Although we sampled populations from Uruguay, we sampled them from the Uruguay River, in‐between Argentina and Uruguay, and speculate that the sample is likely genetically similar to those weevils in Santa Fe, Argentina. Thus, rather than due to multiple introductions, the higher allelic richness in Florida, Texas, and California, USA, may have been due to the temporal proximity of these populations to the initial imported population from the native range . In addition to clarifying the introduction pathways, our population genetic analyses demonstrated the presence of several distinct and broad genetic clusters for each N. bruchi and N. eichhorniae. In the case of N. bruchi, FLOCK and DAPC indicated two main genetic clusters and 11 sub‐clusters For N. eichhorniae, FLOCK and DAPC signified four to six main genetic clusters and 23 sub‐clusters. In comparison, the STRUCTURE program detected two to six distinct broad populations for each weevil species, but did not detect sub‐clustering within these populations . This indicates that significant divergence occurred among and between several of the introduced populations and the native population since the initial introductions in the 1970s. This supports previous studies on invasive species and biological control agents that demonstrate the divergence of populations from the native range but see Franks et al. . Divergence of introduced populations from the native populations likely depends on the time since the initial introduction. For example, we sampled populations almost 50 years after the initial introductions, harvest drying rack whereas Franks et al. sampled in the introduced range just 2 years after the initial releases.
One caveat that we acknowledge is that the genetic divergence between the introduced and native range may have been due to the fact we sampled from Uruguay rather than Argentina, where the actual initial source populations were exported from. However, based on the DAPC and FLOCK analyses, the populations from Uruguay for both species appear to be genetic sources for several of our populations. Thus, we feel confident that the genetic composition from weevils in Argentina compared to those in Uruguay is not very different. In addition to the results demonstrating that genetic drift and inbreeding occurred in several populations, we speculate that divergence has also occurred due to local adaptation to some of the regions of introduction. Many of the introduced regions that we tested in this study have colder climates than that occurring in South America. Recently, Reddy et al. tested the cold‐temperature tolerance and life‐history performance of N. eichhorniae under cool temperature conditions simulating the fall season in Sacramento– San Joaquin River Delta, California. Reddy et al. tested the same populations of N. eichhorniae used in the present study and found that weevils from the population in Australia had a higher fecundity under these cool temperature conditions compared to weevils from California and Uruguay, SA: Kubusi River. These results were surprising as the population in Australia had lower genetic diversity than the other populations, thus suggesting that populations can still adapt to local areas even with moderate levels of genetic diversity.
Furthermore, the present study combined with that of Reddy et al. demonstrates that both genetic composition and life‐history performance may have diverged among these populations. We support the recommendation that population genetic analyses be performed prior to the selection and release of biological control agents . The genetic diversity and genetic composition may have implications for the population growth of the biological control agents and their success in controlling the target weed or pest. Although these weevils have shown tremendous success in reducing water hyacinth in a number of countries , less than optimal levels of control has been found in regions with cooler temperatures, including some of the high altitude areas in South Africa and in the Sacramento–San Joaquin River Delta, in northern California . The lower efficacy of biological control in these regions could be due to climatic mismatch and/or the inability to thrive and adapt to the local area based on the genetic diversity and composition as influenced by importation methods and the selected source populations.Hybrid sunflower seed production involves planting male-fertile parental lines and malesterile parental lines in separate rows within a field. Cross-pollination between the two lines produces a hybrid seed that contains the best traits of each parent. Males generally have multiple flowers per plant, compared with female plants with a single composite flower. Honey bees are relied on to move pollen from the male to female lines. After pollination, when the seeds are set, growers chop the male rows to prevent any male seeds from contaminating the female lines at harvest.
When the female sunflower stalks dry down at the end of the growing season, they are harvested, with yields averaging 1,200 pounds per acre, depending on the yield potential of the female parent . Prices paid to seed growers averaged $1.30 per pound over the past 5-years, depending on the hybrid, with lower yielding types having higher prices . The value of planting seed is about five to ten times that of a commercial oil or confectionary seed crop. Sunflower, and other members of this genus, are native to North America. They were domesticated thousands of years ago and likely favored as an important high-energy food source by Native Americans. For the oil-seed market, sunflower oil is often preferred by the food processing industry because it is stable at high cooking temperatures. The oil also supplies more vitamin E than any other vegetable oil, and the varieties with high levels of oleic oil deliver low levels of saturated fat. The small-seeded oil types are also important for birdseed markets. For the confectionary market, the non-oil, large-seeded white-stripe sunflower types are important for the snack food industry. The United States grows about 1.7 million acres of sunflower, mostly for oil, primarily in the Dakotas, although the production area stretches south to the panhandle of Texas. In most years, about 25 percent of Californiahybrid sunflower seed goes to international markets, though in some years it can be as high as 60 percent. Russia and Ukraine are California’s largest export markets, where more than 50 million acres of sunflowers is grown for oil, followed by China and the European Union.Management practices for sunflower production depend on the growth stage of the plants . Sunflowers reach physiological maturity at the R9 stage and can be harvested once the desired seed moisture content is reached. In the Sacramento Valley, for a March planting time, maturity is reached in 130 to 140 days from the planting date. For April and May plantings maturity is typically reached in 120 days. In the Imperial Valley sunflowers are typically planted in February, with 130 to 140 days to harvest from the planting date. In general, sunflower fields initiate bloom about 60 to 70 days after planting, bloom for about 10 days, and then are harvested about 60 days after bloom. Sunflowers are in the Compositae family. The sunflower head is actually not a single flower but a flower head consisting of numerous small individual florets. The outside florets resembling petals are called ray flowers and are sterile. The numerous disk flowers in the center produce seed and normally are considered ‘perfect’ . However, in hybrid seed production the female lines have cytoplasmic male sterility, so they produce only female florets, enabling cross-pollination with a male line .Sunflower breeding programs for hybrid seed development are primarily driven by private seed companies. Hybrid sunflower varieties are first produced when plant breeders cross-pollinate two different inbred selection lines where each of the parents contain complimentary traits . The first generation of a hybridized plant cross tends to grow with more vigor and produce higher yields and oil content than the parental lines. The hybrids also display better disease resistance, self-compatibility , vertical growing racks and uniformity in height, maturity, and moisture content, which facilitates harvest. The two main types of sunflower varieties are oilseed and non-oilseed . Oilseed varieties have a very high oil content ; they are processed into sunfloweroil, and the resulting meal is used as a high-protein animal feed. Small-seeded oil types are also used for birdseed.
