Cherimoyas originate in the mountains of Peru and Ecuador where the flowers are pollinated by a specific type of beetle. Unfortunately, this type of beetle is not found elsewhere so in gardens cherimoyas may go unpollinated and fruit development may be poor unless the flowers are pollinated by hand.
Each segment of the fruit, with its white flesh containing a single black seed, is the result of one pollination, so the more pollinations the larger the fruit.
The male and female parts are carried in the same flower but, unfortunately, they do not ripen at the same time – the female parts ripen first, then a few hours later the male parts shed their pollen.
The simplest approach to pollination is to use an artist’s paint brush and simply pollinate every flower you can reach. Insert the brush into a flower till the tip of the brush touches the base. Twizzle it around a little to collect a little pollen then move on to the next flower. Insert the brush into each flower, rotate it just a little, then move on to the next flower, again and again.
Deal with as many flowers as you can reach. Simply leave the flowers high on the tree unpollinated, it would be difficult to harvest those fruits anyway.
This approach sets aside the need to identify whether male or female flowers are receptive – simply use the brush on them all and enough flowers should be pollinated to give you a crop.
Start hand pollinating in late spring or early summer, and go round the tree pollinating two or three times at four or five day intervals.