Arthur A. Hower

Effects of root nodules and taproots on survival and abundance of Sitona hispidulus (Coleoptera: Curculionidae) on Medicago sativa

ABSTRACT. 1 The effects of food resources on populations of larvae of Sitona hispidulus (F.), a root and nodule herbivore of Medicago sativa L., were investigated in 1983 and 1984. 2 Density‐dependent mortality among first‐instar larvae indicated that a lack of available root nodules, the primary resource for first instars, contributed to the 66.2±5.2% and 80.7±4.1% mortalities of first‐instar larvae in 1983 and 1984, respectively. Initial densities of larvae entering the soil were 31.7±2.4 and 15.6±1.3 per soil sample (10.5 cm diam. ×15 cm deep) in 1983 and 1984, respectively (±SEM). 3 Survivorship and number of first‐instar larvae per soil core sample were significantly related to number and biomass of nodules per soil sample. Percentage soil moisture was not consistently correlated with survivorship and number of larvae per soil sample. 4 Numbers of second‐ and third‐instar larvae were not consistently correlated with either nodule or taproot biomass. 5 Numbers of fourth‐instar larvae were associated with taproot biomass, suggesting that taproot surface area may also limit population levels of S.hispidulus. No correlations were found between taproot and nodule biomass on any sampling date indicating that high numbers of fourth‐instar larvae associated with larger taproots were not merely due to a greater survival of first instars associated with larger taproots. 6 The study suggests that population levels of nodule‐herbivores are controlled by the availability of root nodules. A sparse distribution of nodules results in high levels of mortality among nodule‐herbivores of Medicago sativa.

Role of the Glandular Trichomes in Resistance of Perennial Alfalfa to the Potato Leafhopper (Homoptera: Cicadellidae)

Abstract Experiments were performed to elucidate resistance of glandular-haired alfalfa, Medicago sativa L., to the potato leafhopper, Empoasca fabae (Harris). The primary objective was to examine the potential role of the glandular trichomes and stem lignification for imparting resistance to this pest. During free-choice foliar discoloration experiments, the resistant alfalfa clone FGplh13 expressed lower levels of injury than the susceptible P5373 after 13 d. No-choice tests performed using nymphal potato leafhoppers showed higher levels of mortality associated with the resistant glandular-haired clone FGplh13, and no nymphs survived after 48 h. Additional no-choice experiments using FGplh13 and P5373 alfalfa with the glandular and nonglandular trichomes intact or removed show that mortality of nymphs and adults decreased after removal of the glandular trichomes from FGplh13. Nymphal and adult mortality and nymphal development time did not differ on FGplh13 with the trichomes removed and P5373 with the trichomes intact or removed. First-instar, and possibly second-instar, potato leafhoppers were entrapped by an exudate produced by the glandular trichomes on the FGplh13 alfalfa. Stem lignification and the number of vascular bundles did not differ between FGplh13 and P5373 alfalfas. The glandular trichomes on FGplh13 alfalfa appeared to provide the major host resistance factor, with resistance to adults being chemically based and resistance to nymphs being chemically and mechanically based. To maintain levels of potato leafhopper resistance, breeders would appear to benefit by continuing to select for the expression of the glandular trichome phenotype.

Infectivity and reproductive potential of the Oswego strain of Heterorhabditis bacteriophora associated with life stages of the clover root curculio, Sitona hispidulus.

The infectivity and reproductive potential of the entomopathogenic nematode Heterorhabditis bacteriophora (Oswego strain), at different concentrations, was studied. Seventy to 80.0% mortality to late instar larvae of the clover root curculio, Sitona hispidulus, and 40.0-76.0% mortality to pupae, was observed at concentrations of 15-100 infective juveniles. There were no significant differences in mortality among nematode concentrations. LC(50) levels of 4.0 and 21.4 nematodes were determined for clover root curculio larvae and pupae, respectively. Nematodes did not cause significant mortality to adult or first instar clover root curculio. H. bacteriophora was able to complete its development and reproduce in 74.0-95.0% of clover root curculio late instar larvae and pupae. Reproductive potential in curculio larvae and pupae ranged from 0 to 7040 infective juveniles per host. Larvae exposed to 100 nematodes had a reproductive potential significantly higher than in those larvae exposed to 15 and 50 nematodes. Reproductive potential in pupae decreased with an increased nematode dose, indicating potential crowding effects. Host larval and pupal mass were positively correlated with nematode progeny production.

