Cyromazine resistance in the house fly (Diptera: Muscidae): genetics and cross-resistance to diflubenzuron

Larvae of a house fly, Musca domestica L., strain collected in a chicken house near Pittsburg, Tex, after a control failure with the poultry feedthrough insecticide cyromazine showed 6.5-fold resistance to cyromazine and 10-fold resistance to diflubenzuron. Adults of the strain showed high levels of resistance to carbaryl, DDT, and diazinon; moderate resistance to cypermethrin and permethrin; and low resistance to dieldrin. In contrast, no resistance to cyromazine was observed in eight laboratory house fly strains with resistance to four groups of conventional insecticides. When the genetics of cyromazine resistance was investigated in crosses to susceptible strains with visible mutant markers, results indicated cyromazine resistance was incompletely dominant over susceptibility and the resistance gene was on chromosome V. The same or a closely linked gene conferred resistance to diflubenzuron. A strain containing only chromosome V from the original resistant strain was resistant to cyromazine and diflubenzuron, but not to other insecticides except for low level resistance to DDT and carbaryl. Resistance to the latter insecticides appeared to be due to a linked, but distinct, gene. Therefore, resistance to cyromazine and probably diflubenzuron appears to be genetically distinct from other types of insecticide resistance.     

Mutations in acetylcholinesterase genes of Rhopalosiphum padi resistant to organophosphate and carbamate insecticides.

Apple grain aphid, Rhopalosiphum padi (Linnaeus), is an important wheat pest. In China, it has been reported that R. padi has developed high resistance to carbamate and organophosphate insecticides. Previous work cloned from this aphid 2 different genes encoding acetylcholinesterase (AChE), which is the target enzyme for carbamate and organophosphate insecticides, and its insensitive alteration has been proven to be an important mechanism for insecticide resistance in other insects. In this study, both resistant and susceptible strains of R, padi were developed, and their AChEs were compared to determine whether resistance resulted from this mechanism and whether these 2 genes both play a role in resistance. Bioassays showed that the resistant strain used was highly or moderately resistant to pirimicarb, omethoate, and monocrotophos (resistance ratio, 263.8, 53.8, and 17.5, respectively), and showed little resistance to deltamethrin or thiodicarb (resistance ratio, 5.2 and 3.4, respectively). Correspondingly, biochemistry analysis found that AChE from resistant aphids was very insensitive to the first 3 insecticides (I50 increased 43.0-, 15.2-, and 8.8-fold, respectively), but not to thiodicarb (I50 increased 1.1-fold). Enzyme kinetics tests showed that resistant and susceptible strains had different AChEs. Sequence analysis of the 2 AChE genes cloned from resistant and susceptible aphids revealed that 2 mutations in Ace2 and 1 in Ace1 were consistently associated with resistance. Mutation F368(290)L in Ace2 localized at the same position as a previously proven resistance mutation site in other insects. The other 2 mutations, S329(228)P in Ace1 and V435(356)A in Ace2, were also found to affect the enzyme structure. These findings indicate that resistance in this aphid is mainly the result of insensistive AChE alteration, that the 3 mutations found might contribute to resistance, and that the AChEs encoded by both genes could serve as targets of insecticides.     

Cross-resistance between dieldrin and fipronil in German cockroach (Dictyoptera: Blattellidae)

The toxicity of fipronil and dieldrin was determined in one susceptible laboratory strain and seven insecticide-resistant field-collected strains of Blattella germanica (L). The Zo960302 and Ga021001 strains were 1,270- and 2,030-fold resistant to dieldrin and 15- and 14-fold resistant to fipronil. The Su960304 and Od010803 strains were 15- and 13-fold resistant to dieldrin and two- and four-fold resistant to fipronil. Three strains showed no or a low level of resistance to dieldrin and fipronil. Crosses were performed between the susceptible strain Danish Pest Infestation Laboratory (DPIL)-SUS and the resistant strains Zo960302 and Su960304 and resistance to dieldrin and fipronil were intermediate compared with the susceptible and the resistant strains. Backcrosses to both of the parental strains showed cosegregation of dieldrin and fipronil resistance. The toxicity of dieldrin and fipronil was correlated when compared at LD50, and 93% of the observed variation in LD50 of fipronil can be ascribed to variation among predictions based on the value of LD50 of dieldrin. The frequency of the A302S substitution in the resistance to dieldrin (Rdl) gene in the highly dieldrin- and fipronil-resistant strains Zo960302 and Ga021001 and the moderately resistant Su960304 was 0.97, 1.0, and 0.38, respectively. We consider the connection between the frequency of the Rdl mutation and dieldrin and fipronil resistance a causal connection and not merely a coincidence.     

