P. J. Murray
Institute of Grassland and Environmental Research, North Wyke Research Station,
Okehampton, Devon, EX20 2SB. UK.
Introduction
Materials and methods
Results
Discussion
Acknowledgement
References
Weevils of the genus Sitona are among the most common and injurious pests of white clover in the UK and can cause severe damage (Mowat and Shakeel, 1989; Clements and Murray, 1991; Lewis and Thomas, 1991). One of the most widespread species in the UK is Sitona flavescens which is distributed widely in the grass growing areas of the country. The adult weevil feeds on the leaves of the plant, cutting the commonly-seen 'U'-shaped notches in edges of the leaf. Previous work (Murray and Clements, 1994) investigated feeding preferences of Sitona spp. for a range of legumes, and Murray and Clements (1993) showed differences in preference among white clover varieties. While the previous work investigated what were essentially antixenotic effects of the plant to the weevil, this present work investigates antibiotic effects of a range of legumes in terms of fecundity and survival of the weevil.
Feeding preference tests. Adult S. flavescens were collected, by sweep-netting, from a pasture of perennial ryegrass/white clover. The weevils were kept in a refrigerator at approx. 5°C until required. Prior to the start of each test the weevils were removed from the refrigerator and starved for 24 h.
Plants of Trifolium repens L., T. hybridum L., T. pratense L., T. fragiferum L., T. incarnatum L., T. subterraneum L., T. dubium Sibth., Lotus corniculatus L., L. uliginosus Schkuhr, Melilotus alba Medicus, Medicago sativa L., M. lupulina L., Pisum arvense L., Vicia sativa L., V. faba L., Coronilla varia L., Anthyllis vulneraria L., Ulex europaeus L., Cytisus scoparius L.(Link), Phaseolus vulgaris L., and P. multiflorus Will., were raised from seed, in potting compost, in a glasshouse at 20°C. The plant species were chosen to cover a range of legumes which were either crop plants or commonly found pasture species which were possibly capable of supporting Sitona feeding.
In no-choice tests ten replicates of each plant species were set up by placing either a single leaf, a pair of leaflets in the case of V. sativa and C. varia, or in the case of V. faba and Phaseolus spp. a disc cut from a leaf, on moistened filter paper in a 90 mm diameter plastic petri-dish, together with one adult weevil chosen at random from the captive population. All the plant material was of a similar physiological age and morphology. To determine the fresh weight of leaf material consumed, five 50 mm2 sections of leaf were removed from each legume species and weighed immediately using a microbalance. The dishes were kept in a constant temperature room at 17°C. After 60 h the weevils were removed, and by placing a transparent grid divided into mm2 over the remains of the leaf, the area of leaf consumed was estimated, and the fresh weight consumption of plant material calculated.
In choice tests leaf material was used as before. A single leaf, leaflet or leaf disc from each plant species were all placed together, and arranged randomly on moist filter paper in a 120 mm diameter glass petri-dish. Five weevils, chosen at random from the captive population were placed in each dish, and ten replicates were established. The dishes were kept under the same conditions and time period as in the no-choice tests. The amount of leaf material consumed was estimated as in the no-choice experiment. All results were analysed using ANOVA.
Fecundity tests. From the captive population of S. flavescens two males and two females were placed on moist filterpaper in a 90 mm plastic petri-dish to which was added an adequate supply of food material. For each plant species five replicate dishes were established. The dishes were kept under the same conditions as in the feeding choice tests. The eggs produced were collected for six days and transferred to clean petri-dishes and incubated at 20°C for 48 h. Eggs of Sitona spp are a whitish yellow when first laid and if fertilized there is a colour change to black within 48h (Jackson, 1920). By incubation it was possible to differentiate between fertilized and unfertilized eggs. Data from this test was analysed by ANOVA.
Longevity tests. Longevity tests were conducted under the same conditions as the feeding preference tests. Adult weevils, chosen at random from the captive population, were placed on moist filter paper in plastic petri-dishes, five to a dish. Food material of one of the legume species was added. Five dishes for each plant species were set up and monitored daily. Uneaten food material and dead weevils were removed daily and the food material replenished. This procedure avoided any weevil mortality due to fungal infection or starvation. The number of dead weevils was recorded daily and the results analysed by the GENSTAT procedure CUMDISTRIBUTION (Brain and Butler, 1988) which fits frequency distributions to cumulative counts. From this analysis it was possible to calculate the time taken for 50% of the population to die (LT50).
Feeding preference tests. In the no-choice tests feeding on Trifolium spp. was significantly (P < 0.001) greater than on all the other legume species, apart from one T. dubium, where there was only a trivial intake of plant material by the weevil. Within Trifolium spp. the intake on T. fragiferum and T. repens was significantly greater than T. incarnatum or T. subterraneum. Over 84% of the total intake over all legume species was of Trifolium spp. In the choice tests a similar pattern of feeding preferences was found. T. fragiferum was preferred significantly (P< 0.001) to any other legume. T. hybridum and T. repens were equally preferred. Little evidence of feeding was seen on any other species. Some 88% of the intake was just from the three species listed above, and 98% was from all species of Trifolium. The total intake per weevil per dish was 0.43 mg h-1. There was a highly significant correlation (r = 0.870, P < 0.001) between intake in the two feeding tests (Table 1).
