Biological control of Culex mosquitoes (Diptera: Culicidae) by the tadpole shrimp, Triops longicaudatus (Notostraca: Triopsidae).

Laboratory oviposition choice tests and behavioral observations indicated that the activity of tadpole shrimp, Triops longicaudatus (LeConte), near the water surface deterred gravid Culex quinquefasciatus Say from ovipositing. In the cities of Oasis and Riverside, Calif., tadpole shrimp significantly reduced the abundance of immature mosquitoes (Cx. tarsalis Coquillett and Cx. quinquefasciatus) probably due to lowered oviposition rates, as well as tadpole shrimp predation. Generally, mosquito oviposition rates in field ponds with tadpole shrimp were lower than that of controls, except when tadpole shrimp were very young (4 d after flooding) or when their abundance had declined late in the flooding period. When analyzed by pond, tadpole shrimp size was correlated inversely with abundance; however, differences in size or abundance did not affect their capacity to reduce mosquito populations.

Comparative functional morphology of the mouth brushes of mosquito larvae (Diptera: Culicidae).

Structural and morphological features of the feeding brushes of fourth instars of four species of mosquitoes were studied using light and scanning electron microscopes. The fine structure of the elements of the lateral palatal brushes, anteromedian palatal brushes, mandibular brushes and combs, and maxillary brushes of each species was investigated and related to feeding behavior. The movement of the mouth brushes in living larvae was observed by stereomicroscopy under laboratory conditions. Mouth brush function and morphology varied among the species studied. The brushes of Culex quinquefasciatus Say, Cx. tarsalis Coquillett, and Aedes aegypti L. were more complex than those of Anopheles albimanus Wiedemann. The main role of the filaments of the lateral palatal brushes (LPB) was to generate and maintain water and particle flow around the larval head. In Ae. aegypti, most of the LPB filaments were provided with stout pointed teeth at the tips which were useful in the brushing of food material. In An. albimanus, the LPB filaments were simple and very closely packed and were used to stir the water surface film to direct the floating materials into the area of the mouth opening. The filaments of the anteromedian palatal brush (APBr) in the Culex species were branched. In Ae. aegypti, the APBr contained long bladelike filaments and short teethlike elements, whereas in An. albimanus, the APBr filaments were all long and bladelike. The morphological structure of the spicules and setae of the mandibular brushes and combs and the maxillary brushes are described. The main function of these brushes is to direct and concentrate particle flow.

Quantitative determination of free amino acids in the hemolymph of autogenous and anautogenous strains of Culex tarsalis (Diptera: Culicidae).

Autogenous and anautogenous strains were selected from the egg rafts of Culex tarsalis Coquillett collected from Coachella Valley, California. In autogenous (21-25 generations) and anautogenous (0-4 generations) strains used for hemolymph free amino acid analysis, the autogenous oviposition rates were 73.6-83.6% and 2.7-3.9%, the proportions of females undergoing autogenous ovarian development 120 h postemergence were 75.6 and 8.9%, respectively. To study the presence and quantities of free amino acids and explore their relationship to autogenous ovarian development, the 6 legs of females from both autogenous and anautogenous strains were removed at the coxo-femoral joints, and hemolymph was collected by centrifuging the bodies of 50-100 females. Proteins in the hemolymph were precipitated with 10% sulphosalicylic acid. On days 1, 4, 7, and 10 postemergence, a total of 17 free amino acids was found in females from the autogenous strain, and a total of 16 was found in females from the anautogenous strain in which asparagine (Asn) was absent. Comparisons of free amino acid concentrations between these 2 strains indicated that there were significant differences in total free amino acids and 13 individual free amino acids except Glu, Gln, Pro, and Met on day 1, total free amino acids and 14 individual free amino acids except Gln, Pro, and on day 4, and total free amino acids and 12 individual free amino acids except Asp, Glu, Gln, Pro, and Met on day 7 postemergence. There were no significant differences in the concentrations of individual and total free amino acids on day 10 postemergence. Female age affected free amino acid content in the autogenous strain, where total free amino acids and 14 individual free amino acid content in the autogenous strain, where total free amino acids and 14 individual free amino acids increased from day 1, peaked on day 4, declined on day 7, and declined further on day 10 postemergence. In the anautogenous strain, only total free amino acids and Ser, Gly, Ala, and Leu exhibited age-dependent changes in concentration, and the magnitude of changes in concentration was less than that in autogenous strain. Conclusively, autogenous ovarian development was the major reason for the changes in hemolymph free amino acid concentration postemergence. The current and previous findings indicate that the free amino acids, Arg, Gly, Ile, Leu, Lys, Phe, Ser, Thr, and Val putatively may be essential for vitellogenesis in mosquitoes.