Pesticide Susceptibility of Two Coccinellids (Stethorus punctum picipes and Harmonia axyridis) Important in Biological Control of Mites and Aphids in Washington Hops

The susceptibility of Stethorus punctum picipes (Casey) and Harmonia axyridis Pallas larvae to pesticides used or with potential for use in Washington hops, was examined in laboratory bioassays. All pesticides tested except the miticide, hexythiazox, the insecticides, chlorpyrifos and pirimicarb, and the fungicide, mycobutanil, produced 100% mortality in S. punctum picipes at concentrations equivalent to field rates. The insecticides, pirimicarb, endosulfan, and thiamethoxam were least toxic to H. axyridis. Bifenthrin, diazinon, dimethoate, methomyl, carbaryl, malathion, phosmet, imidacloprid, and chlorpyrifos were highly toxic. The miticides, abamectin and fenpyroximate were highly toxic, milbemectin was moderately toxic but all other miticides tested were non-toxic. All fungicides had low toxicity. Selection and use of pesticides compatible with natural enemies and conservation biological control in Washington hop production is discussed.     

KNOCKDOWN OF THE IONOTROPIC γ‐AMINOBUTYRIC ACID RECEPTOR (GABAR) RDL GENE DECREASES FIPRONIL SUSCEPTIBILITY OF THE SMALL BROWN PLANTHOPPER,Laodelphax striatellus (HEMIPTERA: DELPHACIDAE)

Insect γ‐aminobutyric acid receptors (GABARs) are important molecular targets of cyclodiene and phenylpyrazole insecticides. Previously GABARs encoding rdl (resistant to dieldrin) genes responsible for dieldrin and fipronil resistance were identified in various economically important insect pests. In this study, we cloned the open reading frame cDNA sequence of rdl gene from fipronil‐susceptible and fipronil‐resistant strains of Laodelphax striatellus (Lsrdl). Sequence analysis confirmed the presence of a previously identified resistance‐conferring mutation. Different alternative splicing variants of Lsrdl were noted. Injection of dsLsrdl reduced the mRNA abundance of Lsrdl by 27–82%, and greatly decreased fipronil‐induced mortality of individuals from both susceptible and resistant strains. These data indicate that Lsrdl encodes a functional RDL subunit that mediates susceptibility to fipronil. Additionally, temporal and spatial expression analysis showed that Lsrdl was expressed at higher levels in eggs, fifth‐instar nymphs, and female adults than in third‐instar and fourth‐instar nymphs. Lsrdl was predominantly expressed in the heads of 2‐day‐old female adults. All these results provide useful background knowledge for better understanding of fipronil resistance related ionotropic GABA receptor rdl gene expressed variants and potential functional differences in insects.     

