Phytochemical screening and insecticidal activities of some medicinal plants against the maize weevil, Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae)

This study emphasizes the phytochemical study of some locally available botanicals against maize weevils. Nine plant parts were collected from six plant species. The test plant powder (200 g) was suspended sequentially in 600 ml of petroleum ether, chloroform, acetone, methanol, and distilled water for 72 h with frequent agitation. Different concentrations of the crude extracts were applied to maize seeds at rates of 10 ml, 15 ml and 20 ml per 100 g. All treatments with different extracts at different rates of application showed significant differences (p < 0.05) in the cumulative mean percentage mortality of the maize weevil. The seed extract of Maesa lanceolata and Croton macrostachyus and the leaf extract of Clausena anisata showed cumulative percent mortality ranged 95.32-98.02% in 28 days after treatment application. There was no significant difference (p > 0.05) among all treatments for the prevention of F1 progeny emergence. In all extracts, Clausena anisata showed 100% inhibition of F1 progeny emergence. All treatments significantly reduced seed weight loss and damage. The treated maize seeds were germinated with an acceptable germination quality. In conclusion, an increased dosage of the extract resulted in significant mortality in maize weevils. The seed extracts of Maesa lanceolata and Croton macrostachyus and Clausena anisata leaf extract were observed to be the most promising botanical in protecting stored maize against maize weevil.

Insecticidal toxicity of essential oil of Nepalese Acorus calamus (Acorales:Acoraceae) against Sitophilus zeamais (Coleoptera:Curculionidae)

Maize weevil (Sitophilus zeamais) (Coleoptera:Curculionidae) is an economic stored grain pest that causes significant damage to various stored products, including maize (Zea mays). In this study, we extracted essential oil from the rhizome of sweet flag (Acorus calamus) (Acorales:Acoraceae) by hydro-distillation and tested insecticidal property of the oil at 7 concentrations (10, 5, 2.5, 1.25, 0.625, 0.3125, 0.15625 and control) against maize weevil (Sitophilus zeamais) at the National Entomology Research Center, Nepal Agricultural Research Council in the year 2020/2021. Three different experiments were conducted: scintillating vial bioassay, repellency test, and exposing weevils to oil treated maize grains. Scintillating vial bioassay showed that higher the concentration of essential oil, lower the time required to cause 50 % maize weevil mortality. Median lethal concentration (LC50) at 3 and 24 h was calculated as 2.29 and 0.16 % of oil concentration in scintillating vial bioassay. When oil is treated to maize grain, LC50 for 10 and 16 days was calculated as 2.77 and 0.23 % of oil concentrations. In the same way, at 10 % concentration maize weevil showed highest repellent activity (98.75 %) as compared to 5, 2.5 and 1.25 % concentrations after 24 h of treatment. Weight loss and grain damage were significantly less in the oil treatments than the control. However, from the perspective of health benefits, Acorus calamus treated maize is still questionable for feed and food purpose. As β asarone has carcinogenic effects at certain level, it needs further residue tests of treated maize to know allowable maximum residue limit (MRL) before consumption as food or feed.

Development of the first high-density linkage map in the maize weevil, Sitophilus zeamais

The maize weevil, Sitophilus zeamais, is a worldwide pest that disproportionately affects subsistence farmers in developing countries. Damage from this pest threatens food security in these communities as widely available and effective control methods are lacking. With advances over the last decade in the development of genetic pest management techniques, addressing pest issues at the ecosystem level as opposed to the farm level may be a possibility. However, pest species selected for genetic management techniques require a well-characterized genome and few genomic tools have been developed for S. zeamais. Here, we have measured the genome size and developed the first genetic linkage map for this species. The genome size was determined using flow cytometry as 682 Mb and 674 Mb for females and males, respectively. The linkage map contains 11 linkage groups, which correspond to the 10 autosomes and 1 X-chromosome found in the species and it contains 1,121 SNPs. This linkage map will be useful for assembling a complete genome for S. zeamais.