Biological fitness of Culex quinquefasciatus (Diptera:Culicidae) susceptible and resistant to Bacillus sphaericus.

Biological fitness of laboratory and field-collected strains of southern house mosquito, Culex quinquefasciatus Say, susceptible and resistant (37- and 31-fold upon selection) to the microbial agent, Bacillus sphaericus, were compared in the absence of B. sphaericus. The resistant strains showed significantly lower fecundity and fertility, but they had significantly higher survival rates than the susceptible strains. The preadult stages from females of resistant strains developed at slightly faster rates than those of the susceptible strains, which could result in a shorter generation time. However, lower fecundity was likely to lead to overall lower population growth rates than in the susceptible strains. Data provided evidence that the resistant strains exhibited fitness disadvantages in the absence of B. sphaericus. We suggest that once resistance to B. sphaericus is detected in the field, its use should be discontinued until the mosquito population becomes susceptible again because of the decline in number of resistant individuals. A strategy of resistance management by rotation of insecticides is discussed.

Nutritional reserves, body weight, and starvation tolerance of autogenous and anautogenous strains of Culex tarsalis (Diptera:Culicidae).

The nutritional reserves and body weight of autogenous and anautogenous strains of Culex tarsalis Coquillett were determined for 4th instars, pupae, females, and males. Starvation tolerance and survivorship of adult females and males also were studied. The autogenous individuals contained significantly greater amounts of total lipids (except the total lipids in 4th instars), total carbohydrates, glycogen, and total proteins, which resulted in a heavier mean body weight of autogenous than anautogenous mosquitoes (except the fresh weight in pupae). Results of the body fluid coefficient determination were inconclusive. Nutritional reserves in the immature stages, especially the pupal stage, were significantly greater (except the total proteins in 4th instars and female adults) than those in the adult stage. There were no significant differences in median longevity between autogenous and anautogenous females, and between autogenous and anautogenous males, provided with distilled water alone after emergence. The median longevity of females and males was significantly different in both autogenous and anautogenous strains. Our findings strongly indicate a relationship between autogenous reproduction and differences in nutritional reserves of autogenous and anautogenous strains.

Factors influencing ingestion of particulate materials by mosquito larvae (Diptera: Culicidae).

Relative ingestion rates of mosquito larvae, as indicated by the number of substrate-filled gut segments per unit time, were determined for Culex tarsalis Coquillett, Aedes aetypti L., and Anopheles albimanus Wiedemann. Among the three species, Ae. aegypti larvae were the most rapid feeders. F50 (median time for complete repletion of 50% of the larvae) was 61, 42, and 100 min for the three species feeding on a wheat flour suspension, respectively. Food particles with nutritive values (dried yeast, wheat flour, fishmeal, or dried blood) were ingested faster than inert particles (kaolin, talc, chalk, or charcoal). The addition of aqueous yeast extracts containing phagostimulants accelerated the ingestion of inert particles. Increasing the concentration of inert particles did not increase ingestion rates. Larval age, water temperature, and starvation, but not larval density, influenced rates of ingestion. Younger instars were more rapid feeders than older instars. First instars of the three species filled their guts with wheat flour approximately two times faster than fourth instars. Increasing water temperature from 18 to 31 degrees C accelerated wheat flour ingestion by fourth instars of Cx. tarsalis, Ae. aegypti, and An. albimanus by factors of 1.9, 1.5, and 1.7, respectively. After starvation for 12 h, fourth instars of Cx. tarsalis and Ae. aegypti increased ingestion of wheat flour about 1.6 and 1.8 times, respectively. In contrast, starvation of An. albimanus larvae for the same period resulted in decreased wheat flour ingestion by 2.2 times when compared with unstarved larvae. These results indicare that, in addition to the chemical factors associated with the food substances, the physiological and environmatal conditions of the larvae play an important role in regulating the ingestion rate of suspended particles by mosquito larvae.