新疆棉铃虫对溴氰菊酯和硫丹 抗性的生化机理研究

通过生物测定和生化分析研究了新疆棉铃虫Helicoverpa armigera (Hübner)敏感种群和室内筛选获得的抗性种群对硫丹和溴氰菊酯的反应及其α-乙酸萘酯酶和乙酰胆碱酯酶的动态活性反应。结果表明,筛选后新疆棉铃虫对硫丹和溴氰菊酯产生的抗性倍数分别为13倍和66倍。两个抗性种群的α-乙酸萘酯酶和乙酰胆碱酯酶比活力均高于敏感种群。相应杀虫剂预处理后,α-乙酸萘酯酶酶活力受到抑制。抗性种群的α-乙酸萘酯酶对底物的亲和力高于敏感种群,但Vmax低于敏感种群。抗性种群的乙酰胆碱酯酶对底物的亲和力显著低于敏感种群,Vmax比敏感种群高。聚丙烯酰胺凝胶电泳显示,两个抗性种群都有一条特异性酶带,其迁移率相近,且均可被甲基对氧磷抑制。因此推测,α-乙酸萘酯酶参与了新疆棉铃虫对硫丹和溴氰菊酯的抗性,具有代谢和阻断作用;乙酰胆碱酯酶对抗性的产生也起到了重要作用。     

Investigation of resistance mechanisms to fipronil in diamondback moth (Lepidoptera: Plutellidae)

The SZ-F strain of Plutella xylostella (L.) (Lepidoptera: Plutellidae) was derived from a field-collected strain (SZ) by 20 generations of continuous selection with fipronil. The selection resulted in 300-fold increase in resistance to fipronil, and 3.5- and 6.5-fold cross-resistance to dieldrin and endosulfan, respectively, in the SZ-F strain compared with the unselected SZ strain. Analysis of detoxification enzyme activities and bioassay with synergists indicated that metabolic mechanisms are not important to fipronil resistance of the SZ-F strain and that the fipronil resistance is most likely attributed to target site insensitivity. The genomic DNA fragments flanking the second membrane-spanning region of Rdl gamma-aminobutyric acid (GABA) receptor gene from P. xylostella, PxRdl, were cloned and sequenced. A single allele of the PxRdl gene (encoding A302 or allele PxRdl-Ala) was found in both the Roth (susceptible) and SZ strains. In addition to the wild-type allele PxRdl-Ala, the resistant SZ-F strain carried a mutant PxRdl allele with the conserved amino acid replacement A302(GGC)--> S302(TCC) (allele PxRdl-Ser). The mutant PxRdl-Ser allele frequency in the SZ-F strain was 30%. After treatment of 20 mg/l fipronil on the SZ-F strain, the PxRdl-Ser allele frequency in the survivors increased to 57%. High frequency of the PxRdl-Ala allele remaining in the resistant SZ-F strain suggested that the A302S mutation in the PxRdl gene is partially associated with fipronil resistance and that other mutation(s) in the PxRdl gene or other Rdl-like subunit(s) may contribute to fipronil resistance.     

European monitoring of resistance to insecticides in Myzus persicae and Aphis gossypii (Hemiptera: Aphididae) with special reference to imidacloprid

The susceptibility to several insecticides of 16 and 8 strains of Myzus persicae Sulzer and Aphis gossypii Glover, respectively, received from different European countries in 2001 was investigated. Most of the strains were derived from places known for their aphid resistance problems to conventional insecticides before imidacloprid was introduced. In many regions and agronomic cropping systems imidacloprid has been an essential part of aphid control strategies for a decade, and therefore the susceptibility of aphid populations to imidacloprid using FAO-dip tests and diagnostic concentrations in a leaf-dip bioassay was checked. Additional insecticides tested were cyfluthrin (chemical class: pyrethroid), pirimicarb (carbamate), methamidophos and oxydemeton-methyl (organophosphates). Diagnostic concentrations (LC99-values of reference strains) for each insecticide were established by dose response analysis using a new leaf-disc dip bioassay format in 6-well tissue culture plates. Virtually no resistance to imidacloprid in any of the field-derived populations of M. persicae and A. gossypii was detected. In contrast, strong resistance was found to pirimicarb and oxydemeton-methyl, and to a lesser extent also to cyfluthrin. Two strains of A. gossypii exhibited reduced susceptibility to imidacloprid when tested directly after collection. However, after maintaining them for six weeks in the laboratory, the aphids were as susceptible as the reference strain. The diagnostic concentration of methamidophos did not reveal any resistance in M. persicae, but did so in four strains of A. gossypii.     

