Introduction

The onion thrips Thrips tabaci Lindeman is an insect pest that belongs to the insect order Thysanoptera. It has specialized mouthparts that are used to punch through leaf tissues and siphon off plant contents leading to loss of chlorophyll and reduced photosynthetic efficiency (Boateng et al. 2014). In cultivated onions, smaller bulb sizes (30 – 50% reduction in bulb yield) could occur during thrips outbreaks (Nault and Shelton 2008). Thrips tabaci is also known as an insect vector of plant pathogens (Diaz-Montano et al. 2011), such as tomato spotted wilt virus or TSWV (Pittman 1927), Iris yellow spot virus or IYSV (Kritzman et al. 2001), Alternaria porri as well as Pantoea ananatis in onion (Thind and Jhooty 1982; Dutta et al. 2014). Thrips are minute and delicate insects that are difficult to handle and mass produce in the laboratory. Their body size, which on average is around 1 mm in length, presents difficulties in handling and observing thrips individuals during experiments. In studies that include evaluation procedures of control technologies (examples are insecticides and natural enemies screening) as well as transmission assays of plant pathogens in the laboratory, the development of practical and effective methodologies that enable the production of thrips in large numbers and uniform age has always been a challenge to researchers (Lewis 1973; Loomans and Murai 1997; Chatzivassiliou et al. 1999). The use of the bean jar method (Tedeschi et al. 2001), potted plants with or without covering (Edelson and Magaro 1988), boxes or containers (Guzmán et al. 1996), petri plates (Gulzar et al. 2021) and membrane method (Murai 2000; Murai and Loomans 2001) are known for maintaining the large number of thrips individuals under laboratory condition. Detached plant parts of host plant species like onion (Moraiet et al. 2017), leek (Chatzivassiliou et al. 1999) and cabbage (Gulzar et al. 2021) are also used as rearing medium for T. tabaci mass production. Another report described the use of plant pollen and germinated broad beans (Murai and Loomans 2001). However, most of these methods are laborious and time-consuming because the plant material such as detached leaves or plant parts easily dries up and therefore needs to be replaced with fresh ones frequently (every 2 to 4 days, for example). Thus, alternative rearing methods which are less laborious and simple to follow should be identified and investigated for T. tabaci. In the Philippines, harvested bulbs of local garlic or Allium sativum L. are usually stored for months by some farmers, and thus are a potential candidate as a rearing medium for T. tabaci. Although T. tabaci is known to feed and reproduce on young garlic leaves, very few studies have reported on the performance of T. tabaci on bulbs of garlic. Therefore, this paper aims to: 1) determine the biological parameters of T. tabaci when fed with garlic cloves and 2) demonstrate that garlic cloves can be used as an alternative rearing medium for T. tabaci in the laboratory.

Materials and Methods

Study Insect

Thrips tabaci was collected in a garlic farm site in Quinarayan, Narvacan, Ilocos Sur, Luzon Islands, Philippines. Infested garlic plants were placed inside resealable plastic bags and brought to the laboratory for processing. Thrips larvae feeding on garlic leaves were collected under the microscope using a fine, slightly moistened brush and transferred individually to healthy, clean garlic plants grown in pots. Cylindrical mylar cages were used to prevent the entry of other insects or arthropods on artificially-infested garlic plants. The adult stage of the field-collected thrips larvae as well as those from the culture were mounted in glass slides and used for taxonomic verification. Thrips tabaci was maintained for at least 15 generations on garlic plants in the laboratory (27ºC and under natural lighting) before using in experiments. The study was conducted in Room 106, National Crop Protection Center, University of the Philippines Los Baños from January until December 2019.

Biological parameters

Experiment 1: Untreated cloves of Ilocos white garlic variety (local variety bought in Sinait, Ilocos Sur) and X variety (imported variety bought in Los Baños, Laguna) were peeled and examined for the presence of other insects under the stereomicroscope. One replicate of each variety consisted of a glass beaker (Pyrex, 300 mL) containing 50 g of peeled, clean garlic cloves. Two pieces of paper towel (circular in shape) were also placed in between the cloves at the bottom, middle and upper locations inside the beaker. Fifteen pupae (near adult emergence) of T. tabaci from the progeny of the original female line were then placed inside each beaker. Two sheets of paper towel and organza cloth were used to tightly seal off the beaker with a rubber band. The experiment was conducted in a Completely Randomized Design (CRD) with seven replications in the laboratory (with 27ºC average temperature). The total number (on garlic cloves and paper towels), the average number per life stage, and the population increase of T. tabaci were recorded 15 days after infestation. Data were analysed using Analysis of Variance (ANOVA) with alpha = 0.05.

