and sap beetles were caught leaving the inflorescences and examined: not a single grain between them.
The inflorescence runs a three-day programme in which the first twenty-four hours are sexually dead. It heats once, not twice. Its sterile flowers are a protein-and-carbohydrate food body that doubles as the thing holding the beetles in place. And a beetle leaving it may be carrying more than a thousand pollen grains, or none.
The result that makes this genus worth a page
An inflorescence's chance of setting fruit rises with the number of beetles in it, peaks at about eight to twelve, and then falls.
In a year when beetles were four times as abundant, the commonest beetle species loaded negatively on fruit set while a scarcer one loaded positively. In a lean year, no species differed from any other. The same insect is a mutualist at low density and something closer to a parasite at high density — and abundance turns out to be a poor guide to which visitor matters.
One naming problem, stated once
The plant studied at La Selva through the 1980s was published as Dieffenbachia longispatha. It was later redetermined as Dieffenbachia nitidipetiolata. A 1999 study then compared the true Dieffenbachia longispatha, in Panama, against that same La Selva plant.
So two bodies of work carry the same name on their covers and are about different species. This page uses the corrected names throughout, and the Sources block records what each paper actually says on its title page.
Part I
The Plant and the Clock: Three Days, and the First One Does Nothing
A dumb cane inflorescence is open for three days. For the first twenty-four hours of that it is not receptive to anything — a finding that contradicted two earlier accounts which had assumed the opposite.
The three days
| When | What the inflorescence is doing |
|---|---|
| Day 1, early evening | The spathe opens. Nothing is receptive. No stigma will take pollen and no anther will shed it for about a day |
| Day 2, about 17:30 | Stigmas become receptive. The single heating episode and the scent belong here — the female stage. Beetles arrive and go into the chamber |
| Day 3, about 17:00–17:30 | Pollen is released while the stigmas are still receptive — which leaves a window in which the inflorescence can pollinate itself |
| Then, for 8–11 months | The spathe closes over the fruit and stays shut while it ripens, green to white to yellow to orange-red |
The plant cannot easily pollinate itself from a neighbouring flower
A single shoot carries two to seven inflorescences, about 4.8 on average — and they open three to twelve days apart, never two at once.
So pollen moving between inflorescences on the same plant is structurally prevented. The only route to selfing is inside a single inflorescence, in that day-three overlap.
The heat
One heating episode, in the female stage, of about 3.3 and 4.1 degrees above ambient, just before seven in the evening.
Two inflorescences, two evenings
Those figures come from n = 2, on two evenings in one season. They are a demonstration that this plant heats, and they are not a species value. Nobody has run a proper thermometry series on any Dieffenbachia.
What is worth noting is the shape: one peak, in the female stage only. That is unlike the two-peak pattern of Philodendron and it is shared with the one other genus in this tribe that has been measured.
The scale of the thing
An inflorescence carries about 77 female flowers and roughly 440 male units above them. Between the two zones sit the sterile flowers — the staminodes — which are the beetles' food and, as Part IV shows, the reason the beetles stay.
And the name problem, stated once
Two research programmes, one name, two species
The plant studied at La Selva in Costa Rica through the 1980s was published as Dieffenbachia longispatha. It has since been redetermined as Dieffenbachia nitidipetiolata.
A 1999 study then compared the true Dieffenbachia longispatha, on Barro Colorado Island in Panama, against that same La Selva plant — and found different principal pollinators at the two sites.
The rest of this page uses the corrected names. The papers themselves still say longispatha on their covers, and a citation that follows them is not wrong — it is just about a different plant than it appears to be.
Part II
Only the Scarabs Carry Pollen: Everything Else Is a Passenger
Plenty of insects turn up at a flowering dumb cane. Sixty- three of the non-scarabs were caught and examined grain by grain. Not one of them was carrying any pollen at all.
The measured zero
Three groups were sampled off the inflorescences and checked: 13 Drosophila, 29 plant bugs and 21 sap beetles. Every one of them came back with zero grains.
This is not an absence of looking. It is the result of looking. The cyclocephaline scarabs are the only animals moving this plant's pollen, and that is true for every species of Dieffenbachia anyone has studied.
A departing beetle can be carrying more than a thousand grains
Across thirteen departing Cyclocephala, the mean load was 1,649 grains. That is a substantial cargo for an animal that will fly, on average, eighty-three metres to the next open inflorescence.