The impact of insecticide resistance in the currant-lettuce aphid, Nasonovia ribisnigri, on pest management in lettuce

Abstract 1 This paper reports on experiments to determine how two different insecticide resistance phenotypes in the aphid Nasonovia ribisnigri (Mosley), which is a major pest of lettuce, change its susceptibility to pyrethroid insecticides and the carbamate pirimicarb. 2 A novel statistical approach determined how the effectiveness of different insecticides was changed by the two resistance phenotypes. This compared the between‐plant distribution of aphid numbers, as opposed to the mean number of aphids per plant. 3 Results from field cage experiments showed that the effect of the resistances differed. Pyrethroid resistance resulted in lower mortality immediately after application of pyrethroids, whereas resistance to pirimicarb shortened the time over which the chemical was effective. 4 The results of laboratory bioassays suggested that these two resistances were not found together in N. ribisnigri. However, the results reported here contradict this assertion. 5 Experiments with insecticide residues showed that reproduction of resistant N. ribisnigri was greater than that of susceptible N. ribisnigri on plants with ageing insecticide residues, even in circumstances where mortality of resistant and susceptible clones of N. ribisnigri were similar. 6 If more than a few aphids are found on a plant then a whole consignment can be rejected for processing. The results reported here suggest that the effect of both insecticide resistances in N. ribsinigri will be to increase the proportion of lettuce heads with an unacceptable number of aphids on them, leading to increased rejection of plants for processing.     

The prospect of using sub-lethal imidacloprid or pirimicarb and a parasitoid wasp, Lysiphlebus fabarum, simultaneously,to control Aphis gossypii on cucumber plants

The broad-spectrum insecticides greatly influence the control of cotton aphids; however, due to frequent chemical control, Aphis gossypii (Hemiptera: Aphididae) has developed resistance against several classes of synthetic insecticides. In this study, we explored the sub-lethal effects of imidacloprid and pirimicarb, two commonly used insecticides for aphid control, on a parasitoid wasp, Lysiphlebus fabarum (Marshall) (Braconidae: Aphidiinae), when simultaneously used to control melon aphid on cucumber plants, as part of a comprehensive study for integrated pest management. Bioassays of imidacloprid and pirimicarb were performed to calculate LC₅₀ with third instars of A. gossypii. The LC₅₀ of these insecticides (110.55 and 250.89 μg/lit, respectively) were used to expose the wasp larvae, pupae, and adult parasitoids on a cucumber leaf. The percent mortality, percent adult emergence, and sex ratio were calculated during each exposure test. Moreover, the body size, egg load, and mature egg size of wasps surviving the insecticide treatments, as well as the sex ratio of the second generation was evaluated. Regardless of the host aphid mortality, none of the insecticides caused mortality of larval stage of the parasitoid. The insecticide application on pupal stage revealed that the percentage of mortality, sex ratio, body size, and egg load of surviving wasps, as well as the sex ratio of their offspring was adversely affected by imidacloprid, but not by pirimicarb. The present study suggests pirimicarb as a preferred insecticide, with less harmful effects on the fitness components of L. fabarum, for integrated pest management of cotton aphids.     

Sublethal effects of two neurotoxican insecticides on Araneus pratensis (Araneae: Araneidae)

Spiders are important predators of several agricultural pests and they play an important role as indicators of ecosystem disturb. In Argentina, soybean crop has increased from the introduction of transgenic soybean resistant to glyphosate. This expansion produced an increase in the use of conventional and non-selective pesticides to control soybean pests. The objective of this work was to evaluate the side effects of subletal concentrations of two neurotoxican insecticides with a different mode of action: endosulfan (Glex, 35%, 25 mg/l a.i.) and spinosad (Tracer, 48%, 30 and 3 mg/l a.i) on Araneus pratensis. The insecticides were applied by ingestion of the treated prey (Musca domestica), and the effects on mortality, prey consumption, web building, mating, ootheca construction and fecundity were determined. Spinosad (30 mg/l a.i.) produced higher mortality than endosulfan (25 mg/l a.i.). Tremors and non-coordinated movements were observed in this treatment. The prey consumption was significantly reduced by the two insecticides (approximately 40% lower than control). The spider web building was significantly affected by the two insecticides, but spinosad had a greater effect. Though mating was not affected by both pesticides, abnormal oothecas and dehydrated eggs were observed. This work reports that sublethal concentrations representing approximately from 25 to 2.5% of the maximum field recommended concentrations (105 and 120 mg/l a.i., respectively) showed negative effects on A. pratensis. The consequences of these effects on role of A. pratensis as a natural mortality factor of soybean pests are discussed.     

