Almost every fruit we grow needs pollinating and in many cases if we want the best crops we cannot rely on nature to do the job for us. Unfortunately vicissitudes of the weather makes outdoor pollination hit and miss some years and the recent scarcity of honey bees has made things worse. Under cover the conditions may be better but in there there are few natural pollinating agents and effectively no wind.
First a bit of botanical revelation; we all know there are male and female flowers and male and female flower parts and that we ought to be able to tell the difference from looking at them. Fortunately you can forget all the pistil/stamens/stigma/style bit if all you want to do is tell a male from female for pollination purposes. The simplest way that works in most cases is- "Does it shed pollen all over your finger? aye, then that bits a male! And a miniature fruit swelling just behind a flower is a sure sign of a female in many cases!
Secondly, and I'm sure you all know this too, not all male flowers or male flower parts can fertilise all other female flowers or female flower parts even if they are of the same species. And most often when they're on the same plant. In many plants the male and female flowers open or mature the relevant parts at different times of day, or days, to try and ensure self pollination does not take place. Many plants are actually self-infertile or self-incompatible to also aid cross pollination and guard against selfing. However others, typically peas and beans, don't appear to care and apparently self-pollinate away. Even so some actual pollination does occur from the male to female parts. In only a very few fruits such as bananas and our temperate varieties of figs are the fruits readily formed with no pollination whatsoever. (In their original home figs are pollinated by a small fly fostered in capri-figs and entering by a small hole in the end.)
Thus we know almost every tree and bush relies on pollination but so rarely do we lend a hand. Yet this simple act which is light work can give us much better sets of fruits in years when they are otherwise lacking. Heavy sets may be more work to thin but are more easily thinned to give a uniform spread of fruits in the best positions. Obviously such work is far more important under cover and when you only have a few varieties but even the commonest garden fruits and nuts can benefit from our intervention.
In particular the nuts are predominantly wind pollinated and are usually left to themselves. However I noticed that my walnut tree had the tendency of flowering the long green male catkins a week or even two before the female flowers were receptive. (Two feathery paddles stick out of the wee walnut like bud.) I collected the opening males and stored them in an envelope in a dry place, then I put them in a small net bag on a very long cane and dusted the females when they opened. I got a real heavy crop wherever I could reach. Now I do the same every year!
Chestnuts are harder to deal with as the males and females form on the same spindly shoot and the opening males are harder to collect before they fall so instead I've tried collecting the pollen on a bit of fur on the end of a long cane and brushing it onto the females as they open which also works well. The hazels seem to do very well by themselves but then I have a copse of several sorts and I'd recommend hand pollination if you have only one or two. Doing it yourself is certainly one good way of ensuring cross pollination and is particularly effective for hazels, and for blueberries but not for sweet corn.
Sweet corn is normally planted in blocks of the same variety as if they are mixed they will cross pollinate only too well and the seeds will become extremely variable making for weird cobs. So the blocks must always be all of the same variety and although they ought to self pollinate the pollen from the males often comes rather too early for the female tassels who are not yet all ready. Thus I collect some of the prolificly made pollen in an envelope and keeping it dry to dust it on a few days later when the females are more receptive.
Such wind pollinated plants usually produce masses of pollen so it is easy to spot, save and apply but flowers that are insect pollinated produce much smaller amounts. (Pollen is not always yellow; pear is often red, poppy is grey black and some pollen is even blue) For insect pollinated flowers a bit of fur or fake fur or a small brush is the usual way of transferring the minute amounts nowadays -I got in PC trouble when I showed the traditional bunnies tail in use on peaches, OOPS, I should have used the end of my plait! (the tail had been on a road victim anyway!) One good brush will do as it does not need to be a separate one for each species as inter-species crosses are unlikely (and interesting if they happen).
Earlier crops such as peaches need the assistance of a brush most as they are flowering when few pollinating insects are about. Indeed the rare honey bees that are left do not fly early enough anyway and it helps immensely if we can encourage more of the bumble and humble bees by providing them nest sites and alternative pollen and nectar sources. But in their absence we must step in with our brush.
It does help to catch the flowers at the right time when some are giving pollen and some receptive. But if you can't tell then ideally do it several times; i.e.. as when a few flowers are open, many are open, most are open, and most are over. And of course you should move from variety to variety and back again each time. Well do what you can. And do try and avoid doing it when the flowers are wet are this will promote disease.
Anyway I find that assisting the earliest outdoor fruits certainly ensures more regular cropping always provided I can then keep the frosts from taking off the flowers and the fruitlets once they have set. It is not often realised that small fruitlets are as sensitive to frost as the blossoms and need the same sort of protection or they fall off after a hard frosty night. Some of us protect our flowers and leave the covers on for days together. If you hang covers permanently over the flowers and fruitlets to prevent frost damage you must then pollinate yourself or you may lose the crop as the barrier will stop the insects who can't reach the flowers. Apricots, peaches and pears are all blossoming so early that hand pollination makes a big difference but the later fruits such as cherries and apples usually need no assistance. Neither do the soft fruits which mostly manage amazingly well, despite flowering so early and without assistance, mind you if you've got the time......
Under cover it is another matter. I have some permanent plantings in my polytunnel to try to build up an ecosystem but with the exception of spiders it is singularly short of flying pollinators just as is almost every greenhouse in the land. However on one occasion after using some rat bait I was unable to find the corpse I could smell. A few flies did find it and some weeks later I found my tunnel visited by a Biblical plague of the now much multiplied devils. It was quite interesting, I got terrific sets on almost every flowering thing and they cleaned all the honey dew off my citrus like magic! Bit too macabre to repeat though.
If you hope to set indoor crops from winter flowers then they need hand pollination with a brush. Lemons and oranges particularly so. Though if you wish for non-stop flowers for perfume then take the set fruitlets off immediately the petals drop and they'll flower more prolificly for much longer. I've found most indoor fruits, especially the earliest ones, really need hand pollinating but as the season warms up later in the year some pollinators do find their way in. Unfortunately for them few find their way out again as a polytunnel is a very good trap! Anyway I rarely have any trouble getting a set for free with the main crop tomatoes, peppers and so on but again the earliest crops are nearly always benefited by hand pollination. Likewise with the melons, in their case they are happy enough to set fruits but then they come unevenly with one hogging all the nutriment. In their case it is best to hand pollinate all the females on one day and then to ruthlessly thin to the pair or so of best ones that set.
Indeed having to practice ruthless thinning on a substantial set on some fruiting plant you've pollinated may seem like you've made a rod for your own back. True, but as I said above, now you have the choice of fruits to select from whereas if you had left it to chance most of them would not have been there in the first place! And left to chance some years there'd be none at all! Especially with indoor crops- I never saw a custard apple till I started hand pollinating the funny tripod like green flowers with a wee brush and now I have one swelling bigger than a hens egg already.