The threat posed to crop production by plant pests and diseases is one the key factors that could lead to "a perfect storm" that threatens to destabilise global food security.

Already, the biological threat accounts for about a 40% loss in global production and the problem is forecast to get worse, scientists warn.

BBC News has asked Dr Matthew Cock, chief scientist for Cabi, a UK-based agri-environment research organisation, to compile a list of the worst plant pests threatening crops around the world.

Image caption Throughout history, the impact of plant pests and diseases, such as potato blight, can devastate lives

Scientists can be difficult; we have so many ways in which things can be measured, analysed or compared that sometimes it seems as if we never agree on anything.

So when I was asked "which are the world's worst agricultural pests?' my answer was simply that the question cannot be answered.

How do we define a pest? What measure would you use? How would you value that measure? Not only do perceptions of the worst problems vary according to geography, they also vary from year to year.

It is an unfortunate fact too that despite a general consensus on the threats from pests and diseases to global production, actual monitoring and evaluation of damage caused globally is very poorly understood.

The following list is by no means definitive therefore, nor a serious attempt to prioritise the threats posed by different agricultural pests.

All we are trying to do is to raise awareness of the immense range of pests and diseases that threaten agricultural crops, the devastation they can cause, and the difficulties in controlling them.

Scientists working in the field may disagree with our nominations; if they do, I welcome them to join the debate and share their own ideas, add a comment to this article or visit Cabi's Plantwise blog.

Worst historical pest - the desert locust

Schistocerca gregaria: a pest since biblical times, they fly in unexpectedly, strip a field bare in an hour and consume a very wide range of crops.

Locust swarms may vanish for many years, only to break out of their endemic regions after periods of abnormally high rainfall.

Another nomination is human beings, Homo sapiens. This species could appear in several categories, but we include it just once here, for its sometimes accidental, sometimes purposeful habit of introducing pests to new habitats where they flourish through lack of natural controls.

A recent example of the latter was the deliberate introduction of witches' broom disease of cocoa, Crinipellis perniciosa, in Brazil. The motive was a social one - to weaken the political power of the powerful cocoa landowners, and it achieved its desired effect: production across the region fell 75%. Brazil went from being the world's third-leading cocoa producer to 13th place.

Hardest pest to control - South American rubber blight

The rubber tree is a native of South America but very little of the commodity is produced there. The principal reason is the fungus Microcyclus ulei has resisted all attempts to control it over more than 100 years.

In the 1920s, it notoriously defeated Henry Ford's attempts to grow rubber for car tyres in the eponymous Fordlandia, Brazil, losing him an investment (in today's terms) of $250m (£156m).

Also worth considering in this category is coffee wilt disease, Fusarium xylarioides. The disease spreads insidiously through the soil and on machetes used to prune the trees.

The only way to halt it is a scorched-earth policy of pulling up all trees in infected plots and then waiting a year before replanting - plus another four years until you get a full harvest again.

Most expensive pest to control - western corn root worm

In terms of the amount of pesticides once used to control Diabrotica virgifera virgifera and the expense of developing a resistant GM-strain, this beetle is a strong contender.

Under control for many years now, maize has been one of the few crops to show steady yield increases and for this the resistant strain can take credit.

Now, though, there are signs that resistance is breaking down.

Pest of greatest human impact - potato blight

The Phytophthora infestans fungus caused the Irish potato famine (1845-1852), during which one million people died and a further million emigrated from Ireland, causing the population to decrease by about 24%.

Another candidate is coffee leaf rust, Hemilaea vastatrix, a fungus that devastated coffee production in Sri Lanka (Ceylon) in the 19th Century and famously led to a switch to tea drinking in the UK.

But tea picking requires more continuous labour than coffee, so the Tamil migrants from India that used to go home after the coffee harvest ended up settling in the country, which led to catastrophic sectarian strife over a century later.

Worst stored product pest - the Khapra beetle

This insect, Trogoderma granarium, is difficult to control because it feeds on a variety of dried materials.

It is resistant to insecticides and can go long periods without food.

Infestations can result in up to 70% grain damage, making products inedible and unmarketable.

Worst climate change threat - mountain pine beetle

The cumulative effect of the current outbreak of Dendroctonus ponderosae in British Columbia, Canada, has killed 13 million hectares of lodgepole pine forest and released an estimated 270 million tonnes of carbon, converting the forest from a carbon sink to a large net carbon source.

Generally, climate change is likely to mean that many wood-boring pests of cold northern climes will become more destructive, since higher temperatures will increase winter survival and possibly enable an extra generation in the summer.

Ironically then, northern forests, seen as a bastion against climate change, will become more threatened by it.

Most imminent threat - wheat stem rust strain Ug99

The Puccinia graminis tritici strain of wheat rust was discovered in Uganda in 1998 and has subsequently spread across Africa, Asia and the Middle East.

In fact, seven races of the Ug99 lineage have now been identified.

New resistant varieties that yield more than current popular varieties are being released and promoted, but a major effort is needed to displace current susceptible varieties with those that have durable resistance.

Most resilient pest - colorado potato beetle

Leptinotarsa decemlineata is a strong candidate for this award, having managed in the space of about 50 years to develop resistance to 52 different compounds belonging to all major insecticide classes (including cyanide).

This beetle therefore has effectively beaten the chemists.

Wherever possible, biocontrol (control by natural enemies) should be part of the strategy because the predator-parasite can more easily keep up in this arms race, one which humans have so palpably lost.

Next steps

The above is at best a very partial list of serious agricultural pests. At any one time, because of weather conditions, mutation to a virulent form, or emergence of resistance to chemical control, a pest will surge into prominence unexpectedly.

What we need is better monitoring and recording of pests in order to alert authorities to take early action, something we at Cabi are very keen to promote through our Plantwise initiative.

Coffee wilt disease, mentioned above, is a case in point - regular monitoring and rapid action could have halted this disease at a cost of a few million dollars at the most.

Instead, we reckon it has now caused at least a billion dollars of lost earnings to African coffee farmers, and it is still spreading.