Insecticide resistance in the malarial mosquito Anopheles arabiensis and association with the kdr mutation

A colony of Anopheles arabiensis Patton (Diptera: Culicidae) from the Sennar region of Sudan was selected for resistance to dichlorodiphenyltrichloroethane (DDT). Adults from the F-16 generation of the resistant strain were exposed to all four classes of insecticides approved for use in malaria vector control and showed high levels of resistance to them all (24-h mortalities: malathion, 16.7%; bendiocarb, 33.3%; DDT, 12.1%; dieldrin, 0%; deltamethrin, 24.0%; permethrin, 0%). Comparisons between the unselected base colony and the DDT-resistant strain showed elevated glutathione-S-transferase (P<0.05) in both sexes and elevated esterases (P<0.05) in males only. The Leu-Phe mutation in the sodium channel gene was detected by polymerase chain reaction and sequencing, but showed no correlation with the resistant phenotype. These results do not provide any explanation as to why this colony exhibits such widespread resistance and further studies are needed to determine the precise mechanisms involved. The implications for malaria vector control in central Sudan are serious and resistance management (e.g. through the rotational use of different classes of insecticides) is recommended.     

Monitoring for resistance to organophosphorus and pyrethroid insecticides in Varroa mite populations

The occurrence of resistance in Varroa mite populations is a serious threat to the beekeeping industry and to crops that rely on the honey bee for pollination. Integrated pest management strategies for control of this pest include the judicious use of insecticides. To monitor field populations of Varroa mite for insecticide resistance, a glass vial bioassay procedure was developed to use in the development of a resistance management strategy. Diagnostic concentrations needed to separate susceptible genotypes from resistant individuals were determined for cypermethrin (0.1 microg per vial), fluvalinate (5.0 microg per vial), malathion (0.01 microg per vial), coumaphos (10.0 microg per vial), diazinon (5.0 microg per vial), methomyl (0.5 microg per vial), propoxur (0.1 microg per vial), and endosulfan (2.5 microg per vial). Resistance to organophosphorus insecticides (malathion, coumaphos) and pyrethroids (cypermetrhrin, fluvalinate) was widespread in both La Media Ranch, TX, and Wewahitchka, FL, from 2007 to 2009. There was no resistance to endosulfan, diazinon, methomyl, and propoxur in field populations of Varroa mite in the two locations where resistance was monitored. The seasonal patterns of resistance in Wewahitchka were different from those of La Media Ranch. In the former location, the frequency of resistance to all insecticides tested decreased significantly from 2007 to 2009, whereas it increased in the latter location. Resistance levels were unstable, suggesting that resistance could be successfully managed. The results validate use of the glass vial bioassay to monitor for resistance in Varroa mite and provide the basis for the development of a resistance management strategy designed to extend the efficacy of all classes of insecticides used for control of Varroa mite.     

Impact of insecticides on the haemogram of Rhynocoris kumarii Ambrose and Livingstone (Hem., Reduviidae)

Abstract:  The haemogram of Rhynocoris kumarii Ambrose and Livingstone comprises prohaemocytes, plasmatocytes, granular haemocytes, cystocytes and oenocytoids. The impact of five insecticides, viz. monocrotophos, dimethoate, methylparathion, quinalphos and endosulfan on the total haemocyte count (THC) and differential haemocyte counts (DHC) was studied. All of the insecticides except endosulfan initially reduced both prohaemocytes and plasmatocytes, increased the granular haemocytes, altered the percentage of cystocytes and oenocytoids and increased the total haemocyte count (THC). On the contrary, endosulfan initially increased the prohaemocytes and plasmatocytes, decreased the granular haemocytes and also the THC. The highest impact on the DHC and THC was caused by methylparathion and monocrotophos and the least impact by endosulfan. Hence, endosulfan is considered as the safest insecticide followed by dimethoate and quinalphos among these five insecticides to use with R. kumarii .     

