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Methods for assessing infestations of sunflower stem weevil
Abstract
The sunflower stem weevil, Cylindrocopturus adspersus LeConte (Coleoptera: Curculionidae), reduces sunflower, Helianthus annuus Linnaeus (Asteraceae), yields by spreading pathogens, damaging vascular tissues, and promoting lodging of sunflower plants. To assess weevil populations for host plant resistance and insecticide field trials, larvae are dissected out of stems and counted; a process that is slow and limits experimental designs. To improve efficiency of sunflower stem weevil sample processing, field-collected sunflower samples were used to evaluate whether digital radiographs (X-rays) of stem sections or population estimates from rearing out overwintering stem weevils are suitable substitutes for dissection of complete stems. Digital X-rays of small stem pieces (15 cm above soil level) split longitudinally were found to explain over 75% of the variation in numbers of weevil larvae from dissected stem samples (50 cm), but required less than one-fifth the time of manual dissection. Use of small emergence boxes to estimate weevil populations was similarly time efficient, but may not be easily related to weevils per plant because of parasitism and other mortality. Results suggest for large field trials with sunflower stem weevils, digital X-rays provide much more time-efficient larval population estimates.
... Entries were planted 17 May into single-row plots 3.7 m long, with 0.76 m between rows and 0.15 m between plants in a row. Five randomly-selected stems per plot were collected at maturity by pulling up plants or prying with shovels, and the lowest 15 cm of stem above soil level were used to obtain a digital radiograph (X-ray) showing weevil larvae within the lower stem; data from this method correlate well with manual stem dissection, but the X-ray method is safer and more time-efficient (Prasifka et al., 2014). ...
... A smaller follow-up trial in Nebraska indicated that the effect of stem diameter is largely environmental; that is, within a hybrid, changes in plant density should predictably affect both stem diameter and numbers of stem weevil larvae. Accounting for the fact that X-rays from the basal 15 cm of stem may include only about half of all the weevil larvae present (Prasifka et al., 2014), both the Kansas and Nebraska trials appeared to have weevil populations below that at which economic damage is likely (Charlet et al., 1985). ...
Sunflower stem weevil, Cylindrocopturus adspersus LeConte (Coleoptera: Curculionidae), larval feeding on pith and vascular tissues can weaken stalks of cultivated sunflower, leading to breakage and loss of harvestable yield. Previous research has suggested host plant resistance (HPR) to this pest is possible, but results have been difficult to interpret because of limited comparisons to cultivated inbreds and hybrids, along with possible confounding effects of stem diameter. Trials in Kansas (2012) and Nebraska (2015) were used to help reassess the value of germplasm with putative resistance to sunflower stem weevil. Field tests in 2012 included 28 inbreds and hybrids and three entries previously identified as resistant. Though a putative resistance source, PI 386230, was infested with fewer weevil larvae per stem than several hybrids, differences primarily appeared to be due to stem diameter, which is strongly correlated with the number of weevil larvae per stem. A follow-up trial in 2015 suggested differences in larval infestation were not attributable to other qualities in groups of germplasm that differ in stem diameter (i.e., males, females or hybrids), but that changes in plant density predictably alter both stem diameter and numbers of stem weevil larvae. Though resistance to sunflower stem weevil and other stem-feeding pests may be possible, the cost and difficulty of developing HPR, declining severity of the pest, and availability of other simple management options are significant disincentives to additional research.
... As a result, 2013 samples were scored using pooled 200 seed samples. Stem sections were placed in cold (6°C) storage until needed, then cut to include the lowest 10 cm above soil level and the number of stem weevil larvae per sample counted from digital X-ray images (MX-20: Faxitron Bioptics LLC, Tucson, Arizona, USA) of each stem (Prasifka et al., 2014). ...
... When a maturity × location interaction was detected, the effect of location was evaluated using a t-test. After X-ray images of stems were examined, it was concluded the levels of infestation were too low to warrant further analysis; previous research suggests < 60 weevils per stem cause little or no damage (Rogers and Jones, 1979), which is approximately 20 weevils per X-ray (Prasifka et al., 2014), several times the level seen at any of the locations. ...
