A flower open after sunset
At its stem tips, senita develops zones covered in long, dense spines. Small flowers open among them in the evening. Their structure and timing connect cactus reproduction with nocturnal insects. Theodore Fleming and Nathaniel Holland studied this association near Bahía Kino in Sonora, Mexico, in 1995 and 1996, combining insect observations with pollination experiments. [Fleming and Holland 1998]
The flowers required pollen from another individual. Hand pollination with pollen from the same plant produced no developing fruits among 27 flowers on eight plants. After cross-pollination, 46.4% of 28 flowers on six plants initiated development. The result demonstrated self-incompatibility in the tested sample and showed that supplying outcross pollen allowed only some flowers to begin fruit development. [Fleming and Holland 1998]
Abdominal scales carry pollen
The female rubs the end of her abdomen over the open anthers. Elongated scales form a brush that retains pollen grains. At another flower, she positions herself head down and rubs her pollen-laden abdomen against the stigma. The authors observed deposition lasting about 18–20 seconds. This is active pollination, in which the insect’s behaviour directly places pollen on the flower’s receptive surface. [Fleming and Holland 1998]
The sex difference was also evident among moths captured at night. Pollen occurred on 64 of 69 females, whereas none of the 50 examined males carried it. Males have a differently structured abdomen without the pollen brush. The association between behaviour and body structure explains the females’ role in pollination. [Fleming and Holland 1998]
The egg and the developing fruit
The female usually lays one egg on a petal. After hatching, the larva moves towards the bottom of the closed flower, chews through the ovary wall and begins eating developing seeds. It later leaves the fruit and enters the stem to pupate. A fruit damaged by a larva drops. Seed loss in these fruits was complete, although the larva left before consuming every ovule or seed. [Fleming and Holland 1998]
The balance therefore depends on the scale of observation. An individual larva-occupied fruit is lost, while pollination by adult females enables many other fruits to develop. In the authors’ calculation, larvae destroyed about 30% of the fruits and seeds attributable to moth pollination. The pollination benefit exceeded damage costs, so the overall association was identified as a mutualism, an interaction benefiting both partners. [Fleming and Holland 1998]
The moth’s contribution varied between years
By restricting animal access to daytime or night-time, the researchers estimated that moths accounted for 75% of fruit set under open pollination in 1995 and at least 90% in 1996. During the cooler mornings of 1995, flowers remained open longer and were also visited by halictid bees. In the warmer year, flowers usually closed before sunrise. Flower-closing time therefore affected access by daytime co-pollinators. [Fleming and Holland 1998]
The study also followed 2290 flowers on 60 plants. Early flower abortion and later larval damage represented distinct stages of loss. The authors associated the first phase with plant resource limitation and the later phase with feeding. Distinguishing these causes allowed the cost of moth presence to be estimated without attributing every unsuccessful fruit to the insect. [Fleming and Holland 1998]
The evolution of obligate pollination mutualisms: senita cactus and senita moth
Fleming T. H., Holland J. N., The evolution of obligate pollination mutualisms: senita cactus and senita moth, Oecologia, 1998, 114(3):368-375, doi: 10.1007/s004420050459
Investigates pollination of Lophocereus schottii by Upiga virescens near Bahía Kino in 1995 and 1996. Behavioural observations, hand pollination and exclusion experiments documented active pollen transfer by females and a variable contribution from bees. The authors estimated pollination benefits and the costs of fruit loss through larval feeding.