Glandular trichomes on perennial alfalfa affect host‐selection behavior of Empoasca fabae

Selected behavior of Empoasca fabae (Harris) (Homoptera: Cicadellidae) was examined to elucidate resistance of commercially‐available glandular‐haired alfalfa to this key forage pest. The overall objective was to assess the effects of the glandular trichomes on the behavior of nymphs and adults. Studies of host‐plant acceptance by E. fabae nymphs found first and third instars to show a higher degree of change in settling location on the glandular‐haired FGplh13 alfalfa than on the nonglandular P5373 alfalfa. Nymphs also cleaned their tarsi more frequently when in contact with the glandular trichomes on FGplh13 alfalfa, and in addition a larger number of nymphs jumped/fell off the surface of FGplh13 alfalfa. The glandular trichomes on FGplh13 also impeded nymphal mobility more effectively than the nonglandular trichomes on P5373. No choice, time‐course analysis of adult host‐plant acceptance behavior found that adults settled less frequently on FGplh13 alfalfa with the glandular trichomes intact, than on either FGplh13 with the glandular trichomes removed, or P5373 alfalfa with the nonglandular trichomes intact or removed. Free choice, time‐course analysis of adult host‐preference behavior determined that at each observation, stems of the nonglandular P5373 alfalfa were preferred over FGplh13 alfalfa. Similarly, at each observation, stems of FGplh13 alfalfa with the glandular trichomes removed were preferred over FGplh13 with the glandular trichomes intact. These data provide additional evidence for the localization of a resistance factor in the glandular trichomes of FGplh13 alfalfa. An antixenotic resistance mechanism also appears to be present, which may function, in part, through a tactile avenue.

Soil and moisture effects on the dynamics of early instar clover root curculio (Coleoptera : Curculionidae) and biomass of alfalfa root nodules

Abstract Movement and survival of early instar clover root curculio, Sitona hispidulus (F.), and the development of root nodules in alfalfa, Medicago sativa L., were examined in relation to soil texture and moisture. Early instars were subjected to three soil textures (loamy sand, loam, and silty-clay loam) and two moisture potential regimes approximating very moist and slightly moist conditions, respectively. Survival of early instars was significantly higher in the slightly moist silty-clay loam (61%) and lowest in the slightly moist loamy sand (23%). These results suggest that cracks 1.0 mm or larger in the silty-clay loam increased access of the first instars to alfalfa root nodules. Production of root nodules was significantly greater in the loamy sand, yet clover root curculio survival in this soil type was lower, indicating that this soil deterred first instar access to critical nodule resources. We tested the ability of first instars to penetrate soils of three different textures and five moisture levels. First instars penetrated moist and slightly moist silty-clay loam and coarse sand significantly better than severely moist and saturated treatments, and also better than loamy sand at any moisture level. This was due to the presence of large cracks and/or pores in the drier silty-clay loam and coarse sand. This information will be useful for identifying soil conditions in alfalfa fields that influence the pest potential of this insect and hence assist in developing more effective integrated pest management strategies.

Temporal and Spatial Dynamics of Empoasca fabae (Harris) (Homoptera: Cicadellidae) in Alfalfa

Abstract We describe the dynamics of potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), populations in a 4-ha alfalfa field over 2 yr. Population growth and spatial structure were strongly influenced by days after cutting. Capture of E. fabae by suction traps above the boundary layer along with sex ratios of in-field populations suggested that immigrants contributed to population growth throughout the second and third alfalfa growth cycles. Initial sex ratios were strongly female biased (1995, 80%; 1996, 90%), with the degree of bias decreasing and approaching a 1:1 ratio through the third growth cycles. A higher proportion of the population was located in the edge relative to the interior plots in three of four alfalfa growth cycles. Spatial correlation between females and males was initially low, but increased as density increased; this correlation also decreased immediately after alfalfa harvest, and significantly increased over time after harvest. These data suggest that dynamic in-field spatial organization exists for E. fabae. Although the entire field was colonized, we hypothesize an edge-biased colonization process, initiated by females for at least the second growth cycle in the northeastern United States, followed by density-dependent movement away from crowded areas of declining host quality.

Population Dynamics, Persistence, and Efficacy of the Entomopathogenic Nematode Heterorhabditis bacteriophora (Oswego Strain) in Association with the Clover Root Curculio (Coleoptera: Curculionidae) in Pennsylvania

Abstract The population dynamics, persistence, and efficacy of Heterorhabditis bacteriophora Poinar (Oswego strain) applied to control the clover root curculio, Sitona hispidulus (F.), were investigated in a Pennsylvania alfalfa (Medicago sativa L.) field. Nematodes established and persisted following application rates of 2.5, 7, and 15 billion infective juveniles per hectare. Significant differences in nematode densities between treatments were not observed beyond 43 d after application, indicating that application rate likely did not affect long-term persistence. In the third field season, plots that received the original 15-billion nematodes per ha treatment were split into clover root curculio-excluded and clover root curculio-present subplots to assess the effect of this insect on nematode persistence. Nematode populations were significantly lower in the curculio-excluded plots by October, suggesting that the nematodes recycled through that host. However, nematode populations were not significantly different by April of the fourth field season. Nematode efficacy throughout the study was inconsistent. In the first field season, emergence of clover root curculio adults was significantly reduced in plots receiving the 15 billion nematode per ha treatment. However, alfalfa taproot ratings for clover root curculio feeding injury indicated that scarring was reduced only in the 7 billion nematodes per ha treatments. No significant reductions in taproot injury were observed in the second field season, but by the third year, significant reductions in root injury were evident in the plots that originally received 7 and 15 billion nematodes per ha.