Ecology of Culex tarsalis (Diptera: Culicidae): factors influencing larval abundance in mesocosms in southern California.

Colonization and succession of mosquitoes and macroinvertebrate predators were studied in 30-m2 ponds (mesocosms) during summer and fall 1987. Larval abundance of Cx. tarsalis Coquillette was lower during the hot, summer months than during the fall. In all studies, larval populations declined markedly 2-3 wk after habitat flooding. Although predator abundances differed in these studies, sometimes by an order of magnitude, the common predators colonized mesocosms in the following order: Triops, hydrophilid beetle larvae, dytiscid beetle larvae, mesoveliids, dragonfly and damselfly naiads, and notonectids. The similarity of the colonization phenologies probably resulted from the vagility of the adult insects and species-specific developmental rates. Stepwise multiple regression was used to identify factors potentially affecting larval mosquito populations. For most studies, coleopteran larvae were related inversely to per capita change in the entire larval population and the third- and fourth-instar subpopulation (i.e., large coleopteran larval populations were associated with large declines in the Cx. trasalis larval population). Maximum water temperatures and pond age (days after flooding) also were identified as significant factors affecting larval abundance and per capita change of mosquitoes. Potentially lethal water temperatures (greater than or equal to 35 degrees C) occurred during the summer; however, the declines in larval abundance of Cx. tarsalis were not restricted to (or obviously associated with) periods of high water temperature. Our results indicated that predation by coleopteran larvae and factor(s) associated with pond age, such as mosquito ovipositional preferences, significantly affected Cx. tarsalis larval populations.

Effect of drying period and soil moisture on egg hatch of the tadpole shrimp (Notostraca: Triopsidae).

Tadpole shrimp (Triops spp.) are potential biological control agents against larval mosquitoes in temporary ponds and flood-irrigated fields. In some rice field situations, however, they may become pests that uproot and eat young rice plants. In cursory observations, it has been reported that tadpole shrimp eggs do not readily hatch on flooding when the soil or substrate containing eggs is moist before flooding. The relationship between drying (moisture content) of soil and tadpole shrimp hatch was determined in studies conducted in mesocosms at the University of California Aquatic and Vector Control Research facilities at Riverside and at Oasis in the Coachella Valley of southern California. In laboratory hatching trials, an increase in hatch of Triops longicaudatus (LeConte) with declining soil moisture content was demonstrated (t = 8.4, P less than 0.001; r2 = 0.76). In field trials in mesocosms at Riverside, egg hatch increased with increased drying period and declining soil moisture content (G = 29.8, P less than 0.01). No hatch of eggs occurred after 3 d of drying, when soil moisture content was high, but a high level of hatching occurred after 7 and 14 d of drying, when soil moisture declined to low levels. At Oasis, soil moisture did not decrease with drying time because of porous soil and capillary action of water from adjacent flooded mesocosms and thick vegetation covering the pond bottoms. Therefore, hatch rates at Oasis were not associated with the length of the drying period (G = 35, P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The future of insect growth regulators in vector control.

Insect growth regulators (IGRs) are diverse groups of chemical compounds that are highly active against larvae of mosquitoes and other insects. The IGRs in general have a good margin of safety to most nontarget biota including invertebrates, fish, birds, and other wildlife. They are also relatively safe to man and domestic animals. The IGR compounds do not induce quick mortality in the preimaginal stages treated. Mortality occurs many days later after treatment. This is indeed a desirable feature of a control agent because larvae of mosquitoes and other vectors are an important source of food for fish and wildlife. On account of these advantages of IGRs and the high level of activity against target species, it is likely that IGRs will play an important role in vector control programs in the future.