The time duration of 15 days was used because of two reasons: 1) approximately 1.5 generations of T. tabaci are completed within this period and 2) cloves without husks become dried after 15 days inside the beaker (Beltran, MJB, unpublished observation). During data gathering, circular paper towels from each beaker were removed first. The number and developmental stage of T. tabaci found on paper towels were counted. Ethanol (85%) was then poured inside the beaker to facilitate easy handling and counting of thrips individuals. All thrips individuals found in the beaker and paper towel were counted and recorded under the microscope.

Experiment 2: One larva (L2) from the stock culture was randomly selected, isolated in a glass test tube (8” x 1.2”), and provided with a young garlic leaf every other day until it reached the prepupal stage. The test tube was sealed off using a paper towel and organza cloth. After verifying the sex of the emerging adult, the female adult was provided with clean, uninfested garlic cloves (Ilocos white) to allow oviposition until it died. Sex determination of the adult stage of the progeny of this virgin, female adult was used as the basis to identify its mode of reproduction. In another setup, a total of 14 prepupae from the progeny of the original, virgin female adult was used to determine the average daily fecundity and total fecundity of T. tabaci. Prepupae were placed individually in a glass test tube and sealed with a paper towel and organza cloth using a rubber band. After verifying the sex of the emerging adult, one clove was provided daily to each female to allow oviposition for a period of 15 days. Each clove exposed to the female for 24 h was collected, sealed in another glass vial (2.2” x 1”) and properly labeled. After five to six days, Ethanol (85%) was poured on each glass vial to facilitate easier handling and counting of thrips larvae emerging from the exposed clove. Fecundity was determined based on the larval stage (L1 to L2) because it is easier to count this stage than the eggs that are partially inserted in the cloves that have dried already.

Results

First, the total number of all life stages of T. tabaci was significantly higher in the Ilocos white cloves as compared to the X cloves (Table 1; Figs. 1 and 2). The life stages considered in this comparison consisted of the larval stages 1 (L1) and 2 (L2), prepupal stage (P1), pupal stage (P2) and female adult (A). Thrips tabaci individuals at the egg stage were not included and no male adult individuals were found. Second, the number of prepupa and pupal stage of T. tabaci was significantly higher in the paper towel found in the base location as compared to other locations for both the Ilocos white and X variety (Table 2 and 3). Third, the number of T. tabaci is significantly higher in larval stage 2 (L2) as compared to the other life stages (Table 4). This is followed by pupal (P2) stage. No difference was found in the two-way interaction between the garlic variety and the life stage of T. tabaci. Fourth, the population increase is significantly higher

Fig. 1: Proportion of different life stages of T. tabaci in Ilocos white cloves 15 days after artificial infestation (clockwise: life stage with largest to smallest proportion)

Fig. 2: Proportion of different life stages of T. tabaci in X variety cloves 15 days after artificial infestation (clockwise: life stage with largest to smallest proportion)

in the Ilocos white cloves as compared to the X cloves 15 days after artificial infestation (Table 5). Fifth, the progeny of virgin, female adults of T. tabaci were all females. And six, the daily fecundity and total fecundity of virgin, female adults (N = 14) on garlic cloves (Ilocos white) were 2.38 ± 1.50 and Table 1: Number of specimens recorded in every life stage and total number of specimens in all life stages of T. tabaci fed with Ilocos white and X varieties in the laboratory

Variety	L1	L2	P1	P2	A	Total number
Ilocos white	65.714	166.714	59.571	149.571	66.857	508.429
X	68.571	135.571	41.286	62.714	18.571	326.714

Table 2: Number of T. tabaci (P1 and P2) on paper towels placed in different locations inside the glass beaker for the Ilocos white variety

Location	Total average number (P1, P2)
Base	76.071^a
Middle	22.500^b
Upper	6.000^b

*Means within the same column followed by a different letter are significantly different from each other (P < 0.05, LSD)

