Companies such as BASF, Dow and Syngenta have "struggled to get new chemistry through the new regulatory process, and due to these regulatory difficulites many of these companies are reluctant to prioritise registrations of new material in Europe over other parts of the world, never mind Ireland," says Dr Hennessy.
"It's a double whammy: the EU declassifying chemistry on one side, and not many new products coming to the market on the other."
The winter of 2018 was the last time cereal growers here could use neonicotinoids, an insecticide effective at tackling aphids on cereal crops. The EU ban came into place due to environmental concerns about their impact on bees.
Experts warn that the move steps up the risks of a considerable fall in yields of wheat and barley.
"Neonicotinoids were vital for the control of harmful insects such as aphids that lead to some of the viruses in crops," says agricultural consultant Pat Minnock.
"There is no replacement being brought in, so there is going to be a major problem for the production of cereals in this country."
British researchers predict that wheat farmers could be looking out at yellowing fields of winter wheat by February and March next year.
The concern is that a lapse in controlling aphids will lead to the spread of barley yellow dwarf virus, which can slash yields significantly.
"Next year (2019) could be the last good year for European wheat harvests for some considerable time," Dr Kim Hammond-Kosack, senior scientist at Rothamsted Reasearch, warned in November.
High aphid numbers can spread barley yellow dwarf virus (BYDV) on winter cereals, which if untreated can reduce yields by 40 to 50pc for a grower.
One step growers can take is to plant winter cereals later in the year, to avoid the flying aphids, but this delay can mean planting in trickier weather or soil conditions and result in patchier crop and lower yields.
"The break-even yield for winter wheat here is around 8.4 tonnes per hectare. Every tonne above this helps pay for overheads, and profitability, so a tillage famer has to achieve high yields in Ireland to make money," says Dr Hennessy.
This new situation without neonicotinoids is going to be difficult, but not impossible, he adds.
The big unknown is what will happen to the aphid population when neonics are removed; control of aphids relied on a combination of neonics and pyrethroids, which are sprayed onto crops.
Teagasc estimates that perhaps 40 to 50pc of the aphid population is resistant to pyrethroids, but crop scientists are unsure what will happen to resistance once pyrethroids alone are sprayed.
The worry is that if resistance increases and there are no other useful tools to control aphids, growing some cereals may become impossible.
"This is always the problem of getting rid of potential pest control tools. You become over-reliant on one or two groups of chemistries to control the pest - we essentially are limited to one group of aphicide for aphid control in cereals now," says Dr Hennessy.
"The more often you use any pesticide, the quicker resistance will build up in the pest population, and over time the control measure may become ineffectual."
Careful use and a combination of other control measures can prolong a pesticide's usefulness.
One alternative is to breed crops resistant to BYDV, but it can take seven to eight years to bring a new variety to market. Scientists across Europe are working to integrate resistance/tolerance to BYDV into cereals, says Dr Hennessy.
Teagasc too is working with on a number of research projects with commercial companies to develop improved breeding populations for cereals that are more suitable in an Irish farming context.
Irish tillage farmers especially must worry about fungal diseases. Our wet maritime climate offers ideal conditions for them to proliferate.
The biggest threat to winter wheat in Ireland is the foliar fungal disease Septoria tritici, which can cause yield losses ranging from 30pc up to 50pc. The infection reduces photosynthesis in the leaf layer, impairing growth and grain fill.
A key fungicide to control septoria is chlorothalonil, a broad-spectrum fungicide that is the active ingredient of Bravo.
Syngenta, which first brought chlorothalonil to the Irish market, told a Teagasc conference last September that the re-registration process for the chemical was becoming more and more difficult.
Dr Steven Kildea, plant pathologist at Teagasc, described chlorothalonil as essential for septoria control.
A Teagasc study found that wheat not treated by chlorothalonil could take a yield hit of around 2.5 t/ha - but up to 5/ha - with resulting reduction in profitability of over 60pc from the crop.
"Essentially the wetter the weather, especially around May, the more septoria you are going to have. In our climate we have moderate to high levels of septoria every year in Ireland," says Dr Hennessy.
In December, the European Commission postponed a decision until March to re-register the fungicide.
"If you cannot control septoria on wheat, you can lose half your crop. So why would you grow it?" asks Pat Minnock.
The margins for cereals crops are tight, so substantial yield declines are not going to cut it with growers.
In the past, chlorothalonil was used in a fungicide mix to best control the disease. Often it is used with the azole and SDHI fungicides.
"We try to use multiple-action fungicides. By having a diverse fungicide mix on crops, the more chance you have of avoiding potentially resistant strains of a fungus," says Dr Hennessy.
Chlorothalonil has been increasingly important in direct disease control and for maintaining yields, however. For this reason, Teagasc recently looked at the impact of its removal on wheat and barley (see box).
Another devastating fungal disease of cereals is ramularia, which starts as small brown spots and hits barley yields especially hard.
"It is very difficult to control," notes Pat Minnock. "Chlorothalonil is a product that gives some beneficial effects, though."
The disease is not such a big deal across in the rest of Europe. Most problems are in Ireland, Scotland, and parts of southern Germany.
"It is very difficult for a small country like ours to keep necessary tools vital for Irish agriculture, compared to a big country like France," says Dr Hennessy, who sees another downside to Brexit here.
"We lose a close ally who is looking for the same types of ag-chem tools, as we share similar weather conditions, disease and produce similar product labels to the UK.
"It is possible there will be less interest by ag-chem companies in bringing new products to a small market like Ireland."
'It's madness - they are tying farmers' hands in knots'
An alternative strategy to using ag-chemicals is to breed more disease resistant varieties, something Teagasc has been doing that for wheat and barley, particularly with a view to increase resistance to septoria and ramularia.
"We have been working hard to identify pre-breeding lines for companies, but this takes years," says Dr Michael Hennessy, head of crops knowledge transfer at Teagasc, as it can take seven to ten years for a breeding programme to succeed.
The situation has been made worse, say many crop scientists, by the decision last July to place gene editing under the same strict regulations as GMOs.
"This was just madness," says Dr Hennessyof the gene editing decision. "They are tying the hands of farmers in knots in terms of producing food."
He points also to the situation with glyphosate:, the active ingredient in the Roundup product. It has become controversial, with links to cancer in the public's mind.
"Everyone has their mind set against it, no matter what information comes out. All the reports I've read, bar one which on my reading has been discredited, said it is one of the safest herbicides available," says Dr Hennessy.
Last November, EU member states agreed on a five-year renewal period. If removed, the notice period for growers would likely be less than two years. If the EU moved against glyphosate, the import of food and fibre grown using the herbicide would be unlikely to be banned given how widely it is used outside of Europe.
"The rules are farcical," says Dr Hennessy. "On one side they want to ban it. On the other side, they want to import (foods made with) it."
Europe has a huge and expanding deficit in protein production, to feed both animals and people. The majority of that protein is grown in South America, which is not subject to the same regulatory regime as European farmers.
This apparent disconnection between various parts of EU regulations will then become more transparent and perhaps lead to very difficult decisions about these imports in future, Dr Hennessy warns.