Evaluation of Semiochemical-Baited Traps for Monitoring the Pea Leaf Weevil, Sitona lineatus (Coleoptera: Curculionidae) in Field Pea Crops.

The pea leaf weevil (PLW), Sitona lineatus L., is a pest of field pea (Pisum sativum L.) and faba bean (Vicia faba L.) that recently invaded the Canadian Prairie Provinces. Although most damage is done by larvae that feed on root nodules, adults are easier to monitor than larvae. Both male and female weevils respond to a male-produced aggregation pheromone and to volatiles released by host plants. The current study tests the attractiveness of synthetic aggregation pheromone, 4-methyl-3,5-heptanedione, and host plant volatiles linalool, (Z)-3-hexenol, and (Z)-3-hexenyl acetate to PLWs in spring when weevils are reproductively active and in fall when weevils seek overwintering sites. Different combinations of semiochemical lures at various doses, released from a variety of devices were tested in pitfall traps. Semiochemical-baited traps captured both male and female weevils in both seasons but the sex ratio varied with season. Weevils did not respond in a dose-dependent manner to pheromone, as all pheromone lures were equally attractive. Pheromone release rate was determined by the release device and not the pheromone dose in the lure. The addition of plant volatiles sometimes increased weevil captures but plant volatiles alone were not attractive to PLW adults. An additional study tested the effect of trap type on weevil capture. Of the 12 different trap types tested, pheromone-baited pitfall traps were most successful in attracting and retaining weevils. Bycatch of other Sitona species was limited to a few specimens of the sweet clover weevil, Sitona cylindricollis Fahraeus.

Sitona lineatus (Coleoptera: Curculionidae) Larval Feeding on Pisum sativum L. Affects Soil and Plant Nitrogen.

Adults of Sitona lineatus (pea leaf weevil, PLW) feed on foliage of several Fabaceae species but larvae prefer to feed on nodules of Pisum sativum L. and Vicia faba L. Indirectly, through their feeding on rhizobia, weevils can reduce soil and plant available nitrogen (N). However, initial soil N can reduce nodulation and damage by the weevil and reduce control requirements. Understanding these interactions is necessary to make integrated pest management recommendations for PLW. We conducted a greenhouse study to quantify nodulation, soil and plant N content, and nodule damage by weevil larvae in relation to soil N amendment with urea, thiamethoxam insecticide seed coating and crop stage. PLWs reduced the number of older tumescent (multilobed) nodules and thiamethoxam addition increased them regardless of other factors. Nitrogen amendment significantly increased soil available N (>99% nitrate) as expected and PLW presence was associated with significantly lower levels of soil N. PLW decreased plant N content at early flower and thiamethoxam increased it, particularly at late flower. The study illustrated the complexity of interactions that determine insect herbivory effects on plant and soil nutrition for invertebrates that feed on N-fixing root nodules. We conclude that effects of PLW on nodulation and subsequent effects on plant nitrogen are more pronounced during the early growth stages of the plant. This suggests the importance of timing of PLW infestation and may explain the lack of yield depression in relation to this pest observed in many field studies. Also, pea crops in soils with high levels of soil N are unlikely to be affected by this herbivore and should not require insecticide inputs.

Solid-stemmed wheat does not affect overwintering mortality of the wheat stem sawfly, Cephus cinctus.

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a key pest of wheat in the northern Great Plains of North America. Host plant resistance in the form of solid-stemmed wheat cultivars is the main control strategy for C. cinctus. This study investigated the effect of novel and traditional solid wheat hosts on the overwintering mortality and cold-hardiness of C. cinctus. Field conditions from 2003-2005 showed that overwintering mortality in various wheat cultivars averaged 8% and was not related to the type of wheat cultivar. Similarly, supercooling points (-22° C) were not influenced by wheat host type. C. cintus are cold-hardy; up to 80% survive 10 days at -20° C and 10% survive 40 days. Its overwintering microhabitat near the crown area of the plant is well insulated for temperatures above -10° C and remains ~ 20° C above ambient minima. These data suggest that winter mortality is a minor factor in the population dynamics of wheat stem sawfly, and despite clear detrimental effects on larval weight and adult fitness, solid-stemmed cultivars do not reduce the ability of larvae to survive winters.

Developmental instability in a stem-mining sawfly: can fluctuating asymmetry detect plant host stress in a model system?

Fluctuating asymmetry (FA) may be a sensitive indicator of the stress experienced by organisms during their development. Its use in this manner is an intuitively appealing, frequently proposed, and potentially powerful tool but remains controversial partially because its underlying premise rarely has been critically tested. Such tests should include direct comparisons among individuals for which levels of FA, stress and fitness have been unambiguously quantified. We assessed the use of FA as a bioindicator of the stress experienced during egg-to-adult development by the stem-mining sawfly, Cephus cinctus Norton. Sawflies were reared in a common garden from seven different wheat cultivars, which were selected to represent a gradient of stem solidness, a key factor imposing stress on sawflies during development. In this model system, stress was quantified by the weight of emergent adults. Fitness was quantified by counting the number of eggs in dissected females, which emerge with their full lifetime complement. FA was measured for wing length, three wing cells, and three wing veins using image analyses. The greatest amount of stress was induced by solid-stemmed cultivars from which the adults were significantly smaller than those developing in hollow-stemmed hosts. In turn, adult weight was positively correlated with fitness. The net effect was a 25-fold variation in sawfly fitness, which gave a reasonable expectation that FA levels would differ across cultivars. However, FA levels of all the traits were similar among cultivars and there was no negative relationship between FA and fitness. These results: (1) document the failure of FA as an indicator of stress in this model system, (2) identify adult weight as a satisfactory indicator of plant-induced stress and sawfly fitness, and (3) add to the growing body of literature questioning the value of FA as a biomonitor tool of developmental stress.