Resistance of Colorado potato beetle (Leptinotarsa decemlineata Say) to commonly used insecticides in Iran

The Colorado potato beetle (CPB, Leptinotarsa decemlineata Say), is a major pest of potatoes in Iran and many other parts of the world. Injury is caused when adults and larvae feed on the foliage and stems of potato plants, resulting in poor yields and/or plant death. Adult beetles can also vector plant diseases. Historically, the CPB been controlled using different insecticides, but it is currently resistant to nearly all classes of insecticides and remains a serious pest in many parts of the world. All of the resistance mechanisms reported in insects have been demonstrated in CPB. L. decemlineata invaded Iran in the early 1980s, probably through the importation of infested potatoes. It has caused significant damage to potato crops in affected areas, and it accordingly remains a major threat to Iranian potato production. Regrettably, no IPM programs have been developed for managing CPB infestations in Iran. Furthermore, there are no organized CPB resistance monitoring programs in Iran, and the recommended insecticides for CPB control, endosulfan and phosalon, have not changed in over 22 years. Anecdotal evidence from local farmers suggests a reduction in the efficacy of control of CPB by commonly used insecticides, probably due to the reduced susceptibility to these insecticides. Given the economic significance of L. decemlineata infestations, the increasing prevalence of resistance in this species, the rate of spread of infestations, and the extent of the area infested, there is an urgent need to develop effective and sustainable integrated pest management programs for CPB in Iran.     

Identification and distribution of a GABA receptor mutation conferring dieldrin resistance in the malaria vector Anopheles funestus in Africa

Growing problems of pyrethroid resistance in Anopheles funestus have intensified efforts to identify alternative insecticides. Many agrochemicals target the GABA receptors, but cross-resistance from dieldrin resistance may preclude their introduction.Dieldrin resistance was detected in An. funestus populations from West (Burkina Faso) and central (Cameroon) Africa, but populations from East (Uganda) and Southern Africa (Mozambique and Malawi) were fully susceptible to this insecticide. Partial sequencing of the dieldrin target site, the ??-aminobutyric acid (GABA) receptor, identified two amino acid substitutions, A296S and V327I. The A296S mutation has been associated with dieldrin resistance in other species. The V327I mutations was detected in the resistant sample from Burkina Faso and Cameroon and consistently associated with the A296S substitution. The full-length of the An. funestus GABA-receptor gene, amplified by RT-PCR, generated a sequence of 1674 bp encoding 557 amino acid of the protein in An. funestus with 98% similarity to that of Anopheles gambiae. Two diagnostic assays were developed to genotype the A296S mutation (pyrosequencing and PCR-RFLP), and use of these assays revealed high frequency of the resistant allele in Burkina Faso (60%) and Cameroon (82%), moderate level in Benin (16%) while low frequency or absence of the mutation was observed respectively in Uganda (7.5%) or 0% in Malawi and Mozambique.The distribution of the Rdl^R mutation in An. funestus populations in Africa suggests extensive barriers to gene flow between populations from different regions.     

