Tipula spp. pest status in Irish and Scottish agriculture: new insights into a long established pest

Abstract

Within the last decade, a record number of chemicalinsecticides have been withdrawn for agricultural use within the EU. Thisfollows the heightened awareness regarding the health and environmental risksassociated with their use. To help farmers combat pest problems and maintainagricultural yield in the absence of chemical control measures, effectivesolutions are needed. For many pests we need to review the control managementstrategies available to farmers, from an integrated pest management (IPM)perspective.

Tipula larvae have beenstudied in many areas globally, as they cause significant damage in multipleindustries including amenity grasslands, turf sod production and agriculture.Two species are known to cause significant agricultural damage; Tipula paludosa and Tipula oleracea. Both actively feed on the roots and shoots ofgrasslands and cereals, severing the plant at key developmental stages andreducing the overall yield. The lifecycle of each species differssubstantially. Tipula paludosa canspend up to 11 months in the soil, actively feeding, whereas T. oleracea is bivoltine, having two larvalcycles of approximately four months each. Therefore correctly identifying thelarval species, its ecology and lifecycle is vital from a pest managementperspective and should always be the first step when designing an IPM approach.

Previously, annual surveys of leatherjacket occurrence inagricultural settings were conducted in England (1940s – 1970s), NorthernIreland (1970-1980s) and Scotland (1980s-2000s). No surveys of this kind havepreviously been conducted in the Republic of Ireland. As part of this thesis,the first national survey of farms across Ireland was undertaken to determine(i) the Tipula larval species ofagronomic importance, (ii) larval distribution in grasslands and cereals and(iii) the soil factors that limit larval survival/occurrence. These findingswere compared to Scottish grassland sites sampled in the same years. Annualsurveys have been ongoing in Scotland for over four decades, providing along-term dataset to compare with Irish data and analyse for reoccurring trendsand population fluctuations. Ireland’s nationalsurvey outlined that grassland farmers were disproportionally affected byleatherjacket infestations, with over 40% of sites in 2019 being above thelatest economic threshold set for grasslands. Cereal fields generally supportedlower densities due to soil cultivation, which is known to reduce larvalpopulations by 50%.

The vast majority of larvae collected across Ireland andScotland were Tipula paludosa, with approximately74% and 91% of samples being identified in each country respectively. Thisspecies has a longer soil dwelling phase, but passes through the mostvulnerable instar phases within two months, showing a need for fast actingcontrol options. Populations in grasslands ranged from 0 to 3,408,750/ha inIreland and 0 to 2,525,000 larvae/ha in Scotland. This was substantially higherthan the maximum population in cereal fields across Ireland (807,660/ha). InIreland, strong positive correlations were observed between larval populationsand sodium (Na) concentration in the grassland soils. This correlates withhigher larval populations occurring along the western seaboard, where sites areexposed to ocean spray drift. Negative correlations were present between larvalcounts and manganese (Mn), possibly relating to differences in managementintensity between sites.

Collaboration with SRUC allowed for linear mixed effectmodelling of leatherjacket survey data collected from Scottish grasslandsacross ten years, 2009 to 2018. Field size had a negative correlation withlarval populations, along with sheep grazing, when compared to no grazing, andprior use of pesticide. It was incorporation of five silage cuts compared tonone that correlated positively with larval counts, emphasising the benefit oflong grass for the gravid female cranefly. Through calculating farm intensityindexes using the suite of variables collected at farm level, a negativecorrelation between high farm intensity and larval counts was observed.

Approaches to control leatherjacket pests were investigated,including the use of different grassland sward cultivars. The aim was identifyoptimum grassland sward compositions for highly infested fields, developingeffective sward management advice. Larval feeding experiments were conductedusing reared T. paludosa larvae andseven different sward types; six plant monocultures and a multi-species sward.Comparisons of larval survival, yield loss and overall sward performance weremade. Larval feeding led to significantly decreased shoot yields (p<0.001)in the white clover swards, while diverse swards were more tolerant. In themultispecies sward, root biomass increased significantly (p<0.001) underlarval feeding, indicating a compensatory effect between the diverse plantspecies.

The soil microbiomes collected from Irish and Scottish surveysites were analysed to provide further context into soil environmentsassociated with larval occurrence. Complex relationships between leatherjacketoccurrence, soil conditions and microbial communities were noted. Soil propertiestied to larval occurrence from the two country leatherjacket survey were alsonoted to be significantly correlated with the structure diversity of bacterialand fungal communities. This highlights the prominent role played by farmmanagement and soil amendment on both larval and microbial occurrence.

It is crucial to thoroughly understand the biology andlifecycle of any pest before implementing management strategies. While thebiology of T. paludosa has beenwell-documented, there is currently limited control options for growers.Through investigating the role of soil properties, soil microbes and grasslandcultivar choice, I aim to provide farmers with an updated IPM toolkit againstthis pest.