And it does not keep
The pollen is trinucleate and short-lived: about a quarter is still viable at twenty-four hours, and a sixth at forty-eight. A beetle that sits in the same inflorescence for a day is not carrying much of use by the time it leaves.
That fact is doing more work than it looks, and Part III is about what it costs the plant.
What the loads do not tell you
Carrying pollen is not the same as delivering it. When single visits by each of the three main beetles were compared directly, the difference in how often they produced fruit was not significant — sixty-five, fifty-two and fifty-three per cent of inflorescences aborted after one visit, regardless of which species had visited.
So the beetle with a dozen grains and the beetle with nine hundred looked about equally effective on a single visit. The difference only appears when you count what happens over a season, which is the next part.
Part III
Abundance Is Not Importance: The Same Beetle, Twice Over
Three beetle species visit this plant in numbers. On a single visit they are equally good at setting fruit. Over a season they are not remotely equivalent, and the most abundant one is the worst.
The trap in the obvious method
The standard way to rank pollinators is to multiply how effective a visit is by how often the animal visits. Do that here and the answer is clear: Erioscelis columbica is the most abundant beetle by a wide margin, its single visits are as productive as anyone's, so it must be the most important pollinator.
The season-long fruit-set data say the opposite.
Success is a hump, not a slope
The probability that an inflorescence sets fruit rises with the number of beetles inside it, peaks at about eight to twelve, and then falls.
In the year beetles were abundant, fruit set rose only to about four beetles before declining. Comparing like with like — restricting to inflorescences with fewer than fifteen beetles — the peak in that year sat at six.
What a four-fold beetle year looks like
| 1982 | 1983 | 1984 | |
|---|---|---|---|
| Beetles collected | 487 | 3,077 | 1,131 |
| Mean beetles per inflorescence | 1.77 | 7.27 | 1.96 |
| Most in one inflorescence | 15 | 39 | — |
And in the crowded year the species pulled in opposite directions
Once the effect of sheer numbers was accounted for, the proportion of Cyclocephala gravis in an inflorescence loaded positively on fruit set, and the proportion of Erioscelis columbica loaded negatively. Cyclocephala amblyopsis had no effect either way.
In the lean year, no species differed from any other.
So the same insect is a useful partner when there are few of them and a net cost when there are many. That is not a statement about beetle species. It is a statement about density — and it means a pollinator's value cannot be read off a visitor list.
Why Erioscelis specifically
Two behaviours, and they pull against each other. It is the flighty one — significantly more of them leave within half an hour of arriving. It is also the loiterer: a third of all Erioscelis recaptures were in the same inflorescence on consecutive days, eating the male flowers or chewing out through the spathe, against about three per cent for each of the other two.
With pollen only a quarter viable after a day, a beetle that stays put is carrying nothing worth carrying by the time it goes.
Two limits, and the second is the more important
Beetles explain very little of it. Every one of the fruit-set regressions has an r-squared between 0.02 and 0.08. Whatever is driving fruit set in this plant, the number and identity of beetles is a small part of it — how many mates are within flying range, and how much the plant can afford, carry the rest.
And nobody has seen the damage. The decline above a dozen beetles is an inference from the curve. In three field seasons, exactly one instance of ovary damage was observed. Dislodged pollen, accumulating waste, and foreign pollen blocking the stigmas are all offered as possible explanations, and none of them has been tested. The hump is real; the reason for its right-hand side is not established.
Part IV
The Tether and the Cost: What Holds the Beetles, and What Selfing Buys
Between the female and male zones sits a band of sterile flowers. They are the beetles' food, and they are also the only thing keeping the beetles on the plant — which was shown by cutting them off.
The staminodes
They are a food body, and a rich one: about a quarter protein, over half carbohydrate, with some fat, at around four hundred calories per hundred grams. A beetle that walks into the chamber has somewhere to be for a day and a reason to stay.
Remove them and the retention collapses to nothing. The plant is not trapping its pollinators — it is paying them to stay put for the twenty-four hours between the stigmas closing and the pollen being released.
