Individual study: The effects of mowing regime on the weevil Zacladus geranii (a beetle dependent on ripening seeds of crane’s-bill Geranium spp.) on a roadside verge near Spaldwick, Cambridgeshire, England
Davis B.N.K. (1973) The effects of mowing on the meadow cranes-bill Geranium pratense L., and on the weevil Zacladus geranii (Payk.). Journal of Applied Ecology, 10, 747-759
Throughout the UK, many plants characteristic of semi-natural grasslands are now frquently confined to road verges. This study conducted in southeast England, aimed to assess the short-term effects of a range of cutting regimes on the performance of a moderately common roadside herb, meadow crane’s-bill Geranium pratense, and on the weevil, Zacladus geranii which depends on its ripening seeds. The weevil is restricted to Geranium and is associated (especially with large-flowered species such as G.pratense). The weevil lays eggs from the beginning of July and their larvae feed on the developing ovules.
A length of road verge (c.8 m wide x 170 m long) was selected along a main road (A604) near the village of Spaldwick (National Grid. ref. TL 138724), west Cambridgeshire. The verge was divided into three sections, and plots were subject to one of six cutting treatments:
T1 - uncut;
T2 - cut once on 3 August;
T3 - cut once on 24 August;
T4 - cut twice (18 May and 13 July);
T5 - cut twice (8 June and 3 August);
T6 - cut twice (18 May and 28 June).
Sampling for adult weevils was undertaken weekly from 19 April using a Dietrick vacuum net. A sample was taken for dissection to determine the sex ratio. Eggs and larvae sampling was undertaken by collecting flowers and seed capsules from each plot every week. Eight flowers or capsules were taken from 1 m² quadrats, selected to represent the range of maturity present in each. On 2 and 23 August, about 200 capsules were taken from each plot about to be mown. From these, 20 per plot were selected at random to estimate capsule size and the proportion showing weevil feeding damage. The capsule length was measured when an egg or larvae was found.Larval age was determined by head measurements of preserved specimens.
The appearance of large numbers of adult weevils was synchronized with Geranium flowering. The first adult was seen on 3 May but numbers in the suction samples remained low until the end of June when 42 were collected. Catches of 20 or more were recorded until 10 August after which numbers fell to zero within two weeks. Between 3 May and 16 August, 61 males and 41 females were collected.
From the 913 seed capsules examined between 5 July and 30 August, 330 eggs and 169 larvae were found. No eggs or larvae were found in a further 85 capsules in September. Eggs were often laid in flowers but were mostly found in capsules of all sizes. Larvae were rarely found in capsules shorter than 18 mm long. On 2 August (20 flowers and capsules taken from each plot scheduled for mowing) proportions containing eggs or larvae were 35% for T2 and 53% for T5. On 23 August, 37% of flowers and capsules from T3 contained weevils. The numbers of developing weevils found were 30, 42 and 27.
Egg-laying probably began in late June. About 4 weeks are required for completion of larval stages, the same time as for seeds to ripen. Mature larvae pupate in the soil.
Few flowers or capsules were evident for a week or more after cutting, thus restricting or preventing egg-laying. Presumably, those eggs/larvae present in flowers/capsules died upon cutting, except perhaps for mature larvae about to pupate. All six mowing regimes adversely affected larvae numbers, including the latest cut (24 August) when few unripe capsules were left. Few eggs or young larvae were present at this time but it is uncertain how many of the numerous third-instar larvae could survive mowing and complete development. Less than 50% had reached the third instar by the first week of August, so that plots cut at this time (T2 and 5) would produce few pupae. The plots cut on both 18 May and 13 July (T4) yielded only a few final-instar larvae at the end of August. Those cut on 18 May and 28 June (T6) had none.
Conclusions: Mowing in May or early June had little or no effect upon crane’s-bill flowering whereas cutting at the end of June virtually eliminated flowering. Progressively later cutting allowed increasing amounts of flowering and ripening of seeds. Although crane’s-bill capsules were available for about 10 weeks (beginning of July to the beginning of September), the numbers of final-instar Z.geranii larvae peaked only towards the second half of August. As the majority of larvae do not pupate until this time, some areas of crane’s-bill should be left uncut between June and the end of August to maintain weevil numbers.