Plant Water Stress Affects Interactions Between an Invasive and a Naturalized Aphid Species on Cereal Crops.

In cereal cropping systems of the Pacific Northwestern United States (PNW), climate change is projected to increase the frequency of drought during summer months, which could increase water stress for crop plants. Yet, it remains uncertain how interactions between herbivore species are affected by drought stress. Here, interactions between two cereal aphids present in PNW cereal systems, Metopolophium festucae (Theobald) subsp. cerealium (a newly invasive species) and Rhopalosiphum padi L. (a naturalized species), were tested relative to wheat water stress. When aphids were confined in leaf cages on wheat, asymmetrical facilitation occurred; per capita fecundity of R. padi was increased by 46% when M. festucae cerealium was also present, compared to when only R. padi was present. Imposed water stress did not influence this interaction. When aphids were confined on whole wheat plants, asymmetrical competition occurred; cocolonization inhibited M. festucae cerealium population growth but did not affect R. padi population growth. Under conditions of plant water stress, however, the inhibitory effect of R. padi on M. festucae cerealium was not observed. We conclude that beneficial effects of cocolonization on R. padi are due to a localized plant response to M. festucae cerealium feeding, and that cocolonization of plants is likely to suppress M. festucae cerealium populations under ample water conditions, but not when plants are water stressed. This suggests that plant responses to water stress alter the outcome of competition between herbivore species, with implications for the structure of pest communities on wheat during periods of drought.

Interactions between Metopolophium festucae cerealium (Hemiptera: Aphididae) and Barley yellow dwarf virus (BYDV-PAV).

Interactions between an invasive aphid, Metopolophium festucae (Theobald) subsp. cerealium, and Barley yellow dwarf virus (BYDV-PAV) were studied under laboratory conditions. M. festucae cerealium is an economic pest of wheat and barley that has recently been found in high population densities in wheat in the Pacific Northwest of the United States. BYDV-PAV is the most prevalent and injurious species of BYDV worldwide and in the Pacific Northwest. Although M. festucae sensu stricto (Theobald 1917) has been reported previously as a vector of some BYDV isolates, there is no confirmed transmission of BYDV by M. festucae cerealium. Two experiments examined the ability of M. festucae cerealium to transmit BYDV-PAV. The first used single aphids caged to indicator plants of a BYDV-susceptible winter wheat cultivar and the second used multiple aphids on each test plant. M. festucae cerealium did not transmit BYDV-PAV in either experiment, whereas transmission by a known BYDV vector, Rhopalosiphum padi L., was consistently high (≥ 93%). A third experiment compared the intrinsic growth rate, days until first reproduction and daily reproduction by M. festucae cerealium on sham-inoculated and BYDV-PAV-infected wheat, but detected no differences. The findings are reviewed in light published data on M. festucae species, BYDV transmission, and the potential pest status of this new invading aphid.

Bumble bee fauna of Palouse Prairie: survey of native bee pollinators in a fragmented ecosystem.

Bumble bees, Bombus Latreille (Hymenoptera: Apidae:), are dominant pollinators in the northern hemisphere, providing important pollination services for commercial crops and innumerable wild plants. Nationwide declines in several bumble bee species and habitat losses in multiple ecosystems have raised concerns about conservation of this important group. In many regions, such as the Palouse Prairie, relatively little is known about bumble bee communities, despite their critical ecosystem functions. Pitfall trap surveys for ground beetles in Palouse prairie remnants conducted in 2002-2003 contained considerable by-catch of bumble bees. The effects of landscape context, remnant features, year, and season on bumble bee community composition were examined. Additionally, bees captured in 2002-2003 were compared with historic records for the region to assess changes in the presence of individual species. Ten species of bumble bee were captured, representing the majority of the species historically known from the region. Few detectable differences in bumble bee abundances were found among remnants. Community composition differed appreciably, however, based on season, landscape context, and elevation, resulting in different bee assemblages between western, low-lying remnants and eastern, higherelevation remnants. The results suggest that conservation of the still species-rich bumble bee fauna should take into account variability among prairie remnants, and further work is required to adequately explain bumble bee habitat associations on the Palouse.

Effects of tillage practices on pea leaf weevil (Sitona lineatus L., Coleoptera: Curculionidae) biology and crop damage: a farm-scale study in the US Pacific Northwest.

The pea leaf weevil, Sitona lineatus L., is periodically a significant pest of pea, Pisum sativum L., in the Palouse region of northern Idaho and eastern Washington, USA. Previous on-station research demonstrated significantly greater adult pea leaf weevil colonization, immature survival, adult emergence and plant damage in conventional-tillage compared to no-tillage plots of pea. In experiments conducted during the 2006 and 2007 growing seasons, aerial and ground adult pea leaf weevil colonization of large-scale commercial pea fields under different tillage regimes in northern Idaho and eastern Washington was examined for the first time. Initial pea leaf weevil feeding damage, immature weevil densities and subsequent adult emergence from the fields were also assessed. During both years, significantly more adult pea leaf weevils were captured in conventional-tillage than in no-tillage fields during the crop establishment period in May. No-tillage soils remained wet longer in the spring and could not be planted by growers until later than conventional-tillage fields. Pea planted under conventional-tillage emerged earlier and had significantly greater feeding damage by the pea leaf weevil than no-tillage pea. Significantly, greater immature pea leaf weevil densities and subsequent adult emergence were observed in conventional-tillage than in no-tillage pea fields. Delayed development of root nodules in the cooler, moister conditions of no-tillage pea fields likely resulted in escape from attack and injury during the critical growth stages that ultimately influence yield. Results indicate that large-scale commercial no-tillage pea fields are less suitable for colonization and survival of the pea leaf weevil and suffer less weevil damage than fields under conventional tillage.