On-farm storage loss estimates of maize in Kenya using community survey methods

Maize is the most important staple in sub-Saharan Africa (SSA), with highly seasonal production. High storage losses affect food security, but good estimations are lacking. A new method using focus group discussions (FGDs) was tested with 121 communities (1439 farmers, 52% women) in Kenya's six maize-growing zones, to estimate the maize losses to storage pests and analyze farmer practices. As control strategies, half of the farmers used chemical pesticides (49%), while hermetic bags (16%) and botanicals (15%) were also popular. Relative loss from weevils in the long rains was estimated at 23%, in the short rains 18%, and annually 21%. Fewer farmers were affected by the larger grain borer (LGB) than by maize weevils: 42% in the long rainy season and 32% in the short rainy season; losses from LGB were also smaller: 19% in the long season, 17% in the short season, and 18% over the year. Total storage loss, from both species combined, was estimated at 36%, or 671,000 tonnes per year. The greatest losses occur in the humid areas, especially the moist mid-altitudes (56%), and with smaller loss in the drylands (20-23%). Extrapolating the point data and overlaying with the maize production map shows the geographic distribution of the losses, with the most important area found around Lake Victoria. FGDs provide convenient and cheap tools to estimate storage losses in representative communities, but a total loss estimate of 36% is higher than is found in other studies, so its accuracy and framing effects need to be assessed. We conclude that storage pests remain a major problem, especially in western Kenya, and that the use of environmentally friendly technologies such as hermetic storage and botanicals needs more attention, both by the public extension service and private agrodealers.

Insecticidal activities of long pepper (Piper retrofractum Vahl) fruit extracts against seed beetles (Callosobruchus maculatus Fabricius, Callosobruchus chinensis Linnaeus, and Sitophilus zeamais Motschulsky) and their effects on seed germination

Bruchid beetles (Callosobruchus maculatus and Callosobruchus chinensis), and maize weevil (Sitophilus zeamais) are important insect pests during the postharvest period. Botanical insecticide is an alternative solution for controlling these insects, and long pepper (Piper retrofractum) has been reported as having insecticidal potential against general insect pests. Film seed coatings with various concentrations of hexane extracts were made for mung bean (Vigna radiata) and corn (Zea mays) seeds. Insecticidal activities of these treatments were assessed at before and after storage period of six months, and seed germination was also evaluated. The hexane extract was subjected to analysis of the bioactive components by using Gas Chromatography-Mass Spectrometry (GC-MS). Results revealed that the hexane extract presented extreme toxicity to both bruchid beetles higher compared to maize weevil at 24 h with LC₅₀ values of 5.57-6.75 and 58.04 μg⋅cm^-2, respectively. Bruchid beetles presented significant response to ethanol, acetone and hexane extracts, whereas maize weevil showed relatively low responsibility. Film seed coating with hexane extract at 1% and 3% concentrations with six-month storage presented high insecticidal activity against bruchid beetles by more than 88% mortality but had low kill rates against maize weevil. The coated mung bean seeds presented non-seed germination effect, whereas high effect was observed on coated corn. Isolation of bioactive components demonstrated that there were 74 compounds, where pentadecane was the main compound. Film seed coating technology for mung bean seed preservation by using 1% hexane extract from long pepper fruit presented to be an extremely effective method to control bruchid beetles without any seed germination effect. It could serve as one of the green insecticides of the future.

Insecticidal activity of a chemotype VI essential oil from Lippia alba leaves collected at Caatinga and the major compound (1,8-cineole) against Nasutitermes corniger and Sitophilus zeamais