Delayed planting is recommended to reduce damage from sunflower insect pests in the United States, including the sunflower moth, Homoeosoma electellum (Hulst) and banded sunflower moth, Cochylis hospes Walsingham. However, in some locations, planting earlier or growing later-maturing hybrids could improve yield or oil content of sunflowers which would partially offset any added costs from insect pests or their management. Because the abundance and distribution of some sunflower insects have changed since recommendations for delayed planting were developed, experimental plots were grown in 2012 and 2013 at sites in North Dakota, Nebraska, Iowa, and Illinois. Sunflowers were planted two to four weeks earlier than normal, including hybrids that flower two to three weeks later than elite commercial hybrids. The sum of seed damaged by sunflower moth, banded sunflower moth, and red sunflower seed weevil, Smicronyx fulvus LeConte, (i. e., total percentage) was influenced by location, but not the relative maturity of tested entries. However, when damage attributed solely to the red sunflower seed weevil was analyzed, more damaged seed were found for late-maturing entries in North Dakota and Nebraska. In addition to the trial data, current pest populations are lower than when delayed planting was first recommended and insecticide use during sunflower bloom is both common and effective. Together, these observations suggest factoring insect pests into planting time decisions may be unnecessary, except for areas with a history of problems with severe pests that cannot be managed using insecticides (e. g., sunflower midge, Contarinia schulzi Gagné).
... After 30 d, the heads were removed, dried, and threshed. The numbers of seeds damaged by S. fulvus larvae were assessed by an X-ray of 200 achenes per head (Peng and Brewer 1995b, with equipment and settings as described by Prasifka et al. 2014). Total numbers of filled seed also were counted to allow weevil performance to be expressed as the number of seeds damaged per female, a measure which adjusts for differences in the size or self-fertility of infested plants. ...
Recent populations of the red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae), have been inconsistent or declining, particularly in North Dakota. Consequently, research on weevil biology, including development of resistant germplasm, has been limited. To determine whether cold storage of diapausing larvae could be improved, nonconstant temperature treatments (fluctuating thermal regime [FTR] and thermoperiod [TP]) were tested versus a constant 6°C for storage up to 365 d. Both alternate temperature treatments produced more adult weevils than constant 6°C for short (42, 91 d) storage, while all temperature treatments were good (≥60% adult emergence) at moderate term (182 d) cold storage, and FTR was best for long (365 d) periods. Excluding the 14-d storage period, which produced too few weevils for comparison, each doubling of cold storage time (e.g., from 42 to 91 d, 91 to 182 d), usually decreased the number of days to 50% relative emergence by ∼10 d. After 365 d of larval storage, emerged S. fulvus adults successfully infested sunflowers in a plant growth chamber, with damage per female similar to that observed in field trials. Compared with previous efforts to store weevil larvae, the method of collection and FTR storage is either more effective (greater adult emergence and reduced parasitism) or more time-efficient, and should permit year-round research using S. fulvus adults. Because successful emergence under FTR was >75% after 365 d, additional research would be required to determine the maximum effective duration of cold storage for S. fulvus .
A laboratory roller mill system was modified to measure and analyze the electrical conductance of wheat as it was crushed. The electrical conductance of normal wheat kernels is normally low and fairly constant. In contrast, the electrical conductance of wheat kernels infested with live insects is substantially higher, depending on the size of the larvae and the resulting contact of the crushed larvae between the rolls. This instrument was designed to detect internal insect infestations in wheat that has a moisture content of 13.5% or less. The laboratory mill can test a kilogram (kg) of wheat in less than 2 min and 100 g in less than 10 s. Hard red winter and soft red winter wheat containing larvae of rice weevils and lesser grain borers of a variety of sizes were tested. On average, the instrument detected 8.3 out of 10 infested kernels per 100 g of wheat containing the larger sized insects (fourth instar or pupae). Kernels infested with medium-sized larvae (second or third instar) were detected at an average rate of 7.4 out of 10 infested kernels per 100 g of wheat. Finally, kernels infested with the small-sized larvae (first or second instar) were detected at a rate of 5.9 out of 10 infested kernels per 100 g of wheat. Under reasonable grain moisture contents there were no false positive errors, or noninfested kernels classified as insect infested. The cost of the mill is low and can lead to rapid and automated detection of infested wheat.