Life history of the bird cherry-oat aphid, Rhopalosiphum padi (Homoptera: Aphididae), on transgenic and untransformed wheat challenged with barley yellow dwarf virus.

The life history of Rhopalosiphum padi (L.) was monitored on transgenic and untransformed (soft white winter wheat plants that were infected with Barley yellow dwarf virus (BLDV), noninfected, or challenged with virus-free aphids under laboratory conditions. Two transgenic soft white winter wheat genotypes (103.1J and 126.02) derived from the parental variety Lambert and expressing the barley yellow dwarf virus coat protein gene, and two untransformed varieties, virus-susceptible Lambert and virus-tolerant Caldwell, were tested. B. padi nymphal development was significantly longer on the transgenic genotypes infected with BYDV, compared with noninfected transgenic plants. In contrast, nymphal development on Lambert was significantly shorter on BYDV-infected than on noninfected plants. Nymphal development on noninfected Lambert was significantly longer than on noninfected transgenics. No significant difference in nymphal development period was detected between virus-infected and noninfected Caldwell. Aphid total fecundity, length of reproductive period, and intrinsic rate of increase were significantly reduced on BYDV-infected transgenic plants compared with BYDV-infected Lambert. In contrast, reproductive period, total adult fecundity, and intrinsic rate of increase on noninfected Lambert were significantly reduced compared with noninfected transgenics. Transgenic plants infected with BYDV were inferior hosts for R. padi compared with infected Lambert. However, noninfected transgenics were superior hosts for aphids than noninfected Lambert. Moderate resistance to BYDV, as indicated by a significantly lower virus titer, was detected in the transgenic genotypes compared with the untransformed ones. Results show for the first time that transgenic virus resistance in wheat can indirectly influence R. padi life history.

Variation in barley yellow dwarf virus transmission efficiency by Rhopalosiphum padi (Homoptera: Aphididae) after acquisition from transgenic and nontransformed wheat genotypes.

The effects of different acquisition access periods (AAPs) and inoculation access periods (IAPs) on the transmission efficiency of barley yellow dwarf luteovirus (BYDV) by Rhopalosiphum padi (L.) (Homoptera: Aphididae) after feeding on transgenic or nontransformed wheat, Triticum aestivum L., genotypes were studied. Three wheat genotypes were tested as virus sources: virus-susceptible 'Lambert' and 'Lambert'-derived transgenic lines 103.1J and 126.02, which express the BYDV-PAV coat protein gene. Lower virus titers were measured in BYDV-infected transgenic plants compared with Lambert. No significant differences in transmission efficiency were detected for R. padi after varying IAPs, regardless of genotype. Transmission efficiency increased with an increase in AAP in all genotypes tested. However, AAPs ranging from 6 to 48 h on Lambert resulted in significantly greater transmission efficiency than similar periods on transgenic 103.1J. Maximum transmission efficiency (70%) was observed after a 48-h AAP on Lambert, whereas the same AAP on 103.1J and 126.02 resulted in a significantly lower transmission efficiency (57%). Contrasts were used to compare the rates of transmission and the theoretical maximum transmission percentage among the different wheat genotypes serving as virus sources. Both parameters were significantly different among genotypes, indicating that viral acquisition from each genotype resulted in a unique pattern of virus transmission by R. padi. The lowest rate of virus transmission after an AAP was observed on 103.1J compared with 126.02 or Lambert. This is likely associated with a lower virus titer in 103.1J. This is the first report of transgenic virus resistance in wheat affecting the transmission efficiency of a virus vector.

A comparison of two methods of assessment of maize varietal resistance to the maize weevil, Sitophilus zeamais Motschulsky, and the influence of kernel hardness and size on susceptibility.