Prevalence and stability of insecticide resistances in field population of Spodoptera litura (Lepidoptera: Noctuidae) from Huizhou,Guangdong Province,China

The field population of Spodoptera litura from Huizhou, Guangdong Province, China was evaluated for resistance to 21 insecticides, including conventional and new chemistry insecticides. Extreme levels of resistance were observed to metaflumizone and emamectin benzoate with resistance factors of 234.1 and 183.3, respectively. Resistance to abamectin was also high (perhaps extremely high) and over 71.9-fold. The Huizhou population of S. litura possessed high resistance to deltamethrin (96.5-fold) and moderate resistance to beta cyfluthrin and lambda cyhalothrin but remained susceptible to bifenthrin. Moderate resistance to chlorantraniliprole (22.3-fold), endosulfan (22.2-fold), tebufenozide (10.7-fold) and thiodicarb (14.3-fold), and low-level resistance to fipronil, indoxacarb and spinosad were also reported in this population. This field population remained susceptible to acetamiprid, chlorfenapyr, chlorfluazuron, hexaflumuron, chlorpyrifos, pyridalyl and spinetoram. The stabilities of resistance to metaflumizone, emamectin benzoate, deltamethrin, chlorantraniliprole and endosulfan were evaluated, the resistance level decreased when the insecticide stress was removed, suggesting stop of the application of insecticides with high level resistance could be implemented into the resistance management. Because S. litura from Huizhou developed resistance to multiple insecticides, integration of different control practices, especially the rotation of insecticides with biocontrol agents, should be performed in the management of this pest. The results suggested the suspension of the application of insecticide to which S. litura had developed high level of resistance in order to mitigate the resistance status, and the use of the insecticides to which this pest remained sensitive, including spinetoram, pyridalyl, indoxacarb, hexaflumuron, chlorfluazuron, chlorfenapyr and bifenthrin, could be incorporated into the alternating application for resistance management.     

Assessment of insecticide resistance in five insect pests attacking field and vegetable crops in Nicaragua

Field populations of Hypothenemus hampei (Ferrari), Plutella xylostella (L.), Spodoptera exigua (Hübner), Helicoverpa zea (Boddie) and Bemisia tabaci (Gennadius) were tested for resistance to several insecticides commonly used in Nicariagua. Assays were conducted to estimate the LD50s or LC50s and the corresponding resistance ratios. A diagnostic concentration was used to discriminate between susceptible and resistant strains of H. hampei. The tests with >6,000 H. hampei adults collected from six different sites indicate the absence of resistance to endosulfan. Resistance to cypermethrin, deltamethrin, chlorfluazuron, thiocyclam, and methamidophos was documented in six field populations of P. xylostella. High levels of resistance to cypermethrin and deltamethrin, but moderate levels of resistance to chlorpyriphos and methomyl, were also documented in two field populations of S. exigua. Moderate levels of resistance to cypermethrin, deltamethrin and chlorpyriphos were also documented in three field populations of H. zea. Moderate to high levels of resistance to bifenthrin, methamidophos and endosulfan were documented in four field populations of B. tabaci. The presence of significant correlations between LD50s or LC50s suggests the occurrence of cross-resistance or simultaneous selection for resistance by different insecticides with different modes of action. Our data could not differentiate between these two possibilities. Because insecticides will continue being used in Nicaragua, a resistance management program is urgently needed. The implementation of integrated pest management tactics must be accompanied by specific regulations for pesticide registration. In the future, pesticide registration regulations in Nicaragua should include periodic resistance monitoring. The mechanisms to cover the costs of resistance monitoring and resistance management should also be established.     

The effects of low doses of three insecticides on activity, feeding, mating, reproductive performance and survival in Glossina morsitans morsitans (Glossinidae)

Glossina morsitans morsitans Westw. surviving 48 hr after topical doses of dieldrin, endosulfan or permethrin which killed 15% or 30% during this period in an initial bioassay, were investigated for subsequent effects of the insecticides. Males surviving 48 hr after exposure to dieldrin and endosulfan were subject to further mortality which reached 70%–80% by day 15. Permethrin had some influence on longevity but only later than day 30 after treatment. Females were less susceptible than males to these effects. Treated flies mated readily but early separation occurred more frequently in pairs including dieldrin or endosulfan-dosed males. This premature separation may impede insemination and ovulation. Mortality associated with mating in flies exposed to dieldrin and endosulfan in these experiments is attributed mainly to enforced activity following matual disturbance under the experimental conditions. Reproductive performance in treated females surviving mating was largely unaffected, except for a small reduction in the number of pupae produced by permethrin-treated females and some indication of reduced survival of pupae from females exposed to dieldrin. Minor effects of the solvent (butanone) and the insecticides on activity and on feeding were also demonstrated. These results show that permethrin has relatively little effect on flies surviving at 48 hr after exposure to a low dose, compared with dieldrin or endosulfan. It is suggested that the intrinsic toxicity of insecticides measured by conventional laboratory bioassay (based on mortality at 48 hr after dosing) may prove misleading if effects of low doses similar to those described here, make a significant contribution to the impact of insecticides on tsetse populations in the field.