A 7-yr field study evaluated 61 oilseed sunflower, Helianthus annuus L., accessions and 31 interspecific crosses for resistance to attack by naturally occurring populations of three stem-infesting pests, the sunflower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae); a longhorned beetle, Dectes texanus LeConte (Coleoptera: Cerambycidae); and a root boring moth, Pelochrista womonana (Kearfott) (Lepidoptera: Tortricidae), at two locations in the central Great Plains. Germplasm with potential sources of resistance to attack from all three stem-infesting species were revealed. Accessions PI 650558, PI 386230, and PI 431516 were consistent in averaging low densities of stem weevil larvae per stalk among lines tested, and PI 497939 exceeded 25 weevil larvae per stalk in only 1 yr of 5 yr of trials. Several interspecific crosses also had consistently low densities of C. adspersus larvae per stalk. Populations of both D. texanus and P. womonana were variable over years, but differences among the lines tested were evident in many trials, revealing potential for developing resistant germplasm. Four accessions (PI 386230, PI 431542, PI 650497, and PI 650558) had low larval densities of C. adspersus and P. womonana in addition to reduced percentage infestation by D. texanus. Results showed potential for developing resistant genotypes for these pests. The prospect of adding host plant resistance as an integrated pest management (IPM) tactic would provide another tool for reducing economic losses from stem-infesting insect pests of sunflower in the central Great Plains.
There are four major insect pests attacking achenes of sunflower in the Great Plains of North America (Charlet et al., 1987); the red sunflower seed weevil, Smicronyx fulvus LeConte (Coleoptera: Curculionidae), the gray sunflower seed weevil, S. sordidus LeConte (Coleoptera: Curculionidae), the banded sunflower moth, Cochylis hospes (Walsingham) (Lepidoptera: Cochylidae), and the sunflower moth, Homoeosoma electellum (Hulst) (Lepidoptera: Pyralidae). Identification of achenes damaged by larvae of these species is important for assessment of yield loss caused by each species (Peng and Brewer, 1995). Achenes infested by the gray sunflower seed weevil are easily identified because they are enlarged and protrude above the surrounding achenes on intact sunflower heads and the kernel is entirely consumed (Brewer, 1991). Descriptions of achenes infested by larvae of the other three species are similar (Carlson, 1967; Oseto and Braness, 1979; McBride and Charlet, 1991) except for the location of feeding/exit holes on the surface of the achenes infested by the red sunflower seed weevil and banded sunflower moth (Charlet and Gross, 1990). The red sunflower seed weevil oviposits in the developing achenes. The larvae feed on the developing kernels and when mature chew emergence holes in the pericarp (hull) (Oseto and Braness, 1979). The banded sunflower moth and sunflower moth oviposit on the bracts and florets. Early instars feed on the pollen and florets, later instars penetrate into achenes and feed on the developing kernels (Charlet et al., 1987, Charlet and Gross, 1990). In this study, we describe achenes infested by the red sunflower seed weevil, banded sunflower moth and sunflower moth and provide characters for separation of achenes infested by each of these pest species.
Plant population had no significant effect on the infesation level of Cylindrocopturus adspersus (LeConte) larvae infesting the stalks of cultivated sunflower under dryland conditions. However, the mean number of larvae per plant was significantly (5% level) reduced by both delayed planting dates and reduced preplant soil water. Delayed planting dates also significantly reduced the avg number of larvae per plant and percentage infestation in irrigated sunflower. Application of insecticide to control adults reduced the number of larvae infesting stalks of irrigated sunflower, reduced plant lodging, and resulted in significant (5% level) increases in both the quantity and the quality of seeds produced in plants that did not lodge.
Systemic insecticides applied to soil at planting reduced the number of larvae of sunflower stem weevil, Cylindrocopturus adspersus (LeConte), infesting stems of cultivated sunflower (Helianthus annuus L.). Carbamate insecticides were more effective than the organophosphate insecticide terbufos in reducing C. adspersus larval numbers in 1981. Carbamates aldicarb and carbofuran at the higher rates tested lowered larva densities 32 and 79%, respectively, compared with the control. At eight locations in 1983, aldicarb and carbofuran lowered larva densities an average of 63 and 81%, respectively, compared with untreated plots. In 1983, there was a significant difference between number of larvae and percent lodging of plants in treated and untreated plots. A significant difference was observed between seed yield of plots treated with carbofuran and untreated plants (1983). As a management strategy, use of systemic insecticides applied to soil at planting can be beneficial in lowering population densities of sunflower stem weevil larvae, reducing lodging, and preventing yield lossesin areas where C. adspersus has caused economic damage.
