An in-field screen for early detection and monitoring of insect resistance to Bacillus thuringiensis in transgenic crops

We present a field-based approach to detect and monitor insects with resistance to insecticidal toxins produced by transgenic plants. Our objective is to estimate the phenotypic frequency of resistance in a population by relating the densities of insects on genetically transformed plants to densities on nontransformed plants. We focus on European corn borer, Ostrinia nubilalis (Hübner), in sweet corn, Zea mays L., expressing Cry1Ab from Bacillus thuringiensis subsp. kurstaki Berliner to illustrate principles underlying the method. The probability of detecting one or more rare, resistant larvae depends on sample size, the density of larvae on nontransformed plants, and an assumed frequency of resistant phenotypes in a given population. Probability of detection increases with increases in sample size, background density, or the frequency of resistant individuals. Following binomial probability theory, if a frequency of 10(-4) is expected, 10(3)-10(4) samples must be collected from a B. thuringiensis (Bt) crop to have at least a 95% probability of locating one or more resistant larvae. In-field screens using transgenic crops have several advantages over traditional laboratory-based methods, including exposure to a large number of feral insects, discrimination of resistant individuals based on Bt dosages expressed in the field, incorporation of natural and Bt-induced mortality factors, simultaneous monitoring for more than one insect species, and ease of use. The approach is amenable to field survey crews working in research, extension, and within the seed corn industry. Estimates of the phenotypic frequency of resistance from the in-field screen can be useful for estimating initial frequency of resistant alleles. Bayesian statistical methods are outlined to estimate phenotype frequencies, allele frequencies, and associated confidence intervals from field data. Results of the approach are discussed relative to existing complementary methods currently available for O. nubilalis and corn earworm, Helicoverpa zea (Boddie).     

Nitrogen mineralization by bacterial-feeding nematodes: verification and measurement

Bacterial feeding nematodes excrete N assimilated in excess of that required for growth. Because metabolic and developmental rates differ among nematode species, we hypothesized that their contribution to N mineralization in soil would differ. Sand-column microcosms amended with an organic substrate, bacteria, and with or without bacterial-feeding nematodes, were leached at 3-d intervals. Cumulative N, as NH ₄ ⁺ or NO ₃ ^- , leached from columns containing nematodes was consistently greater than from columns without nematodes. Maximum N-mineralization rates for populations of rhabditid nematodes, which predominated in field soils early in the summer were at lower temperatures than those for cephalobid nematodes, which predominated later in the summer. For an organic substrate with C-to-N ratio of 11:1, rates of N mineralization among species of different body size were similar, ranging between 0.0012 and 0.0058 g-N nematode^-1 d^-1, mainly as NH ₄ ⁺ . Smaller nematodes mineralized more N per unit of body weight than larger nematodes. We hypothesized that at low C-to-N ratios of the organic substrate, bacterial growth is C-limited and N-immobilization will be minimal; at high C-to-N ratios bacterial growth will be N-limited and there may be rapid immobilization of newly-mineralized N. Consequently, net N mineralization in the presence of nematodes will be lower when the organic substrate has a high C-to-N ratio. In experiments with different nematode species, net mineralization and the nematode contribution to mineralization generally decreased with increasing C-to-N ratio, consistent with the hypothesis; however, there were exceptions.     

Goldspotted oak borer effects on tree health and colonization patterns at six newly‐established sites

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Economic threshold for soybean aphid (Hemiptera: Aphididae)

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), reached damaging levels in 2003 and 2005 in soybean, Glycine max (L.) Merrill, in most northern U.S. states and Canadian provinces, and it has become one of the most important pests of soybean throughout the North Central region. A common experimental protocol was adopted by participants in six states who provided data from 19 yield-loss experiments conducted over a 3-yr period. Population doubling times for field populations of soybean aphid averaged 6.8 d +/- 0.8 d (mean +/- SEM). The average economic threshold (ET) over all control costs, market values, and yield was 273 +/- 38 (mean +/- 95% confidence interval [CI], range 111-567) aphids per plant. This ET provides a 7-d lead time before aphid populations are expected to exceed the economic injury level (EIL) of 674 +/- 95 (mean +/- 95% CI, range 275-1,399) aphids per plant. Peak aphid density in 18 of the 19 location-years occurred during soybean growth stages R3 (beginning pod formation) to R5 (full size pod) with a single data set having aphid populations peaking at R6 (full size green seed). The ET developed here is strongly supported through soybean growth stage R5. Setting an ET at lower aphid densities increases the risk to producers by treating an aphid population that is growing too slowly to exceed the EIL in 7 d, eliminates generalist predators, and exposes a larger portion of the soybean aphid population to selection by insecticides, which could lead to development of insecticide resistance.     