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Résumé Les effets latents de trois insecticides: dieldrine, endosulfan et perméthrine sont étudiés sur Glossina morsitans morsitans en utilisant les survivants après un délai de 48 h., à la suite d'applications topiques de doses qui tuent 15% à 30% des insectes pendant ce même laps de temps. Les mâles survivant 48h après l'application de dieldrine et d'endosulfan, montrent une mortalité ultérieure qui atteint 70%–80% au 15ème jour. La perméthrine a également une certaine influence sur la longévité, mais seulement au-delà du 30° jour suivant le traitement. Les femelles sont moins sensibles à ces effets que les mâles. Les mouches traitées s'accouplaient facilement mais la séparation survenait plus fréquemment pour les couples dont les mâles avaient été traités avec la dieldrine et l'endosulfan. Cette séparation prématurée peut empêcher l'insémination et l'ovulation. La mortalité associée à l'accouplement chez les mouches exposées à la dieldrine et à l'endosulfan dans ces expériences est principalement attribuée à l'activité accrue liée au dérangement mutuel dans les conditions expérimentales. Les performances de la reproduction chez les femelles traitées, survivantes après l'accouplement sont en grande partie inchangées, excepté en ce qui concerne une petite réduction du nombre de pupes produites par les femelles traitées à la perméthrine et quelque indication d'une survie réduite des pupes provenant de femelles traitées à la dieldrine. Des effets mineurs du solvant (butanone) et des insecticides sur l'activité et l'alimentation des mouches ont aussi été observés. Ces résultats montrent que la perméthrine a relativement peu d'effet par comparaison avec la dieldrine et l'endosulfan sur les mouches ayant survécu 48h après une application d'une faible dose. Il est suggéré que la toxicité intrinsèque des insecticides, mesurée par les méthodes conventionnelles de laboratoire (basées sur la mortalité 48h après l'application) peut se révéler erronée si des effects de doses faibles, semblables à ceux décrits ci-dessus, contribuent de façon significative à l'impact des insecticides sur les populations de Tsé-Tsé dans la nature.

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The work was financed by the Ministry of Overseas Development.     

Resistance to dieldrin + fipronil assorts with chromosome inversion 2La in the malaria vector Anopheles gambiae

Cyclodiene insecticide resistance in malaria vector mosquitoes of the Anopheles gambiae species complex (Diptera: Culicidae) has been reported previously from several parts of Africa. We report resistance to dieldrin, a cyclodiene, in two laboratory strains of An. gambiae Giles sensu stricto code-named Ian P20 from Nigeria (1979) and CIG from Cote d'Ivoire (1997). Dieldrin resistance levels were high in adult female mosquitoes (40-75% survived exposure to 4% dieldrin for 1 h) and was closely linked with chromosomal paracentric inversion 2La. This inversion did not occur in Hardy-Weinberg proportions, but showed an excess of heterozygotes in both strains, which may account for the high levels of resistance. This linkage also suggests that dieldrin resistance in Ian P20 is dominant. After subsamples of strain Ian P20 were exposed for 1 h to dieldrin 4% or fipronil 2% (discriminating concentrations), the resultant mortality-rates (61% and 65%) were not significantly different. Most survivors after fipronil treatment also survived subsequent exposure to dieldrin (46/50=92%). This apparent cross-resistance between insecticides of two classes (cyclodiene and phenyl pyrazole) has implications for the management of insecticide resistance in wild populations of the An. gambiae complex.