The mangold fly (Pegomya hyoscyami) is an occasional pest of beet crops. The larvae mine extensively within the leaves, producing characteristic blisters and reducing green leaf area. This reduces plant vigour and can, in severe infestations, kill young plants. The adults usually first emerge in April or May, laying eggs on the underside of leaves. Depending on conditions there can be two or three generations per year.
Current neonicotinoid seed treatments provide good control of early infestations but in recent years damaging invasions have occurred after the seed treatments have lost their efficacy. These later infestations usually occur in August and September when the third generation emerges, however the second generation is also thought to have caused significant damage in the last 2-3 years.
While this pest tends to affect a relatively small area of sugar beet production ( approximately 1-2%) each year, fields that are affected can lose up to 50-70% of the crop canopy. This is very likely to have a significant impact on yield, although the precise relationship between canopy loss due leaf miner and yield is not known. The impact of the miner can also cause additional operational costs as a result of disruption to the harvesting schedule, as affected fields often need to be harvested first because damaged crops have less photosynthetic potential and are prone to frost damage.
If the use of neonicotinoid seed treatments were restricted due to concerns about effects on non-target organisms, damage from earlier generations may once again assume greater importance (Dewar, 1992). Young plants are more vulnerable to leaf mining as they less able to tolerate the loss of leaf area. Additionally, the blisters resulting from the feeding damage increases the risk of damage from herbicides.
To control infestations of leaf miner using foliar insecticides, treatments need to be applied before larvae enter the leaves. This means sprays must be accurately timed to occur after egg hatch and before the larvae begin feeding within the leaves. The primary aim of this project is develop an egg hatch prediction scheme to enable growers to time sprays more precisely against later generations of the pest. Experiments will be
conducted in controlled environments investigating the influence of a range of constant temperatures on egg development. Data from these experiments will be used to develop a day-degree model to predict the time from oviposition until egg hatch. This will provide growers with a tool to assist in timing sprays more precisely to coincide with the egg hatch of the pest. Additionally, if neonicotinoid seed treatments were to
become unavailable, growers also need to time sprays against the particularly damaging first pest generation. In this case, the egg hatch prediction scheme could potentially be adapted to assist in targeting early infestations.
Outcomes / Key Message For Growers And Industry
In 2015, Dursban WG was shown to provide the best control of mangold fly and a significant yield improvement compared to the untreated control plots. However, regulatory restrictions were introduced in 2016 that meant this insecticide could not be used in sugar beet. Hallmark Zeon was shown to be capable of providing significant reductions in pest damage.
In 2016, three experimental products provided consistent control of mangold fly but no yield response was found, possibly due to low levels of pest infestation in the untreated control plots.
Adult activity centred on the Wash in both years but was worst in south Lincolnshire and coastal north Norfolk in 2016.
Patterns in the timing of adult activity differed widely between 2015 and 2016, likely due to differences in weather and environmental conditions.
Large variation in natural enemy numbers were found between sites and across the season.
Hallmark Zeon has been shown to be effective at reducing damage from mangold fly but sprays need to be timed accurately to target hatching larvae before they enter the leaf. This is difficult to achieve without close monitoring of the pest.
A number of experimental products show good potential for providing better control of mangold fly other than Hallmark Zeon but further work is needed to confirm this.