Table 3: Number of T. tabaci (P1 and P2) on paper towels placed in different locations inside the glass beaker for the X variety

Location	Total average number (P1, P2)
Base	43.425^a
Middle	8.500^b
Upper	2.047^b

*Means within the same column followed by a different letter are significantly different from each other (P < 0.05, LSD)

Table 4: Average number of T. tabaci per life stage using garlic cloves as rearing medium 15 days after the date of artificial infestation in the laboratory

Life stage	Average number (Ilocos white, X variety)
L1	67.143^c
L2	151.143^a
P1	50.429^c
P2	106.143^b
A	42.714^c

*Means within the same column followed by a different letter are significantly different from each other (P < 0.05, LSD)

Table 5: Total number and increase in the number of Thrips tabaci from day 0 to day 15 on cloves of Ilocos white and X variety after 15 days

Variety	Replicates	Average number		Average population increase (day 15-day 0)
Variety	Replicates	Day 0	Day 15	Average population increase (day 15-day 0)
Ilocos white	7	15.000	508.429	493.429^a
X	7	15.000	326.714	311.714^b

*Means within the same column followed by a different letter are significantly different from each other (P < 0.05, LSD)

Table 6: Daily and total fecundity of virgin- Thrips tabaci on Ilocos white garlic cloves in 15 days

Variety	Replicates	Daily fecundity	Total fecundity
Ilocos white	14	2.38 ± 1.500	35.640 ± 23.380

35.64 ± 23.38, respectively (Table 6).

Discussion

Results suggest that Ilocos white is more suitable as a medium for the development and reproduction of T. tabaci as compared to the X variety. The local variety then appeared to be more promising as an alternative rearing host plant than the imported one. Moreover, the population increase in terms of numbers from 15 individuals to 508.429 individuals indicates that the increase was almost 33 times higher than the original number for a rearing medium of garlic cloves that weighs 50 grams. Another point is that the base location is the most preferred location for T. tabaci pupation. Thrips larvae that are about to enter the prepupal stage crawled into crevices, such as in the folds of the paper towel found in the bottom of the beaker. This behaviour, which is widely known in several thrips species, facilitated the easy collection of the prepupa and pupal individuals which concentrated in the paper towel. Another advantage of using garlic cloves as a rearing medium for T. tabaci is the production of a large number of the active feeding stage of the pest, which is the larval stage (L2). In the literature, daily and total fecundity of T. tabaci on leaves of garlic are 3.1 ± 0.11 and 58 ± 3.09, respectively (Basri et al. 2019). On the other hand, pupa (P2) can be collected in advance from the cultur if the target stage for use in experiments is the adult stage. However, not a single male individual was collected in the offspring. This suggests that this T. tabaci population is reproducing via thelytoky – an asexual mode of reproduction where unfertilized eggs develop into females. Thelytoky is known as the most common mode of reproduction in this species (Kendall and Capinera 1990).

Conclusion

This study has shown that garlic cloves could be used as an alternative rearing medium for the onion thrips Thrips tabaci. A local variety, Ilocos white cloves, was shown to have a greater potential as a rearing host than the imported, designated as X variety. Biological parameters such as the total number on garlic cloves and on paper towels, the number per life stage, as well as population increase in number of T. tabaci supported this conclusion. On the other hand, the fecundity of T. tabaci when fed with garlic cloves is somewhat close in value when compared to the fecundity of T. tabaci when fed with garlic leaves.

Acknowledgements

Appreciation is also extended to the following: Mr Joenard Calamba for assistance during the collection of plant samples in the field; Ms Evelyn N. Salazar and Mr Ronnie B. Geli for assistance in the laboratory; Mr. Ray Montanez for statistical analysis of data and Dr. Cecilia Reyes for taxonomic verification of thrips samples.

Author Contributions

The author identified the research problem, designed and conducted the experiment. Preparation of manuscript for publication was done solely by the author.

Funding Source

This study was supported by the Department of Agriculture -Bureau of Plant Industry research project entitled Biological and molecular characterization of garlic viruses towards the development of disease management strategies.

Conflict of Interest

The author declares that there is no conflict of interest.

Data Availability

The data is available with the author.

Ethics Approvals

Not applicable in this paper.

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