Mosquito control then, now, and in the future.

This is a memorial lecture honoring the late Professor Stanley B. Freeborn of the University of California. In the spirit of his life-long academic and research interests in mosquitoes and mosquito-borne diseases, I am presenting here the evolution of vector control technology, especially that pertaining to mosquitoes and mosquito-borne diseases during the 20th century. Vector control technology in the first half of this century was relatively simple, utilizing source reduction, larvivorous fish, petroleum hydrocarbon oils, and some simple synthetic and botanical materials. During the 2nd half of this century, however, various classes of synthetic organic chemicals, improved petroleum oil formulations, insect growth regulators, synthetic pyrethroids, and microbial control agents were developed and employed in mosquito control and control of other disease-vectoring insects. Among these groups of control agents, petroleum oil formulations have endured to be used through the whole century. It is likely that petroleum oil formulations, insect growth regulators, and microbial control agents will provide the main thrust against vectors at least during the first quarter of the 21st century. It is also possible that effective tools through the development of vaccines and molecular entomology techniques might become available for the control of vectors and vector-borne diseases during this period of the 21st century.

Effect of the insect growth regulator methoprene on the ovipositional behavior of Aedes aegypti and Culex quinquefasciatus.

Laboratory and field tests were conducted to determine the attractiveness of the insect growth regulator methoprene to ovipositing mosquitoes when presented at concentrations used in mosquito control programs. Laboratory experiments indicated that ovipositing Culex quinquefasciatus and Aedes aegypti were not attracted to methoprene. In field experiments, Culex stigmatosoma and Cx. quinquefasciatus larval dip counts in experimental ponds treated with methoprene briquets were not significantly different from untreated control ponds. Water taken from these methoprene-treated ponds was not attractive when compared with water taken from untreated ponds to ovipositing Cx. quinquefasciatus in laboratory experiments. These studies provide strong evidence that methoprene is not attractive to ovipositing mosquitoes at concentrations within the range of field applications.

Delayed mortality and morphogenetic anomalies induced by the microbial control agent Bacillus thuringiensis ser. (H-14) in Culex quinquefasciatus.

Sublethal concentrations of Bacillus thuringiensis ser. H-14 were applied to early 4th-instar larvae of Culex quinquefasciatus to assess mortality and morphogenetic aberrations in larvae, pupae and adults. At the 24 h LC10, LC25, LC50 and LC80, additional mortality occurred in surviving larvae beyond a 24 h exposure period. The cumulative mortalities increased daily and the overall mortality of larvae up to 7 days posttreatment were 12, 73, 82 and 96% at the indicated concentrations, respectively. Delayed mortality also occurred in the pupal and adult stages. Morphogenetic aberrations were noted in dead larvae and pupae but were rare in the adults. These aberrations are categorized and described. There was little or no delayed effect on the survivorship or fecundity of adults, but in all the treatments the number of emerging males was higher than females. The sex ratio in check adults was 1:1.

Mode of existence and seasonality of midge larvae (Diptera: Chironomidae) in man-made lakes in the Coachella Valley, southern California.

Over the past 2 decades, numerous man-made ponds and lakes constructed in country clubs and on golf courses in the Coachella Valley have become ideal habitats for various chironomid species. Large numbers of adult midges emerging from these water bodies pose nuisance and economic problems. During 1992 and 1993 we initiated comprehensive studies on the nature and scope of the nuisance midge problem in the valley. We sampled on a biweekly basis 2 lakes supplied with well water, 2 supplied with tertiary effluent water, and one supplied with a mixture of these 2 sources, to determine the midge larval fauna and the mode of existence, seasonal abundance, and population trends of these midges. Climbers, clingers, portable sand tube builders, and tube builders on plants predominated in well-water habitats with submerged vegetation and detritus bottoms. Tube builders and burrowers predominated in the tertiary water, which characteristically had a detritus bottom, devoid of vegetation. Habitats holding a mixture of the 2 water types with sandy bottoms supported midge larvae known to be sprawlers.

