Quantifying the Influence of Larval Density on Disease Transmission Indices in Culex quinquefasciatus, the Major African Vector of Filariasis

Larval crowding is one of the abiotic factors affecting biological fitness in mosquitoes. This study aims at elucidating, quantitatively, the influence of more larval crowding on aspects of fitness in Culex quinquefasciatus mosquito. To this end, day-old larvae of the species were reared in 4 density regimens equivalent to 1 larva in 1.25, 2.5, 5, and 10 mL of distilled water. Developmental indices, adult fitness indices, and accumulation and utilisation of teneral reserves for metamorphosis were determined at these density regimens. The results revealed varying significant negative effects of larval density on all fitness indices measured for the species. The study also revealed high utilisation of teneral reserves for metamorphosis at high larval densities. The information generated will be useful in making informed-decisions in allocating scare resources for vector control, although field trials are advocated to establish these laboratory findings.

Assortments of Digestive Enzymes Induced in First Instar Larvae of Busseola fusca Feeding on Different Plants

The stem borer Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is an important pest of maize and sorghum in sub-Saharan Africa. This insect has oligophagous feeding habits, feeding mostly on maize and sorghum with a narrow range of wild Poaceous plant species. We hypothesised that first instar B. fusca larvae, the critical stage for successful establishment on a host plant, can establish and then grow on a particular plant as a result of induction of a complement of digestive enzymes that mediates host acceptance at first instars. A fast semi-quantitative analysis of potentially digestive enzymatic activities present in the first larvae previously fed for 4 days on leaves of host and non-host plants was performed using the API-ZYM kit system able to detect a multiplex of enzyme activities. Regardless of the plant species, the larvae exhibited higher activities of the carbohydrate metabolising enzymes than of aminopeptidases and proteases. In addition, highest activities of carbohydrates degrading enzymes were exhibited by larvae that consumed leaves of the most preferred plant species of B. fusca. Conversely, esterases were only detected in neonate larvae that consumed leaves of the less preferred and non-host plants. No alkaline phosphatase and lipase activities were detected. The significance of these results was discussed in terms of food requirements of first instar larvae when settling on a plant.

Description and Biological Studies of a New Species of Metaphycus Mercet, 1917 (Hymenoptera: Encyrtidae), A Parasitoid of Capulinia linarosae Kondo & Gullan

The guava cottony scale, Capulinia linarosae Kondo & Gullan (Hemiptera: Eriococcidae), is an important pest of guava, Psidium guajava L. (Myrtaceae) in northern Colombia and Venezuela. A species of Metaphycus (Hymenoptera: Encyrtidae) is the only known primary parasitoid associated with this insect pest. The parasitoid is herein described as M. marensis Chirinos & Kondo, sp. nov., based on morphological characteristics of the adult female and male. Biological studies on adult longevity, fecundity, host preference, and sex ratio were conducted. The maximum longevity of the female and the male were 8.0 and 6.5 days, respectively, when fed with diluted honey. On average, a fed mated female laid approximately 40 eggs. Adult females of M. marensis were shown to prefer to parasitize 11- to 15-day-old adult females of C. linarosae and do not parasitize first-instar nymphs of the host eriococcid. The female-to-male sex ratio of the parasitoid was 2.24: 1. When ovipositing females of M. marensis were given only small-sized individuals (second-instar nymphs) of C. linarosae, generally the resulting progeny was a single male wasp. This parasitoid species has arrhenotokous reproduction and is a facultative gregarious parasitoid. These results show a short adult longevity, as well as a relatively low fecundity of the female compared with studies conducted on other Metaphycus species. This study provides essential baseline information for future biological control programmes for C. linarosae.

Effects of Volatiles from Clavigralla tomentosicollis Stål. (Hemiptera: Coreidae) Adults on the Host Location Behavior of the Egg Parasitoid Gryon fulviventre (Crawford) (Hymenoptera: Scelionidae)