The confectionary types usually produce a larger black and white striped seed and are used in a variety of food products ranging from snacks to bread. Most of the oilseed sunflower hybrids planted in North America fall in the mid to high oleic fatty acid content range. These edible oils are stable, with no trans fats, are naturally low in saturated fats, and resist rancidity in long-term storage. Hybrids with high linoleic acid content produce a polyunsaturated oil, which is considered to be less healthy and not preferred by the U.S. food manufacturing industry. However, the bulk of the rest of the world produces sunflower oils high in linoleic fatty acid content. In some years, 60% of the seed production acreage is for export, with a predominance of high-linoleic types. Commercial sunflower seed is virtually all proprietary and owned by companies such as NuSeed, DOW-DuPont-Pioneer, Syngenta, Limagrain, and Bayer-Monsanto, who contract with growers to produce the seed. There are no genetically modified sunflowers currently on the market. Because the Helianthus genus is native to North America, there is concern regardinguncontrolled gene flow of GMO types into wild sunflowers. Herbicide-tolerant hybrids such as Clearfield with imazamox tolerance and ExpressSun with tribenuron tolerance were derived through conventional breeding, using wild sunflowers that already exhibited these genetic traits.Hybrid sunflower seed production must follow strict standards to meet California state export mandates for all certified sunflower fields. These include crop rotation, weed and disease control, simultaneous bloom of male and female lines, and field isolation in time and space to ensure varietal purity . Standards are set by the University of California Division of Agriculture and Natural Resources California Crop Improvement Association and the California Department of Food and Agriculture . To meet the standards, fields are certified by two agencies, CCIA and the county agricultural commissioner in the county where the sunflower seed is produced. CCIA inspects fields three times during the season for genetic purity, appropriate field isolation including no wild or volunteer sunflowers, and weed control. The first inspection occurs during prebloom , the second during early bloom , and the third during mid to late bloom . When “off types” are found, the seed companies will send in crews to rogue the fields for other varieties, to within prescribed tolerances. Any pollen-shedding females, called “shedders,” will also be rogued. Additional roguing may be required if either the seed company agronomist or the CCIA inspector continue to find excessive levels of off-types, shedders, or diseased plants in the field. CCIA inspects fields for noxious weeds that fall into two classifications: prohibited and restricted. The list of plants prohibited under the California Seed Law , including seeds and propagules, are updated periodically and listed on the CCIA website . There is zero tolerance in certified seed production for either prohibited or restricted weeds in both the field and seed lots . CCIA may reject fields and refuse certification due to unsatisfactory appearance caused by weeds, poor growth, poor stand, disease, insect damage, and any other condition that prevents accurate inspection or creates doubt as to the identity of the variety. For non-certified seed production, restricted weeds, as well as weed seeds that are difficult to separate from sunflower seed, must be controlled and are subject to tolerance standards after harvest. Certification standards for hybrid sunflower seed production require a 1.25-mile isolation from volunteer sunflowers, wild sunflowers, ornamental sunflowers, and sunflower fields containing a different male line. Failure to accomplish adequate isolation within the prescribed tolerances prior to bloom will subject harvested seed to a mandatory grow-out and possible rejection of seed, depending on the grow-out results for identifying seed purity. To ensure isolation during the growing season, seed companies coordinate planting of seed fields off season, using an electronic isolation map hosted by CCIA . This map, however, does not maintain a comprehensive listing of wild sunflower populations, so companies will generally avoid planting in regions historically known to have large populations of wild sunflowers, such as the Sacramento Delta region and the San Joaquin Valley. County agricultural commissioner biologists inspect sunflower hybrid seed fields twice during the season for diseases. The first occurs during prebud formation , when they primarily look for virus symptoms. The second field inspection is between full bloom and seed maturity , when they inspect for a range of diseases .