Records of crop damage caused by Bibionidae are reviewed together with evidence for their beneficial activities, natural enemies and control methods. Bibionids are sporadic and infrequent pests. Although grass and cereals are most vulnerable, a wide range of crops is attacked. Damage is most severe after grass or when plants are under stress. The presence of organic matter during ovjposition increases the likelihood of damaging populations. Many natural enemies exist but their impact on adult or larval populations has not been quantified. Given the pattern of attacks and lack of approved insecticides, cultural control methods are more appropriate than chemical. The role of adult bibionids in pollination and of larvae involved in soil processes could be of greater importance than any damage caused by the group.

The distribution of bibionid larvae in agricultural grassland was investigated. They tend to occur sporadically and at low population density. More detailed sampling within a field showed that numbers were highest adjacent to a hedgerow with most being found 6–11 m from the hedge. Log variance/log mean relationships were established for counts of Bibio johannis and Dilophus febrilis larvae collected from soil cores in grassland. Both species were highly aggregated. An area of grassland (160 cm × 80 cm) was divided into 128 soil blocks and B. johannis larvae extracted. Almost all larvae were associated with aggregations. The two‐phase mozaic method of Pielou (1974) was applied to larval counts from different sized concentrically collected soil cores. Larval patches were not randomly distributed. Time‐lapse video studies of bibionid larvae were made and showed both aggregation and processional behaviour. This was associated with chemical stimulation which is probably used to maintain larval groupings in the field. There was evidence for two levels of aggregation. The first is due to groups of larvae arising from eggs laid by a single female and the second, larger scale, aggregation is due to factors that influence the distribution of adult flies.

Three extraction methods were compared for the recovery of bibionid larvae from grassland soil samples. Only wet-sieving followed by flotation in a saturated salt solution yielded bibionids. No larvae were recovered either with modified Tullgren funnels or by slow immersion of soil cores into a saturated salt solution. The efficiency with which larval bibionid populations can be estimated is poor. Generally, smaller cores yielded more larvae per volume of soil. Most bibionids were found in the top 4cm of soil. It is concluded that 10cm diameter and 6-8cm deep soil cores are an acceptable compromise between efficiency and sampling effort but sample size will largely be determined by the resources available for processing of samples.

SummaryBibionid larvae were extracted from soil samples collected from June 1985 to March 1987. Each larva was identified, weighed and measured for maximum head capsule width. Both Bibio johannis and Dilophus febrilis have six larval instars. Bibio johannis grew slowly in late summer and early autumn but rapidly from October to January. Early and late instar larvae of D. febrilis occurred simultaneously and it was concluded that this species was not bivoltine but may exist as two separate cohorts with distinct flight periods.

Bibionid flies were sampled in grassland in Northern Ireland from 4 April to 13 June 1985 using water and sticky traps with a range of characteristics. Nearly 70 000 bibionids were taken, of which 89-5% were males. Bibio johannis (L.) accounted for 96% of the catch, the rest comprising Dilophus febrilis (L.) (3%) and D. femoratus Meigen (1%). Trapping was carried out adjacent to a hedgerow and in the centre of the field. Differences in the abundance of the flies at these two locations occurred for all three species. Yellow was the best colour for maximizing catches. Greater numbers of B. johannis (63%) were taken in water than sticky traps, but over 76% of Dilophus spp. were caught on the latter. Increasing the diameter of water traps tended to enlarge the catch (principally B. johannis), but catch per unit area was relatively constant. The height of sticky traps above the ground affected the catch of all three species, indicating different flight levels.