Yuling Ouyang

Spirodiclofen and spirotetramat bioassays for monitoring resistance in citrus red mite, Panonychus citri (Acari: Tetranychidae)

BACKGROUND Citrus red mite, Panonychus citri (McGregor), is a key pest of San Joaquin Valley California citrus. Spirodiclofen was registered for mite control in 2007, and spirotetramat for scale control in 2008. Because of the potential for resistance to spirodiclofen to develop in spider mites, and cross-resistance to spirotetramat used for other citrus pests, bioassay methods for resistance monitoring were developed. RESULTS The responses of four populations of adult female, egg and larval stages of P. citri to spirodiclofen were compared to determine the most robust bioassay method for this pesticide. Adult females responded with a higher LC(99) and larval stages exhibited higher control mortality and a lower slope of response compared with the egg stage. Thus, the egg stage was found to be the most suitable stage for testing. Egg production and egg shape were significantly affected by spirodiclofen treatment of adult female mites. Bioassays with the related compound spirotetramat revealed that P. citri egg hatch was less affected by this compound, requiring the assessment of mortality to be extended to 11 days after treatment when the hatched larvae succumbed to the pesticide. Discriminating concentrations of 10 ppm for spirodiclofen and 31.6 ppm for spirotetramat in an 11 day bioassay were tested against eight field populations of P. citri, and 99-100% mortality resulted. CONCLUSION These results provide a baseline for the response of P. citri to spirodiclofen and spirotetramat that will aid resistance management in California citrus.

Effects of Spirotetramat on Aonidiella aurantii (Homoptera: Diaspididae) and Its Parasitoid, Aphytis melinus (Hymenoptera: Aphelinidae)

ABSTRACT Laboratory and field studies were conducted to measure the effects of spirotetramat on life stages of California red scale, Aonidiella aurantii (Maskell), and a primary parasitoid, Aphytis melinus DeBach. Organophosphate-resistant and -susceptible populations responded similarly to spirotetramat, suggesting there is no cross-resistance between these insecticide classes. First and second instar male and female A. aurantii were 10- and 32-fold more susceptible to spirotetramat (LC50 = 0.1–0.2 ppm) compared with early third (LC50 = 1.5 ppm) and late third instar females (LC50 = 5.3 ppm). The LC99 value indicated that late stage third instar females would not be fully controlled by a field rate of spirotetramat; however, spirotetramat would reduce their fecundity by 89%. Field applications of spirotetramat in two water volumes and using two adjuvants (oil and a nonionic spray adjuvant) showed similar reduction in A. aurantii numbers, even though the higher water volume demonstrated more complete coverage. These data suggest that this foliarly applied systemic insecticide can be applied in as little as 2,340 liters/ha of water volume, minimizing application costs, and that the two adjuvants acted similarly. The endoparasitoid, A. melinus, was unaffected by the field rate of spirotetramat when it was applied to the host when the parasitoid was in the egg or larval stage. Adult A. melinus showed 2 wk of moderate reductions in survival when exposed to leaves with field-weathered residues. Spirotetramat is an integrated pest management compatible insecticide, effective in reducing A. aurantii stages and allowing survival of its primary parasitoid A. melinus.

Manipulation of the Predacious Mite, Euseius tularensis (Acari: Phytoseiidae), with Pruning for Citrus Thrips Control

The interior and exterior of citrus trees were pruned in late winter or summer to manipulate the timing and population development of the predacious mite, Euseius tularensis Congdon, and reduce scarring caused by citrus thrips, Scirtothrips citri. Pruning the interior of trees in late winter almost doubled the number of predacious mites in spring on the exterior of the trees where citrus thrips are most active. Pruning the interior or exterior of trees in summer resulted in higher predacious mite densities in the fall and accelerated population development by 2 wk in spring the following year. Pruning the exterior of trees in late winter did not affect mite densities until a year later. Pruning significantly lowered the percentage of heavy thrips scarring only in the winter-exterior treatment. E. tularensis may play a significant, however, not always regulatory role in reducing citrus thrips scarring damage. Pruning may be a more practical method of managing mite populations than mass-rearing and releases.