Larvae of a stem weevil, Cylindrocopturus adspersus (LeConte), overwintered in sunflower stalks in North Dakota, South Dakota, and Minnesota in 1979 and 1980. Population levels averaged 2.0 and 1.6 larvae per stalk in 1979 and 1980, respectively. The highest number of overwintering larvae per stalk occurred in North Dakota. About half of the stalks from each sampled field were infested with larvae. There was no correlation between stalk diameter and number of larvae per stalk. Overwintering sites were in the upper root crown and in the base of the plant stalk, with half the larvae between soil level and 4 cm above soil level. Overwintering sites averaged 5.9 and 4.6 cm above the soil level in 1979 and 1980, respectively.
Initial oviposition sites of Cylindrocopturus Adspersus (Le Conte) were in the glabrous zones around the cotyledonous (first node) and second nodes, but they moved up the main stalk below successive leafaxils as the growing season progressed. Fcmale C. adspersus oviposited at a mean height of 4.5 cm above the first node area in 1980 and 20.7 cm above the first node in 1981, which encompassed utilization of the lower 6 and 17% of the stcms in 1980 and 1981, respectively. Oviposition a greater distance above the cotyledonous node in 1981 than in 1980 probably was due to increased weevil populations, and to increased competition for egg deposition sites. Fifty percent of oviposition occurred on a date separated by only 5 days in successive years.
Insect damage in wheat adversely affects its quality and is considered one of the most important degrading factors in Canada. The potential of near-infrared (NIR) hyperspectral imaging for the detection of insect-damaged wheat kernels was investigated. Healthy wheat kernels and wheat kernels visibly damaged by Sitophilus oryzae, Rhyzopertha dominica, Cryptolestes ferrugineus, and Tribolium castaneum were scanned in the 1000–1600 nm wavelength range using an NIR hyperspectral imaging system. Dimensionality of the acquired hyperspectral data was reduced using multivariate image analysis. Six statistical image features (maximum, minimum, mean, median, standard deviation, and variance) and 10 histogram features were extracted from images at 1101.69 and 1305.05 nm and given as input to statistical discriminant classifiers (linear, quadratic, and Mahalanobis) for classification. Linear discriminant analysis and quadratic discriminant analysis classifiers correctly classified 85–100% healthy and insect-damaged wheat kernels.
A non-destructive, real time device was developed to detect insect damage, sprout damage, and scab damage in kernels of wheat. Kernels are impacted onto a steel plate and the resulting acoustic signal analyzed to detect damage. The acoustic signal was processed using four different methods: modeling of the signal in the time-domain, computing time-domain signal variances and maximums in short-time windows, analysis of the frequency spectrum magnitudes, and analysis of a derivative spectrum. Features were used as inputs to a stepwise discriminant analysis routine, which selected a small subset of features for accurate classification using a neural network. For a network presented with only insect damaged kernels (IDK) with exit holes and undamaged kernels, 87% of the former and 98% of the latter were correctly classified. It was also possible to distinguish undamaged, IDK, sprout-damaged, and scab-damaged kernels.
Computed tomography, an imaging technique commonly used for diagnosing internal human health ailments, uses multiple x-rays and sophisticated software to recreate a cross-sectional representation of a subject. The use of this technique to image hard red winter wheat, Triticum aestivm L., samples infested with pupae of Sitophilus oryzae (L.) was investigated. A software program was developed to rapidly recognize and quantify the infested kernels. Samples were imaged in a 7.6-cm (o.d.) plastic tube containing 0, 50, or 100 infested kernels per kg of wheat. Interkernel spaces were filled with corn oil so as to increase the contrast between voids inside kernels and voids among kernels. Automated image processing, using a custom C language software program, was conducted separately on each 100 g portion of the prepared samples. The average detection accuracy in the five infested kernels per 100-g samples was 94.4 +/- 7.3% (mean +/- SD, n = 10), whereas the average detection accuracy in the 10 infested kernels per 100-g sample was 87.3 +/- 7.9% (n = 10). Detection accuracy in the 10 infested kernels per 100-g samples was slightly less than the five infested kernels per 100-g samples because of some infested kernels overlapping with each other or air bubbles in the oil. A mean of 1.2 +/- 0.9 (n = 10) bubbles (per tube) was incorrectly classed as infested kernels in replicates containing no infested kernels. In light of these positive results, future studies should be conducted using additional grains, insect species, and life stages.