Scientists from Scotland's Rural College (SRUC) are warning that this spring there is not only a high risk of damage to spring cereals from Leatherjacket grubs, but also concerns that infestation will damage fields remaining in permanent pasture.

Leatherjacket is the name given to the grubs of the crane fly or "daddy-long-legs". They are up to 50mm long, with tough, rubbery, dark olive skins and no obvious head and no legs. They live just below soil level and each year eat the roots of grasses and other plants from August through to June. When larvae numbers reach 1 million per hectare they can nibble as much as 2.5 tonnes of dry matter per hectare.

SRUC has conducted an annual survey since the mid 1970s. Grub densities fluctuate from year to year, but over the past 20 years they have consistently risen higher, linked, it is believed, to climate change and wetter, milder autumns.

Leatherjacket populations in many of Scotland's fields have, as predicted, continued to build from the low levels observed two years ago and the medium levels seen last year. According to the latest annual survey, undertaken in west and central Scotland between November 2016 and February 2017, the average density recorded across the fields in the survey was 1.6m grubs per hectare.

As a youngster I remember my father mixing a stomach poison insecticide with bran, to act as a bait, and then spreading it on a field of oats that had become badly infested. While I remember my father considered he had been successful in controlling the infestation with what was probably fenitrothion, such chemicals are now banned.

Early insecticides such as DDT and gamma-HCH were also effective against leatherjackets, and a wide range of other insect pests, but their use was banned because of the long-term persistence of these chemicals and their breakdown products in the environment.

Spraying chlorpyrifos, an organophosphate chemical, was the chemical control method of choice for decades since, provided it was applied at the right soil temperature, it could be relied upon to reduce grub densities markedly irrespective of soil type. However, concerns about human health safety levels led to the ban of the chemical last year. Paradoxically, the UK is the only country in the EU to do so, despite having the biggest problem with leatherjackets.

Chlorpyrifos is an organophosphate insecticide that originates from nerve gases developed during WW2. It is acutely toxic and can cause systemic illnesses to people not wearing protective clothing, by inhibiting the body's ability to produce cholinesterase, an enzyme necessary for the proper transmission of nerve impulses.

UK farmers now have no economically viable control method other than rolling - which is largely ineffective - and ploughing followed by thorough cultivations that can reduce populations by 50 per cent. Birds like rooks and starlings feed on the grubs, but can't be relied on to control severe infestations.

There are of course other biological control methods. Some initial trials with spraying garlic conducted by the Soil Association have shown some potential, but these will need to be repeated and "costed" in different field situations with different starting densities of grub before they can be considered a viable method of control in some situations.

Another method of combating leatherjackets biologically is to use insect-parasitic nematodes such as Steinernema carpocapsae. After application, the nematodes go in search of the leatherjackets and enter them through naturally occurring body openings. Under favourable circumstances the leatherjackets die within a few days. Unfortunately, while parasitic nematodes may be of use to those with lawns, they are too expensive for field-scale operations.

The problem of controlling leatherjackets is a prime example of the effects of politicians withdrawing plant protection products (PPP) from the market. That's a result of the EU introducing hazard-based regulation in 2009 to replace the previous risk-based regulation. The effect of this has been a massive withdrawal of existing PPP from the EU, and a halving of the number of new active ingredients under development.

In the past these substances were poorly regulated, but they are now highly regulated throughout the EU. Indeed, many argue that EU regulation is now choking off development of new PPP and thereby risking the continued productivity of our farms.

Not only do farmers now have fewer tools to deal with pests and diseases, those that remain are at greater risk of being less effective, as pests and diseases develop resistance to them through greater exposure.