Chlorpyrifos Bioassay and Resistance Monitoring of San Joaquin Valley California Citricola Scale Populations

ABSTRACT The responses to chlorpyrifos of six populations of citricola scale, Coccus pseudomagnoliarum (Kuwana) (Hemiptera: Coccidae), were tested using a leaf dip bioassay, and two- to nine-fold resistances were found. LC50 responses of nymphs ranged from 7.5 to 68.9 ppm and LC90 responses ranged from 20 to 222 ppm chlorpyrifos. A population tested monthly during August-October showed up to 3.5-fold differences in LC50 responses but no differences in LC90 responses as scale size increased. A diagnostic concentration of 178 ppm chlorpyrifos was used to test 93 populations from throughout the San Joaquin Valley California during 2006–2009 by using a leaf dip bioassay. Of the populations tested, 41% showed >20% survival after exposure to the diagnostic concentration of chlorpyrifos, indicating resistance problems. Research is needed to relate the level of survival of the scales in the bioassay to the field efficacy of the insecticide. Tulare County citrus growers applied a higher number of organophosphate and carbamate insecticides during the 15-yr period from 1994 to 2008, and these orchards showed a higher average scale survival of chlorpyrifos and a higher number of locations with resistant scale compared with the other San Joaquin Valley counties. Chlorpyrifos resistance is a significant issue for citricola scale management because biological control is ineffective in the San Joaquin Valley and the alternative neonicotinoid and insect growth regulator (IGR) insecticides require more frequent application.

Postharvest Control of Western Flower Thrips (Thysanoptera: Thripidae) and California Red Scale (Hemiptera: Diaspididae) with Ethyl Formate and Its Impact on Citrus Fruit Quality

ABSTRACT The postharvest control of arthropod pests is a challenge that the California citrus industry must overcome when exporting fruit overseas. Currently, methyl bromide fumigation is used to control postharvest pests on exported citrus, but it may soon be unavailable because of use restrictions and cost of this health-hazard ozone-depleting chemical. Ethyl formate is a natural plant volatile and possible alternative to methyl bromide in postharvest insect control. The objectives of this study were 1) to evaluate the mortality of third instar California red scale [Aonidiella aurantii (Maskell)] (Hemiptera: Diaspididae) and adult western flower thrips [Frankliniella occidentalis (Pergande)] (Thysanoptera: Thripidae) under a wide range of ethyl formate concentrations, 2) to determine the ethyl formate concentration required to reach a Probit 9 level of control for both pests, and 3) to test the effects of ethyl formate fumigation on the quality of navel oranges [Citrus sinensis (L.) Osbeck] and lemons [Citrus limon (L.) Burman f.] at 24 h after fumigation, and at different time periods to simulate shipping plus storage (5 wk at 5°C), and shipping, storage, handling, and shelf-life (5 wk at 5°C, plus 5 d at 15°C, and 2 d at 20°C). The results indicate that ethyl formate is a promising alternative to methyl bromide for the California citrus industry, because of successful control of adult western flower thips and third instar California red scale and no deleterious effect on fruit quality at any of the evaluated periods and quality parameters.

Food Suitability and Population Dynamics of Lorryia formosa (Acari: Tydeidae)

Abstract Lorryia formosa Cooreman (Acari: Tydeidae) is a species of mite commonly associated with citrus in many countries including the United States. A survey report in 1957 suggested phytophagous nature, while other studies claimed that L. formosa populations are associated with honeydew producing insects and sooty mold and it acts as a sanitizing agent. We investigated the effect of various diets on the survival and progeny production of L. formosa on excised leaves and the survival and potential to cause feeding damage to leaves of potted plants in a greenhouse study. A 2-yr field survey of a mandarin orchard was also conducted to elucidate the seasonal infestation, damage potential and population structure of L. formosa in a natural habitat. Results showed that all L. formosa adults and immatures died in less than 14 d on excised leaves, did not survive beyond 7 d on potted citrus plants alone, and caused no observable feeding damage to leaves or fruit. When sugar water, honeydew, or cottony cushion scale, Icerya purchasi Maskell (Hemiptera: Margarodidae), was present, adults and immatures survived the duration of the experiments and produced additional generations. The field survey showed that all stages of L. formosa were present in a mandarin orchard throughout the year and insecticide applications affected but did not eliminate mite populations. Fruit generally had a greater percentage infestation of mites (44.8 ± 4.0) than leaves (16.0 ± 4.7). These studies confirmed that L. formosa cannot sustain a population on leaf tissue alone and is nondamaging to citrus in California.