Effects of organic enrichment on temporal distribution and abundance of culicine egg rafts.

The effect of organic enrichment of the larval habitat on mosquito oviposition was determined in 2 replicated field experiments. In the first study, Culex stigmatosoma and Culex quinque-fasciatus oviposition (as measured by the number of early instar larvae) reached a maximum peak soon after flooding and then declined to a substantially lower equilibrium level. A group of ponds that later received supplemental organic matter had a significant increase in oviposition soon after the additional organic matter was added. In the second experiment, a similar ovipositional pattern was noted by the sampling of egg rafts along transects and determining larval abundance by the dipping technique. In the second experiment a mixture of Culex stigmatosoma and Culex tarsalis larvae was collected.

Field evaluation of new water-dispersible granular formulations of Bacillus thuringiensis ssp. israelensis and Bacillus sphaericus against Culex mosquitoes in microcosms.

A variety of formulations of Bacillus thuringiensis var. israelensis de Barjac (B.t.i.) and Bacillus sphaericus Neide (B.s.) have been studied for mosquito control under laboratory and field conditions. High efficacy, specificity, low risk of development of resistance, long shelf-life, and transportability, as well as the safety to nontarget organisms of these 2 microbial agents have been well documented. Some of the currently available formulations of B.t.i. and B.s. have low potency per unit mass. Research and development efforts are focusing on commercializing formulations with high potency and low minimum effective dosage that are suitable for long-distance shipment. To achieve this goal, new water-dispersible granule (WDG) formulations of both microbial agents were prepared and made available by Abbott Laboratories for evaluation. The newly developed WDGs of B.t.i. and B.s. with high potency dispersed readily in water with gentle agitation. These WDGs were evaluated and the minimum effective dosages were determined in microcosms against natural populations of Culex mosquitoes. The minimum effective dosage for B.t.i. WDGs with 4,000 International Toxic Units (ITU)/mg was 0.27-0.53 lb/acre which yielded significant control for up to 7-12 days. The minimum effective dosage for B.s. WDGs with 350-630 ITU/mg was 0.05-0.10 lb/acre, which yielded significant control of immature mosquitoes for up to 14-20 days.

Activity and biological effects of neem products against arthropods of medical and veterinary importance.

Botanical insecticides are relatively safe and degradable, and are readily available sources of biopesticides. The most prominent phytochemical pesticides in recent years are those derived from neem trees, which have been studied extensively in the fields of entomology and phytochemistry, and have uses for medicinal and cosmetic purposes. The neem products have been obtained from several species of neem trees in the family Meliaceae. Six species in this family have been the subject of botanical pesticide research. They are Azadirachta indica A. Juss, Azadirachta excelsa Jack, Azadirachta siamens Valeton, Melia azedarach L., Melia toosendan Sieb. and Zucc., and Melia volkensii Gürke. The Meliaceae, especially A. indica (Indian neem tree), contains at least 35 biologically active principles. Azadirachtin is the predominant insecticidal active ingredient in the seed, leaves, and other parts of the neem tree. Azadirachtin and other compounds in neem products exhibit various modes of action against insects such as antifeedancy, growth regulation, fecundity suppression and sterilization, oviposition repellency or attractancy, changes in biological fitness, and blocking development of vector-borne pathogens. Some of these bioactivity parameters of neem products have been investigated at least in some species of insects of medical and veterinary importance, such as mosquitoes, flies, triatomines, cockroaches, fleas, lice, and others. Here we review, synthesize, and analyze published information on the activity, modes of action, and other biological effects of neem products against arthropods of medical and veterinary importance. The amount of information on the activity, use, and application of neem products for the control of disease vectors and human and animal pests is limited. Additional research is needed to determine the potential usefulness of neem products in vector control programs.

Evaluation of two new insect growth regulators against mosquitoes in the laboratory.