What selfing costs
The plant can pollinate itself. Its own pollen works, and in the day-three overlap it has the opportunity. But every measure says it is a poor substitute.
| Treatment | Result |
|---|---|
| Self-pollinated by hand | 3 of 14 inflorescences succeeded; 4.64 fruits each |
| Cross-pollinated by hand | 13 of 28 succeeded; 13.21 fruits each |
| Bagged, no beetles got in | 55 of 57 aborted |
| Bagged, but beetles chewed their way in | only 6 of 20 aborted |
| Male flowers removed | 12 of 12 aborted — no seed without pollen, so no apomixis |
Selfed fruit also took longer to ripen — about 297 days against 271 — on a plant that is already carrying its fruit inside a closed spathe for the better part of a year.
And it is not getting enough pollen as it is
Seven inflorescences given extra pollen by hand all set fruit, at 65.9 per cent, against 40.5 per cent for a hundred and sixty-nine left to the beetles.
On the standard definition, this plant's fruit set is pollinator-limited. Whatever the beetles are doing, they are not saturating it.
One tension the sources leave open
Bagged and unvisited inflorescences aborted 55 times out of 57, which reads as almost no spontaneous selfing. But the later analysis found that bagged unvisited inflorescences gave about 29 per cent fruit set, and its regressions predict sixteen to twenty per cent even with no beetles at all.
Both figures are printed here and neither is dropped. Some self-pollination happens without visitors; how much is not settled.
Sources
Sources, and What Kind of Evidence Each One Is
Four papers carry this page, and three of them are one researcher's work on one Costa Rican forest. That concentration is the single most important thing to know about the evidence.
- Young, H. J. (1986). Beetle pollination of Dieffenbachia longispatha (Araceae). American Journal of Botany 73(6): 931–944. The foundation of the page. The three-day clock, the thermogenesis, the flower counts, the breeding system, the staminode-removal experiment, the pollinator-limitation test and the zero-pollen result are all here. Grade: field experiment at scale — 1,017 marked plants, 4,843 marked beetles, three seasons, 129 bagged inflorescences. Note: the species is Dieffenbachia nitidipetiolata (Croat 2004), not longispatha. The thermogenesis figures are the weakest thing in it: n = 2, on two evenings.
- Young, H. J. (1988). Differential importance of beetle species pollinating Dieffenbachia longispatha (Araceae). Ecology 69(3): 832–844. All of Part III, and the per-species pollen loads in Part II. Grade: three-season observational study with regression analysis, and unusually honest about its own explanatory power — every fruit-set regression has an r-squared between 0.02 and 0.08. Same species and site as above. Its headline — that pollinator importance is density-dependent and non-monotonic — is the reason this genus is worth a page. Its damage mechanism is explicitly not shown: one observed instance of ovary damage in three years, and three untested hypotheses.
- Gibernau, M. (2015). Floral biology, pollination ecology and genetics of Dieffenbachia (Araceae) — a review. Aroideana 38: 19–28. The genus frame in the Opening and Part I: the species totals, the five-species evidence base, the tribal placement, the staminode composition and the pollinator roster across all studied species. Grade: review synthesis — it argues and compiles, it does not measure. Useful precisely because it makes the thinness of the evidence explicit. Note that its rendering of one two-site comparison is the reviewer's own computation of means and ranges; cite the primary for those numbers.
- Beath, D. N. (1999). Dynastine scarab beetle pollination in Dieffenbachia longispatha (Araceae) on Barro Colorado Island (Panama) compared with La Selva Biological Station (Costa Rica). Aroideana 22: 63–71. Used on this page for one thing: it is the study that puts true Dieffenbachia longispatha beside the La Selva plant and finds different principal pollinators at the two sites, even though a shared beetle species occurs at both. Grade: two-site observational comparison. Note that its own concluding interpretation is superseded and is not repeated here; and its chi-squared tests cannot be reconstructed from what it prints, and one of them is commonly misread.
Where this leaves the genus
Five species out of perhaps a hundred and forty. Three of the four papers above are the same researcher, on the same plant, in the same forest, in the early 1980s. The most recent first-hand fieldwork on the shelf is from 1999.
What is missing, and it is a lot
No scent chemistry. Two sites have different principal pollinators and the presumed reason is a difference in floral odour — which nobody has analysed for any Dieffenbachia.
No real thermometry. The genus's entire heat record is two inflorescences on two evenings.
Nothing on the houseplant. Dieffenbachia seguine is grown by the million and has two brief field records, in Mexico and French Guiana. What the plant on your windowsill does when it flowers has never been studied where it lives.