Demography of soybean aphid (Homoptera: Aphididae) at summer temperatures

Soybean aphid, Aphis glycines Matsumura, is now widely established in soybean, Glycine max L., production areas of the northern United States and southern Canada and is becoming an important economic pest. Temperature effect on soybean aphid fecundity and survivorship is not well understood. We determined the optimal temperature for soybean aphid growth and reproduction on soybean under controlled conditions. We constructed life tables for soybean aphid at 20, 25, 30, and 35 degrees C with a photoperiod of 16:8 (L:D) h. Population growth rates were greatest at 25 degrees C. As temperature increased, net fecundity, gross fecundity, generation time, and life expectancy decreased. The prereproductive period did not differ between 20 and 30 degrees C; however, at 30 degrees C aphids required more degree-days (base 8.6 degrees C) to develop. Nymphs exposed to 35 degrees C did not complete development, and all individuals died within 11 d. Reproductive periods were significantly different at all temperatures, with aphids reproducing longer and producing more progeny at 20 and 25 degrees C than at 30 or 35 degrees C. Using a modification of the nonlinear Logan model, we estimated upper and optimal developmental thresholds to be 34.9 and 27.8 degrees C, respectively. At 25 degrees C, aphid populations doubled in 1.5 d; at 20 and 30 degrees C, populations doubled in 1.9 d.     

Trophic interactions between bacterial-feeding nematodes in plant rhizospheres and the nematophagous fungus Hirsutella rhossiliensis to suppress Heterodera schachtii

Trophic exchanges in soil food webs may suppress populations of pest organisms. We hypothesize that the suppressive condition of soils might be enhanced by manipulating components of the food web. Specifically, by enhancing populations of bacterial-feeding nematodes, propagule density of the nematophagous fungus Hirsutella rhossiliensis should increase and constrain populations of Heterodera schachtii, a plant-parasitic nematode. The rhizospheres of Crotalaria juncea and Vicia villosa stimulated population growth of the bacterial-feeding nematode, Acrobeloides bodenheimeri, but not of the nematodes Caenorhabditis elegans or Rhabditis cucumeris. The rhizospheres of Tagetes patula, Eragrostis curvula, and Sesamum indicum had no effect on any of the bacterial-feeding nematodes investigated. Acrobeloides bodenheimeri was most susceptible to parasitism by the nematophagous fungus H. rhossiliensis with 35% of individuals being parasitized in a laboratory assay. In three separate trials, parasitism of H. schachtii by H. rhossiliensis was not enhanced when populations of A. bodenheimeri were amplified in a suitable rhizosphere.     

Allopatric populations of the invasive larch casebearer differ in cold tolerance and phenology

1. Traits of non-native insect herbivores may vary spatially due to local genetic differences, rapid post-introduction evolution, and/or novel host plant associations. 2. Populations of larch casebearer, Coleophora laricella Hübner, originally from Europe have likely been isolated for > 60 years in North America on eastern larch, Larix laricina (Du Roi) K. Koch, and western larch, Larix occidentalis Nutt. 3. This study investigated cold tolerance and phenology of larvae collected from eastern larch in Minnesota, and western larch in Oregon, Idaho, and Montana, U.S.A. 4. Mean supercooling points of larvae from Minnesota were up to 10 °C lower than supercooling points of larvae from Oregon, Idaho, and Montana. 5. At ambient environmental conditions in spring, overwintering larvae from Minnesota required a mean (± SE) of 172 ± 19 degree-days above 5 °C to break winter quiescence and actively wander, significantly more than required by larvae from Oregon (66 ± 4), Idaho (64 ± 1), and Montana (60 ± 2). 6. Across all assays and despite substantial latitudinal and elevational variation among western larch sites, no significant differences in any traits were detected among larvae collected from western larch. 7. Spatial variation in cold tolerance and phenological traits of larch casebearer may be attributable to insect genetic differences and/or host plant effects, but exact mechanisms remain unknown. Differences in thermal biology between regions may result in disparate effects of climate change on insect populations and should be accounted for when forecasting insect dynamics across large spatial scales.     

Cold hardiness of Helicoverpa zea (Lepidoptera: Noctuidae) pupae

An insect's cold hardiness affects its potential to overwinter and outbreak in different geographic regions. In this study, we characterized the response of Helicoverpa zea (Boddie) pupae to low temperatures by using controlled laboratory measurements of supercooling point (SCP), lower lethal temperature (LT(50)), and lower lethal time (LLTime). The impact of diapause, acclimation, and sex on the cold hardiness of the pupae also were evaluated. Sex did not significantly affect the SCP, LT(50), or LLTime. However, the mean SCP of diapausing pupae (-19.3°C) was significantly lower than nondiapausing pupae (-16.4°C). Acclimation of nondiapausing pupae to constant temperatures from 10 to 20°C before supercooling also produced a significantly lower SCP than nondiapausing pupae held at 25°C. The LT(50)s of nondiapausing and diapausing were not significantly different, but confirmed that H. zea pupae are chill-intolerant because these lethal temperatures are warmer than the corresponding mean SCPs. Diapausing pupae survived longer than nondiapausing pupae at the same, constant, cold temperatures, a finding consistent with the SCP results. Both of these results suggest enhanced cold hardiness in diapausing pupae. When laboratory results were compared with field temperatures and observed distributions of H. zea in the contiguous United States, the laboratory results corroborated what is currently perceived to be the northern overwintering limit of H. zea; approximately the 40(th) parallel. Moreover, our research showed that areas north of this limit are lethal to overwintering pupae not because of low temperature extremes, but rather the length of time spent at near-zero temperatures.     

Cold tolerance of the invasive larch casebearer and implications for invasion success

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