The epidemiology of a new leatherjacket pest (Tipula oleracea) of winter cereals in northern Britain

Abstract

Leatherjackets are the larvae of craneflies, or daddy long-legs, and are known pests of spring sown cereals in northern Britain. The factors causing damaging populations are well established. The timing of the crop is such that seedlings coincide with the later stages of larval growth when consumption per leatherjacket is at its maximum and so relatively few leatherjackets (>280,000 ha-1) can cause yield loss that is worth controlling with an insecticide. The biology and behaviour of the species involved, Tipula paludosa, favours existence in a stable habitat such as grassland which allows numbers to increase over a few seasons. Thus, generally, only those spring sown crops which follow grass are likely to have leatherjacket populations above the economic threshold.

In the mid 1980s a number of farmers reported autumn damage by leatherjackets in winter cereal crops, sometimes leading to crop destruction. There were a number of features of these attacks that suggested that T. paludosawas not responsible. Prominent amongst these was the observation that relatively large larvae (third and fourth instars) were to be found in November; at this time of year T. paludosa larvae are in their second instar. Additionally, very large populations (> 2m ha-l) were recorded and these were unexpected in cereals, even though such numbers are sometimes seen with T. paludosa in grassland. There was thus reason to believe that the attacks on winter cereals were not attributable to T. paludosa.

A survey of crops with reported autumn leatherjacket damage revealed that 84% had been preceeded by a crop of winter oilseed rape. This argued strongly that an explanation for this new pest problem should be sought in a study of the interactions of leatherjackets with rape rather than an investigation of their effect on winter cereals. Such a study was complicated by the inability to consistently distinguish between leatherjackets of the closely related species T. paludosa, T. oleracea and T. subcunctans. A means of reliably identifying larvae was a precursor to understanding the problem.

Thus the project set a number of objectives:

To differentiate between agriculturally important leatherjackets at all stages of development.
To develop easy and reliable sampling methods for leatherjackets within cereal crops and stubbles.
To investigate records of the occurrence of leatherjackets and leatherjacket damage in winter cereals and identify predisposing factors.
To investigate the influence of oilseed rape upon leatherjacket population dynamics.
To elucidate the annual cycle of the new pest species.
A study of protein bands from eggs, larvae, pupae and adults of T. paludosa and T. oleracea using isoelectric focussing showed that there was a consistent difference between the two species. This held true for all stages, irrespective of whether specimens contained food or were parasitised, and the technique worked for specimens collected from regions of Europe. Under this system, T. subcunctans could not be separated from T. paludosa but this was not important since this species is rare and overwinters as eggs; the lifecycle is sufficiently different that it cannot be implicated in damage to either winter or spring cereals.

Application of the isoelectric focussing method to leatherjackets recovered from winter cereal crops proved that they were T. oleracea and confirmed that T. paludosa was not responsible for the damage.

It was thought that it may be necessary to be able to identify leatherjackets in the field. Attempts to use the difference in proteins between the two species to develop an antibody- based system for this purpose were, however, unsuccessful.

A series of 15 experiments was made to compare different sampling methods for use in arable crops. The most effective and easiest to use was brine flotation. This technique involves the insertion of 10cm diameter plastic pipes into soil and the application of brine solution. Any leatherjackets encompassed by the pipes float to the top where they can be counted. Brine flotation is unreliable on cultivated soils but is effective later in the season when soils are more compact.

Leatherjacket distribution studies were carried out in Scotland and Northern Ireland. They showed that whilst T. paludosa was easily found in agricultural land, T. oleracea is only rarely encountered as larvae. It was not possible to discover the larval habitat of T. oleracea. The occurrence of this species in dense populations, as seen in winter cereals, is therefore unusual.

Samples collected from winter oilseed rape and nearby fields of winter wheat yielded T. oleracea larvae in 20% of the rape fields but none in the wheat during November. In the spring, leatherjackets were found in the wheat but more rape fields contained them and by September they were detectable in 73% of the rape fields. Numbers of leatherjackets recovered over this sampling period increased in the rape but not in the wheat.

Laboratory studies showed that T. oleracea larvae could survive on a diet of oilseed rape and would grow faster on rape than if fed wheat.

By the time of the first flight period of the year for T. oleracea (May/June), the rape canopy has closed over. Field experiments showed that the canopy impeded the vertical movement of adult craneflies but not horizontal movement. The rape crop therefore prevented the normal dispersal behaviour of T. oleracea.

These results provide an explanation for the new pest problem of leatherjacket damage to winter cereals. The female T. oleracea is a relatively strong flier and chooses to lay eggs in seedling rape crops in the autumn. The eggs hatch and the larvae are able to thrive within the crop. By the time they are full grown and pupate, the rape canopy is sufficiently developed to prevent emerging adults escaping from the crop; they are therefore obliged to mate and lay their eggs beneath the canopy. Thus a second generation occurs within the rape crop and these are the leatherjackets that are present, often in large numbers, when the following winter cereal is drilled.

This new leatherjacket pest has arisen because oilseed rape has been introduced into arable rotations and this is the key to understanding the problem. By the time that leatherjacket damage is seen in the winter cereal, it may be too late to effect control. Farmers are therefore recommended to sample rape stubbles before cultivations begin. The brine flotation method provides an easy to use and robust sampling technique with which to estimate leatherjacket numbers. Adopting this approach anticipates leatherjacket attack and maximises the time available to apply control measures.