Fecundity tests. Significantly (P<0.01) more eggs per day were laid by weevils feeding on T. repens, T. hybridum T. fragiferum, P. vulgaris and V. saliva than on the other species of legumes. 58% of the eggs were laid on Trifolium spp. There was no correlation between the number of fertile eggs and unfertile ones. There was, however a significant correlation between the number of fertile eggs and the food intake in both of the feeding tests (r = 0.865, P < 0.001, choice; r = 0.700, P < 0.001, no-choice) (Table 1).
Longevity tests. The LT5o survival times for the weevils on the various legume species are shown in Figure 1. Weevils on T. hybridum, T. fragiferum, T. repens and L. corniculatus survived longest. Shortest LT50 times were on T. dubium, M. saliva, C. scoparius, U. europaeus and C. varia. The LT50 times were significantly correlated with food intake in the choice (r = 0.462, P < 0.05) and no-choice (r = 0.527, P < 0.05) tests.
Table 1. Food intake (mg fwt. weevil-1 h-1) in choice and no-choice feeding tests; number of fertile eggs and LT50 times of Sitona flavescens fed a range of legume species. Numbers followed by different letters are significantly different (P < 0.001)
|
|
No-choice mg weevil-1 h-1 |
Choice mg weevil-1 h-1 |
Fertile eggs/ |
|
Trifolium fragiferum |
0.327a |
0.148a |
7.4ab |
|
Trifolium repens |
0.321a |
0.117b |
13.0a |
|
Trifolium hybridum |
0.222ab |
0.116b |
13.0a |
|
Trifolium pratense |
0.184b |
0.023c |
1.8b |
|
Trifolium incarnatum |
0.182b |
0.001cd |
0.4b |
|
Trifolium subterraneum |
0.138b |
0.006cd |
0.2b |
|
Phaseolus multiflorus |
0.064c |
0d |
1.8b |
|
Lotus corniculatus |
0.043c |
0d |
1.8b |
|
Pisum sativum |
0.039c |
0d |
2.4b |
|
Lotus uliginosus |
0.029c |
0d |
0.8b |
|
Melilotus alba |
0.026c |
0d |
1.6b |
|
Vicia faba |
0.022c |
0d |
3.0b |
|
Medicago lupulinum |
0.017c |
0d |
0.6b |
|
Phaseolus vulgaris |
0.010c |
0d |
3.4b |
|
Coronilla varia |
0.007c |
0d |
1.0b |
|
Medicago sativa |
0.006c |
0.008cd |
1.9b |
|
Anthyllus vulneraria |
0.003c |
0d |
1.2b |
|
Vicia sativa |
0.002c |
0d |
4.8ab |
|
Trifolium dubium |
0.002c |
0d |
0.4b |
|
Cytisus Scoparius |
0c |
0d |
0.2b |
|
Ulex europaeus |
0c |
0d |
0.6b |
Previous work (Murray and Clements, 1994) identified S. flavescens as being Trifolium spp.-dependant in feeding studies. Both feeding tests in the present work confirmed this, especially in the choice tests, where feeding was restricted to just three species. In all experiments there was significant correlation between feeding preference and biotic effect, indicating that food type was of prime importance in the physiology of the insect. The non-preference, reduced survival and fecundity on all but Trifolium spp. make it unlikely that S. flavescens could utilize common legume species to survive periods of clover deprivation due to ploughing and reseeding etc. In many previous experiments (e.g. Murray and Clements, 1994) T. dubium has always tended to be rejected a potential food source. The present work confirms this, and the short survival times of the weevil on T. dubium suggest that there may be a toxic effect causing the death of the insect rather than mere autostarvation which appears to be the case on many of the other food sources. Identification and exploitation of the resistance mechanisms involved could be important in engineering resistance to S. flavescens in white clover breeding programmes.
I acknowledge financial support from MAFF and BBSRC during the execution of these studies
Brain P. and Butler R.C. (1988) Cumulative count data. Genstat Newsletter 22, 38-47.
Clements R.O. and Murray P. J (1991) Pest and disease damage to white clover at widespread sites in England and Wales. Proceedings British Grassland Society Symposium: Strategies for Weed, Disease and Pest control in Grassland. 10:21-10:22.
Jackson D.J. (1920) Bionomics of weevils of the genus Sitones injurious to leguminous crops in Britain. Annals of Applied Biology 7, 269-298.
Lewis G.C. and Thomas B. J. (1991) Incidence and severity of pest and disease damage to white clover foliage at 16 sites in England and Wales. Annals of Applied Biology 118, 1-8.
Mowat D.J. and Shakeel M.A. (1989) The effect of some invertebrate species on persistence of white clover in ryegrass swards. Grass and Forage Science 44, 117-124.
Murray P.J. and Clements R. 0. (1993) Feeding preferences among eleven cultivars of white clover (Trifolium repens) by adults of two species of sitona weevil (Coleoptera; Curculionidae). Tests of Agrochemicals and Cultivars No. 14 (Annals of Applied Biology 122, Supplement). 134 - 136.
Murray P.J. and Clements R.O. (1994) Investigations of the host feeding preferences of Sitona weevils found commonly on white clover (Trifolium repens) in the UK. Entomologia experimentalis et applicata 71, 73 - 79.