The egg parasitoid Gryon fulviventre is a potential biological control agent of Clavigralla tomentosicollis, a coreid pod-sucking pest of Vigna unguiculata. The host location behavior of naive parasitoid females was studied using a four-armed olfactometer. Two strains of G. fulviventre parasitoids from Burkina Faso and Benin were exposed to odors provided by healthy and infested pods as well as C. tomentosicollis females and males. The time spent in each odor zone was recorded to determine the preference of parasitoid females. Results show that odors from healthy pods, infested pods, and pest females did not attract the parasitoid. However, a significantly attractive response of both strains of G. fulviventre was recorded in the presence of volatiles from males of C. tomentosicollis. Moreover, experiments testing G. fulviventre females’ behavior when simultaneously exposed to volatiles from cowpea pods (healthy and infested) and increasing numbers of C. tomentosicollis males revealed a significantly higher attraction of parasitoid females of both strains by volatiles from ten males of C. tomentosicollis. The results suggest that the males of the insect pest emit a pheromone used as kairomone by parasitoids to locate their host. The conditions determining this attractiveness at field level and its impact on host-searching efficiency are discussed.

Effects of Entomopathogenic Fungi on the Biology of Spodoptera litura (Lepidoptera: Noctuidae) and its Reduviid Predator, Rhynocoris marginatus (Heteroptera: Reduviidae)

Entomopathogenic fungi (EPFs), Isaria fumosorosea and Beauveria bassiana, are efficient biological agents in the management of multiple arthropod pests. In this study, the effects of both EPF species on various life stages of Spodoptera litura (F.) (Lepidoptera: Noctuidae) and its natural enemy Rhynocoris marginatus (Fab.) (Hemiptera: Reduviidae) were determined under laboratory conditions. I. fumosorosea significantly ( P < .05) reduced the growth rate of the third and fourth instar larvae of S. litura. For relative consumption rate (RCR), the maximum impact was recorded for I. fumosorosea, which reduced the RCR of the larvae. The larvae of S. litura treated with I. fumosorosea showed significantly lower efficiency of conversion of ingested food (ECI) and the larval mortality rate (58.0%) was also higher compared with B. bassiana (33.3%). Similarly, I. fumosorosea had a significant effect on the pupal formation of S. litura; however, no significant effect was found on adult emergence percentage. To determine the effect of EPF-infected prey on the adult predator, their handling time, predatory rate, consumption rate, and the survival rate were recorded. No significant effect of EPF species on the predation rate was found. Furthermore, no significant difference was found in the survival rate of predators fed on either EPF-infected prey or healthy larvae. The interaction of these EPFs with a reduviid predator suggested that both EPF species, especially I. fumosorosea, could be used together with the predator to boost the biological control of S. litura in commercial crops.

The Diet for Edible-Nest Swiftlets: Nutritional Composition and Cost of Life Stages of Megaselia scalaris Loew (Diptera: Phoridae) Bred on 3 Commercial Breeding Materials

Megaselia scalaris (Loew) is one of the best-known diets for the swiftlet. Previous studies have addressed the problem of some mass rearing conditions for this insect; unfortunately, the details of the nutritional composition of the life stages and cost of the breeding materials were insufficiently reported, even though this information is crucial for farming the edible-nest swiftlet. We aimed to investigate the nutritional composition of the life stages of M scalaris on a cost basis using 3 common commercial breeding materials: chicken pellets (CPs), fish pellets (FPs), and mouse pellets (MPs). Modified Association of Official Analytical Chemists (AOAC) proximate and mineral analyses were carried out on the insect’s third instar larvae, pupal, and adult stages to determine the nutritional composition. Regardless of the breeding materials, the adult stage of M scalaris had significantly higher crude protein than the other stages; the pupae were rich in calcium, which is required for egg production; and the third instar larvae had the highest amount of crude fat compared with the other stages. Regarding the energy content, there were no significant differences among the stages according to the breeding materials. In terms of nutritional cost, CP was the most economic breeding material and generated the highest amount of nutrients per US dollar (US $). Different life stages of M scalaris were used by the swiftlets by supplying the required nutrients, and future studies should focus on effective diet feeding methods.

