The first and second instar larvae were grouped into a single ‘larvae’ observation and recovered males and females were counted separately as well as grouped into an ‘adults’ observation . There were clear differences among the mean number of eggs and larvae found across the different plant types . ANOVA indicated a significant interaction between the plant and the developmental stage present on the respective plant . The interaction indicates there was a differential treatment impact with respect to the number of eggs laid and the number of larvae found. The presence of a significant effect of developmental stage indicates that the number of eggs and larvae in clip cages were not similar, i.e. egg mortality was higher with some plants versus others . A correlation coefficient of 0.75 was calculated between the mean numbers of eggs and hatched larvae. Tukey’s HSD rendered a significant plant effect for the total number of eggs and larvae present in the clip cages . More eggs and larvae were found on four types of plants, i.e. citrus , sumac , Star and Jewel . The blueberry varieties Showchaser and O’Neal had significantly lower total number of eggs and larvae and numbers on Emerald and O’Neal were intermediate . The Jewel variety data were slightly skewed, however,planting blueberries in pots as the mean number of eggs and hatched larvae were negatively correlated . Roughly 25% of males and less than 50% of female citrus thrips were recovered from the plants in the bug dorms 14 days post release. The between replicate bug dorm adult survivorship was relatively low.
Of ca. 60 adults of each sex released, male survivors per bug dorm ranged from 13-21 with a mean of 15.4 ± 3.3 [SE]; female counts ranged from 20-37 with a mean of 28.2 ± 6.4 [SE]. The Star variety had the highest mean numbers of adults collected in comparison with other varieties . Data from the bottom two thirds of the plant were pooled together, as there were few thrips collected and eggs found on the lowest third. There were more eggs located on the top third of the plant than the bottom two thirds of the plant across all plant varieties. There were also more larvae found on the top third of the plant than the bottom two thirds, but no differences in larvae found on the bottom two thirds of the plant than eggs present in those areas, for all plant varieties . The Star variety had the numerically highest numbers of total eggs and larvae per plant compared with any other variety. The likelihood ratio chi-square test indicated a significant effect of egg count on the top third of the plant as well as a significant effect of larval count on the two plant locations, i.e. the top third also having the highest count . More eggs were found on the top third of the blueberry plants and there was a higher percent of larvae present on the top third of the plant , regardless of plant variety. However, in the choise test, the Star variety had the highest percent of eggs found and larvae present. The Snowchaser variety had the lowest numbers of thrips counted overall, with fewer eggs, larvae and adults collected on that variety. The Misty variety had fewer larvae present than eggs laid and the number eggs laid were comparable to levels on Emerald.
Under the conditions of these trials , citrus thrips egg hatch typically would take place within 4-9 days , therefore numbers of motile insects counted in both studies would be larvae from the first generation and surviving adults. The primary focus of this work was to determine if there was a difference in the number of eggs laid on the different types plants and our data clearly show this was the case . The no-choice oviposition test showed that the highest number of eggs and larvae were found on citrus and sumac and the blueberry varieties Star and Jewel. Intermediate levels were found on Emerald and Misty, and the lowest numbers on Snowchaser and O’Neal. The Tukey HSD separation for total counts of eggs and larvae group Jewel with the citrus, sumac, and Star, but these data may be skewed due to the negative correlation between the number of eggs laid on the Jewel variety and the number of larvae present. The choice oviposition test also showed a correlation between the number of eggs located and the number of larvae present on the plant. Evaluation of count means showed a difference in citrus thrips numbers on the Star variety over the other blueberry varieties in the test. When given the choice to move about the plants, the citrus thrips’ activities at the time of isolation and counting appear to be similar for Emerald and O’Neal with a mean of less than 10 adults recovered from those plants. In the choice oviposition tests, the negative correlation between egg and larval counts is also apparent for the Jewel variety. Some explanations for this negative correlation could be: first instars eclosing from the egg suffered high mortality, the larvae did not or cannot feed and/or develop well on this variety, and/or the larvae dispersed away from the plant onto other plants. If the latter were the case, then one would expect to see one or more other blueberry varieties with higher numbers of larvae present, but that was not the case and given the fact the canes did not remain in contact with one another, it is unlikely that high levels of larval dispersion to different pots took place.