Biological activity of two new IGRs was studied under laboratory conditions against 2nd- and 4th-instar larvae and pupae of Anopheles quadrimaculatus, Aedes aegypti and Culex tarsalis. The IGR S-21149, an oxime [0-(2-(4'-Phenoxyphenoxy)ethyl propionaldoxime], produced an overall mortality or inhibition of emergence of 95% (when 4th instar treated) at 0.0047, 0.0013 and 0.00041 mg/liter in the three species, respectively. The EI95 values for the second IGR S-31183, a pyridine compound, [1-(4'-Phenoxyphenoxy)-2-(2'pyridyloxy) propane] were 0.017, 0.0026 and 0.00032 mg/liter for the three species, respectively. Both materials in general were less active against 2nd than 4th-instar larvae, except that the 2nd- and 4th instars of An. quadrimaculatus were equally susceptible. Pupae treated at up to 0.1 mg/liter showed no mortality in the pupal or the ensuing adult stages.

Field evaluation of controlled release pellet formulation of methoprene against chironomid midges in man-made lakes.

The duration of the efficacy of 4% methoprene pellets against emergence of chironomid midges, Chironomus stigmaterus Say, Goeldichironomus amazonicus (Fittkau) and Tanypus imperialis Sublette, in man-made lakes supplied with tertiary effluent water in Palm Desert, California, was dose dependent. For C. stigmaterus and T. imperialis, the rate of 4.5 kg/ha of the controlled release pellet formulation of 4% methoprene yielded > 90% control for 3 wk and 75% control in the 4th week, the rate of 3.4 kg/ha provided > 90% control for 2 wk, and the rate of 2.25 kg/ha provided control for 1 wk. Goeldichironomus amazonicus was under 90% control for 2 wk at the rate of 4.5 kg/ha and 1 wk at the rates of 3.4 kg/ha and 2.25 kg/ha.

Selection-dependent trends of autogeny and blood feeding in an autogenous strain of Culex tarsalis (Diptera:Culicidae).

During successive en masse selection of autogeny in Culex tarsalis from parental to 20th generation, the autogenous oviposition capacity changed as follows: mean number of egg rafts/female and mean number of eggs/female increased significantly, from 0.11 to 0.56 and from 6.2 to 20.2, respectively; the mean size of the egg rafts decreased significantly, from 56.2 to 36 eggs/raft; autogeny rates increased significantly, from 18.6 to 65.2%; mean number of autogenous follicles/female decreased significantly, from 57.4 to 34; the hatching rate of autogenous egg rafts reduced significantly, from 88.3 to 75.4%; and the feeding rates of the females on a blood meal source during the 7th day postemergence increased significantly, from 62.8 to 71.1%. The autogeny rate determined by ovarian dissection was higher than that indicated by actual oviposition. As the age of females increased during the observed 10-day oviposition period, the mean number of egg rafts/female, the mean number of eggs/female, and the mean number of eggs/raft decreased successively, and scattered "light color" egg rafts became more common. Successive en masse selection enhanced autogeny and strengthened the bloodfeeding tendency in this facultative autogenous species.

Influence of tadpole shrimp, Triops longicaudatus (Notostraca:Triopsidae), stocking rate on Culex tarsalis development in experimental field microcosms.

The effectiveness of 5 tadpole shrimp (TPS) stocking rates to reduce cohorts of Culex tarsalis was studied in field microcosms (0.81-m2). Larval/pupal abundance in microcosms containing greater than 5 TPS/m2 was significantly (P less than 0.05) lower than that of controls lacking tadpole shrimp due to predation. Adult mosquito abundances captured in emergence units above the microcosms stocked with tadpole shrimp were significantly (P less than 0.05) lower than that of controls at rates greater than or equal to 10 TPS/m2. Tadpole shrimp growth during 17 days of this study was inversely proportional to their stocking rate, and a linear relationship between size and stocking rate was plotted. Tadpole shrimp stocking rates also influenced rate of mosquito development, causing significantly (P less than 0.05) shorter periods for 50% emergence where shrimp were present when compared with that of controls. Adult male Cx. tarsalis emerged significantly earlier then females in microcosms stocked at 5 TPS/m2, while no significant (P greater than 0.05) differences were detected between the sexes at the remaining predator stocking rates.