Microbial-Based Double-Stranded RNA Production to Develop Cost-Effective RNA Interference Application for Insect Pest Management

RNA interference (RNAi) is a convenient tool to identify and characterize biological functions in organisms. Recently, it has become an alternative to chemical insecticides as a biologically based control agent. This promising technology has the potential to avoid many problems associated with conventional chemical insecticides. In order for RNAi application to be practical for field use, a major hurdle is the development of a cost-effective system of double-stranded RNA (dsRNA) production for a large quantity of dsRNA. A handful of research reports has demonstrated microbial-based dsRNA production using L4440 vector and HT115 (DE3) Escherichia coli for application to vertebrate and invertebrate systems. However, the dsRNA yield, production efficiency, and biological purity from this in vitro system is still unclear. Thus, our study detailed biochemical and molecular tools for large-scale dsRNA production using the microbial system and investigated the production efficiency and yield of crude and purified dsRNAs. An unrelated insect gene, green fluorescent protein (GFP), and an insect neuropeptide gene, pyrokinin (PK) identified from Drosophila suzukii, were used to construct the recombinant L4440 to be expressed in the HT115 (DE3) cell. A considerable amount of dsRNA, 19.5 µg/mL of liquid culture, was isolated using ultrasonic disruption followed by phenol extraction. The sonication method was further evaluated to extract crude dsRNA without the additional phenol extraction and nuclease treatments and also to reduce potential bacterial viability. The results suggest that the ultrasonic method saved time and costs to isolate crude dsRNA directly from large volumes of cell culture without E coli contamination. We investigated whether the injection of PK dsRNA into flies resulted in increased adult mortality, but it was not statistically significant at 95% confidence level. In this study, the microbial-based dsRNA production has potential for applied RNAi technology to complement current insect pest management practices.

Thermal Biology and Seasonal Population Abundance of Bactrocera dorsalis Hendel (Diptera: Tephritidae): Implications on Pest Management

Since the first detection of Bactrocera dorsalis in Botswana in 2010, the establishment, spread, and response to prevailing Botswana microclimates under rapidly changing environments remain unknown. This study investigated the presence, seasonal population abundance, and thermal biology of B. dorsalis in Botswana. We measured B. dorsalis thermal tolerance vis critical thermal limits (CTLs) and lethal temperature assays (LTAs) to understand how temperature largely impacts on fitness and hence invasive potential. Seasonal monitoring results indicated B. dorsalis establishment in the Chobe district (its first area of detection). Trap catches showed continuous adult flies’ presence all year round and high average monthly trap catches as compared with other districts. Furthermore, B. dorsalis was detected south of Botswana, including Kgatleng, Kweneng, South-east, and Southern districts. Critical thermal maxima (CTmax) to activity for adults and larvae were 46.16°C and 45.23°C, whereas critical thermal minima (CTmin) to activity for adults and larvae were 9.1°C and 7.3°C, respectively. Moreover, we found an improved CTmin for larvae at a slower ramping rate, indicating potential rapid cold hardening. The lower lethal temperature (LLT) and upper lethal temperature (ULT) assays revealed a reduction in survival at all the developmental stages as severity and duration of both temperature extremes increased. Microclimatic temperatures recorded in Botswana showed that environmental temperatures fall within the thermal breath of B. dorsalis activity measured here, indicating a potential conducive climate niche for the insect pest across the country, albeit other factors, e.g., host availability, play a significant role. These results therefore suggest that Botswana microclimatic temperatures aided B. dorsalis activity and invasion pathway are thus significant in mapping invasions and pest risk analysis, and may also aid in designing pest management strategies.

Application of Organic Fertilizers Affect the Citrus Leafminer, Phyllocnistis citrella (Lepidoptera: Gracillariidae) Infestation and Citrus Canker Disease in Nursery Plantations

Citrus leafminer (CLM), Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is one of the most important insect pests of Pakistan’s citrus nursery stock and caused extensive damage to young flushes. The organic compost is a widespread technique used to manage insect pests and plant diseases. Different composts (biofert, tara root and vermicompost) at 0.5 and 0.25 kg/plant concentration in comparison to NPK fertilizer at 0.4 and 0.2 g/plant were evaluated for CLM infestation and the associated citrus canker disease in nursery plantations of Citrus reticulata Blanco. Application of biofert at 0.5 kg/plant reduced the CLM infestation up to 54.5% during Fall-2016 and 39.1% during Summer-2017 in comparison to control treatment. The CLM larval density was also found lower by the application of biofert followed by vermicompost during both seasons. Both concentrations of biofert followed by vermicompost at 0.5 kg/plant resulted in remarkable protection against citrus canker disease in both flushes. The incidence of canker associated with CLM infested leaves was also studied and found lower by the application of biofert and vermicompost compared with control treatment. Conclusively, the soil amendment using biofert and vermicompost affects the CLM population and canker infection in nursery plantations. These organic fertilizers can be used in future citrus IPM programs as a tool to suppress the CLM population and citrus canker disease.