Fifty-two maize varieties were screened for resistance to infestation by the maize weevil, Sitophilus zeamais, using assessment methods proposed by Dobie (J. Stored Products Res. 10 (1974) 183-197) and Urrelo et al. (J. Stored Products Res. 26 (1990) 100). The two methods gave similar assessments of maize susceptibility to S. zeamais. The Dobie method is preferred due to the lower total time required for assessment of relative susceptibility of maize varieties. The greatest disadvantage of the Urrelo method is the intensive labour requirements in the early stages of a trial, when numbers of eggs have to be counted, although it has the advantage that the assessment may be terminated upon emergence of the first F(1) adult. Two explanatory variables, kernel size and hardness, were investigated to determine whether they may be used as indicators of resistance. Results suggested that kernel size is the more important in determining resistance to attack by S. zeamais, with large kernels appearing to show greater resistance than small ones. Contrary to expectations, of the varieties tested, including local, hybrid and improved open pollinated (IOP) varieties, the local varieties were generally more susceptible. This may be related to kernel size, as all IOPs and hybrids tested had large kernels, whereas the majority of the local varieties had small ones. No clear relationship between weevil susceptibility and kernel hardness could be detected, although there was an indication that differences associated with kernel size varied depending on kernel hardness. None of the varieties tested showed high levels of resistance to attack by S. zeamais.

Biotype composition of Hessian fly (Diptera: Cecidomyiidae) populations from the southeastern, midwestern, and northwestern United States and virulence to resistance genes in wheat.

Twenty-three Hessian fly, Mayetiola destructor (Say), populations collected in the southeastern (Alabama and Mississippi), midwestern (Indiana), and northwestern (Idaho and Washington) United States from 1995 to 1999 were evaluated for biotype composition based on response to Hessian fly resistance genes H3, H5, H6, and H7H8 in wheat, Triticum aestivum L. Biotypes L and O, combined, made up at least 60% of all Alabama populations. Biotype L was predominant in the northern third of Alabama and biotype O in the southern two-thirds of the state. Based on biotype data, wheat cultivars with H7H8 resistance should be highly effective in central and southern Alabama. Fifty-four percent of the Mississippi population consisted of biotype L, and the remaining virulent biotypes (B, D, E, G, J, and O) ranged in frequency from 1 to 17%. The Mississippi population also contained 4% of the avirulent biotype GP. Only biotypes D and L were found in Indiana populations, but biotype L was predominant. Hessian fly populations from Idaho and Washington contained one or more of the virulent biotypes D-H, J, and L-O; however, only biotypes E, F, and G occurred at frequencies > 12%. The avirulent biotype GP made up 25-57% of Idaho and Washington populations, a much higher percentage than found in populations from the eastern United States. Although the highest level of virulence in Idaho and Washington populations was found to resistance genes H3 and H6, the frequency of biotype GP would indicate that the currently deployed gene H3 would provide a moderate to high level of resistance, depending on location. Nine of the populations, plus populations collected from the mid-Atlantic state area in 1989 and 1996, also were tested against the wheat cultivar 'INW9811' that carries H13 resistance to Hessian fly biotype L and two Purdue wheat lines with unidentified genes for resistance. The H13 resistance in INW9811 was highly effective against all populations tested from the eastern and northwestern U.S. wheat production areas, except Maryland and Virginia. Population studies also indicated that wheat line CI 17960-1-1-2-4-2-10 likely carries the H13 resistance gene, based on the similarity of its response and that of INW9811 to eight fly populations. Continued monitoring of biotype frequency in Hessian fly populations is required for optimal deployment and management of resistance genes in all wheat production areas.

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats.

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.

Eight decades of maize streak virus research.

Maize streak virus (MSV) (genus Mastrevirus; family Geminiviridae) causes what is considered the most important and widespread disease of maize in sub-Saharan Africa. Maize streak was named by Storey in 1925. Since his classical work on the virus and its leafhopper vectors of the genus Cicadulina China, MSV has been the subject of intensive research. Aspects concerning the geographical distribution, virus diversity, molecular characterization, vector-virus relationships and resistance breeding are reviewed. Special emphasis is placed on recent studies of MSV ecology and epidemiology in West Africa.

Interactions Between Fusarium verticillioides, Aspergillus flavus, and Insect Infestation in Four Maize Genotypes in Lowland Africa.

ABSTRACT An experiment was designed to compare cycles of selection of four maize genotypes for ear- and grain-quality characteristics, interactions with Aspergillus flavus and Fusarium verticillioides infection, and insect ear infestation in two seasons. Mean infection levels by A. flavus and F. verticillioides were significantly higher in inoculated rows than in the controls. The F. verticillioides-inoculated rows had significantly more coleopteran beetles and lepidopteran borers per ear than the controls and A. flavus-inoculated rows. Genotypes and cycles of selection within genotype were not different with respect to number of insects or percent fungal incidence in the ear, but they were different for husk extension, field weight, 100-grain weight, and grain density. Inoculation with either fungus resulted in significantly higher percentage of floaters (i.e., loss of grain density) and lower grain weight than the controls. Aflatoxin (B1 and B2) in A. flavus-inoculated rows averaged 327 ppb in the first season and 589 ppb in the second (dryer) season. Fumonisin levels in F. verticillioides-inoculated rows did not differ between seasons, with an average of 6.2 ppm across seasons. In the noninoculated control rows, fumonisin was significantly higher in the first (5.3 ppm) than in the second (3.1 ppm) season. For all genotypes, husk extension and yield parameters decreased in the fungal-inoculated treatments. General ear-rot scoring was significantly correlated with incidence of F. verticillioides in kernels and grain-weight loss but not with A. flavus in the grain.