Lippia alba is an aromatic shrub known to produce a diversity of essential oils, which can be classified into chemotypes. This study reports on the insecticidal activity of essential oil from L. alba leaves collected at Caatinga and its major compound against termite Nasutitermes corniger and maize weevil Sitophilus zeamais. The chromatographic analysis revealed the presence of 19 compounds, with 1,8-cineole being the most common (70.01%). When ingested, the oil promoted the mortality of N. corniger (LC₅₀: 18.25 and 8.4 nL/g for workers and soldiers, respectively). The compound 1,8-cineole was also termiticidal for workers (LC₅₀: 13.7 nL/g). The oil inhibited the activity of N. corniger exoglucanase, xylanase, and proteases. Toxicity by ingestion to S. zeamais was detected for the oil (LC₅₀: 0.297 μL/g) but not for 1,8-cineole; however, both the oil and 1,8-cineole showed anti-nutritional effects. Fumigant effects of the oil and 1,8-cineole against S. zeamais (LC₅₀ of 78.0 and 13.64 μL/L in air, respectively) were detected. This is the first record of a chemotype VI oil from L. alba collected at Caatinga and the first report of the insecticidal activity of a chemotype VI oil. Our study demonstrates that essential oil from L. alba and 1,8 cineole have the potential for the development of natural insecticides.

Evaluation of early-generation tropical maize testcrosses for grain-yield potential and weevil (Sitophilus zeamais Motschulsky) resistance

Smallholder maize farmers in Africa experience pre- and post-harvest production stresses either individually or in combination at different stages of the crop cycle. The maize weevil is among the major post-harvest storage pests. A strategy to address this problem is to develop and promote high yielding maize germplasm with resistance to multiple stresses. A study was conducted to: 1) assess yield and agronomic performance of testcross hybrids developed from early generation lines; and 2) assess the response of the testcross hybrids to infestation with Sitophilus zeamais. Fifty-eight drought-tolerant testcross hybrids were evaluated for agronomic performance and weevil resistance at four environments in Uganda in 2016. Hybrid G39 (L2/T2) had the best grain yield performance; it significantly out-performed the best check by 11.4% in all environments. Hybrid grain from field trials was subjected to Sitophilus zeamais infestation in a choice and no choice test under laboratory conditions. Hybrids G56 (L49/T2) and G58 (L51/T2) had the least weevil damage and were rated as resistant to Sitophilus zeamais. The numbers of damaged kernels, number of exit holes and ear aspect were positively correlated with the grain weight loss. The results suggest possibilities for simultaneous selection for high grain yield and storage insect pest resistance among drought-tolerant genotypes. Use of high-yielding and resistant maize hybrids to storage insect pest should be promoted for increased maize production and managing post-harvest losses due to the maize weevil in smallholder farming communities in Africa.

Transcriptome profiling reveals differential gene expression of detoxification enzymes in Sitophilus zeamais responding to terpinen-4-ol fumigation

Plant essential oils with high bioactivity provide environmental friendly alternatives for synthetic pesticides. Melaleuca alternifolia essential oil and its main constituent terpinen-4-ol have high insecticidal activity to Sitophilus zeamais Motschulsky. Terpinen-4-ol may be metabolized by human and insect cytochrome P450s. However, little is known about how insects systemically respond to terpinen-4-ol. In this study, we used an RNA-seq approach to evaluate the global gene expression of S. zeamais after terpinen-4-ol fumigation. Duplicates of fumigated and control groups, for a total of four libraries, were collected for sequencing. A total of 36,117 unigenes with an average length of 1036 bp were generated in the de novo assembled transcriptome. Comparative analysis of S. zeamais libraries identified 592 differentially expressed genes (DEGs), of which 308 and 284 genes were up- and down-regulated in response to terpinen-4-ol fumigation, respectively. GO and KEGG functional analyses were performed for up and downregulated DEGs separately, showing these DEGs were enriched for terms related to catalytic activity, carbohydrate metabolism, and xenobiotics biodegradation and metabolism. DEGs encoding enzymes for detoxification were detected, including sixteen cytochrome P450s (P450s), eight glutathione S-transferase (GSTs), fourteen esterase (ESTs), ten UDP-glucuronosyltransferase (UGTs), and two ATP-binding cassette transporter (ABC transporter) genes. Real-time quantitative PCR confirmed that ten P450s, three GSTs and one EST were up-regulated dramatically after exposure to terpinen-4-ol at different concentrations and over a time course. The results provided a transcriptional overview of the changes in a stored-grain pest in response to terpinen-4-ol fumigation. The analysis revealed the expression levels of detoxification genes were altered, especially for P450s, and provided candidate genes for understanding systemic metabolic responses to terpinen-4-ol in insects.