There are several possibilities which explain the results: 1) females chose to lay eggs preferentially in some varieties over others, 2) egg survivorship influenced the numbers counted/found, 3) first instar egg eclosion varied across variety, 4) differential larval survivorship and development on the plants, and 5) larval dispersion, possibly to other plants, but possibly off the plants with mortality ensuing in unevaluated portions of the bug dorm. The Chi-square values for plant location were significant indicating that there were disparate numbers of eggs laid and larvae counted between the two locations, with the top third of the plants, regardless of variety, preferentially chosen over the bottom two-thirds. This was likely due to the amount of flush growth present on the top third of the plants. Also, given data in our studies, thrips activities appear higher on the top third of the plant; this is likely a good area to sample when looking for thrips on blueberries. Combining all the data, our studies clearly show that the Star variety had more citrus thrips than the other blueberry varieties, and numbers of oviposited eggs found on Star, Jewel, citrus and sumac were similar. This quite possibly is due to Star’s parentage. The Star variety is a variety that produces flowers the earliest and for the longest period during the season and while yield may not be as high as some of the other varieties, it is planted as a season long fruit bearer. Because the high bush blueberries are hybrids of one another and the nurseries have proprietary rights to the genetics of these varieties, we are therefore unsure how closely related the varieties we tested are. Snowchaser was the variety least preferred by the citrus thrips based on our data, however, to our knowledge it is not a variety currently grown commercially in California. There are countless factors that need to be considered to fully understand insect host choice, including but not limited to, host plant finding, host plant acceptance, host plant relatedness, resource concentration, resource availability and host use . The insect’s ability to locate the host plant, settle, feed, reproduce and successfully produce the next generation that achieves reproductive maturity involves countless steps and subtle interactions that are beyond the scope of the work presented here. We provide a platform for further work on the distribution, abundance and avenues for research related to citrus thrips as a pest of blueberries in California. We anticipate citrus thrips will remain a pest of concern to the blueberry industry of California as the industry continues to expand. Citrus thrips, Scirtothrips citri , is a significant insect pest of citrus and mango fruits and has been recognized as a major pest of California citrus since the 1890s . In the USA,blackberries in containers citrus thrips are known from Arizona, California, Texas and somewhat recently, possibly Florida , whereas in Mexico they are reported only from northern Mexico . Based on its past distribution, several authors have reported that citrus thrips is native to southwestern North America and northwestern Mexico . Citrus thrips is primarily a pest of citrus in California, particularly in the San Joaquin and Coachella valleys. They can have a broad host range, including, but not limited to, alfalfa, rose, grape, laurel, cotton, date, fir, lucerne and various grasses, pecans, and other ornamentals. Citrus thrips have been collected from over 55 different plant species .
Their native host plant is hypothesized to be Quercus or more likely Malosma laurina Abrams which was likely one of citrus thrips more common native host plants in southern California and northwestern Mexico prior to the introduction of citrus. In the SJV of California, S. citri has recently broadened its known host range and become a significant pest of high-bush blueberries . Scirtothrips citri was not known in Florida until 1986 where it was first detected in grape surveys . A collaborator was aware that in Florida, S. citri is not often collected from or abundant in several crops it is notorious for attacking in other regions of the Americas , but it is the most common thrips species he has collected from native vegetation and weeds. Species identifications from slide-mounted specimens can be unreliable or inconsistent and alternative or additional methods of identification may be necessary. Morphological identification suggests that S. citri is present in California, Arizona and Florida, but given that it is not a pest on several crops one might expect in Florida, further investigation is necessary to determine if S. citri is actually a cryptic species complex. The development of molecular genetic techniques , predominantly analysis of mitochondrial DNA , has significantly contributed to an understanding of natural genetic diversity and speciation . Genetic markers offer additional methods of species determination and delineation, especially when coupled with morphological identifications . These approaches are especially useful in groups that demonstrate a mixture of diverse ecological traits coupled with a conserved morphology. Given the distribution of S. citri in major citrus growing regions of North America and the level of its pest status in those regions, re-evaluation of morphological and molecular identifications was deemed necessary. The goals of this work were to investigate the haplotypic variation among S. citri populations based on phylogenetic analysis of the mitochondrial and ribosomal DNA, and to identify possible cryptic species complexes within the Scirtothrips attacking citrus. The collection records for all specimens used in this study are listed in Table 1. Specimens were collected from various parts of California, Arizona, Texas, Florida , Mexico, Nicaragua and Turkey. Specimens from Turkey were included in this analysis as it is an under-represented area of the world and at the time of collection from citrus, the collector believed the specimens to be citrus thrips. Specimens were collected into 95% ethanol by beating the live thrips onto a white piece of paper, touching a clean 5/0 Princeton paint brush into the ethanol filled collection vial, touching the ethanol imbibed paint brush tip to the live insect so that the insect stuck to the paint brush tip and then depositing it passively into the collection vial. After collection, all specimens were stored at -20°C until analysis. Some of the collections contained Frankliniella occidentalis and Neohydatothrips burungae but these collected groups were not included in our analysis.spinetoram and this may result in reduced E. hibisci mortality. Growers should be aware of the data presented herein when deciding upon a pesticide rotation management plan, which reduces avocado thrips resistance evolution. Each of the four recommended products have different features with respect to the efficacy of thrips control, concurrent control of avocado mite pests, and persistence of impacts on predaceous mites and other natural enemies . Thrips were removed from ethanol and allowed to air dry on filter paper for 2 min.