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NORTH AMERICAN BEAN THRIPS

Caliothrips fasciatus (Pergande) -- Thysanoptera, Thripidae

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North American bean thrips (referred to here as bean thrips), Caliothrips fasciatus (Pergande) (Thysanoptera: Thripidae) are small insects (Figure 1) that were first collected from orange leaves in Yuba County California in 1894 (Bailey 1933). Bean thrips adults and larvae feed mainly on mature leaves and occasionally the skin of immature fruit. Female thrips deposit eggs into leaf material. Upon hatching from eggs, larvae feed and pass through two larval instars before reaching the propupal and pupal stages. As second instar larvae reach maturity they abandon host plants to fall into the soil where they transition into propupae and pupae. Following pupation, winged adults emerge from soil and likely fly back onto host plants to mate and feed. Sex determination for bean thrips is similar to other thrips, unfertilized eggs produce male thrips and fertilized eggs result in female offspring. Field populations of bean thrips have a sex ratio that is about 35% male and 65% female. Bailey’s (1933) breeding experiments in the lab indicated that female thrips that mate once and naturally live about 20 days stop producing female offspring after about 10 days and subsequent progeny are male only. If once-mated females naturally live for more than 50 days, female offspring production ceases after about 30 days and only males are produced thereafter. These results suggest that females need to mate frequently to continuously produce female offspring from fertilized eggs. 

Economic Damage

Economic damage is caused by direct feeding injury by thrips larvae and adults. Bailey (1933) notes that larvae are responsible for more feeding damage than adults as they tend to be numerically superior on host plants, they are less active which results in larval feeding being limited to confined areas, and larvae tend to feed gregariously. Feeding damage is manifested as silvering on leaves, and when feeding damage is heavy, premature defoliation can occur. This has been observed for heavily infested pear trees and following defoliation events new growth and immature fruit are subjected to “sun-scald” over summer and the tree productivity is not as vigorous the following season (Bailey 1933). In the absence of leaves, thrips will feed on immature fruit which results in scarring and downgrading at packinghouses. Bailey (1933) notes that in once instance up to 15% of pear fruit were damaged at an orchard in the Berryesa Valley, Napa County California in 1930. Additional cosmetic damage results from the deposition of black fecal droplets on fruit surfaces by feeding larvae and adult thrips.

Bean thrips adults have been recorded from a very large and diverse variety of plants (Table 1). Many potential feeding and breeding hosts are common weeds (e.g., prickly lettuce, Lactuca serriola) and native California plants that may be found in or around citrus orchards. Thrips are notoriously poor fliers and Bailey (1933) states that bean thrips are capable of only flying about 1-1.5 meters (3-4 feet) in a zig-zag or spiral pattern before landing. The pattern of dispersal in the field is irregular and is dependent on the direction and strength of the wind and the location of the source infestation (Bailey 1933). Because bean thrips are poor fliers incapable of controlled linear flight over long distances an effective cultural control practice to reduce the likelihood of adults reaching fruit to overwinter in may be good weed sanitation in orchards. Bean thrips infesting weeds on the orchard floor in close proximity to trees bearing fruit may simply need to fly a couple of feet to find overwintering sites (Figure 3). Weeding would remove potential host plants located within orchards, which are sometimes immediately under fruit bearing trees. Controlling weeds within 10-20 meters (30-60 feet or further from the boundary) of orchard margins could reduce migration rates into orchards and subsequent contamination of fruit in border rows. Orchards that have poor weed management tend to have high levels of bean thrips activity (Bailey 1933). 

Conclusions

Bean thrips contamination of California-grown navel oranges and increasingly mandarins with navels that are exported is a perennial quarantine problem for citrus growers. There are no reliable reports of bean thrips having established populations outside of North America, the assumed native range. Consequently, there are no records of this thrips being a pest anywhere else in the world. In this regard, bean thrips appears to be a pest unique to California (and possibly a few other western states, like Idaho and Utah [Bailey 1937]) but only during the 1930’s. It is uncertain as to why bean thrips is no longer a significant agricultural pest in California (or elsewhere). One possible reason as to why bean thrips has failed to establish in countries receiving California citrus contaminated with bean thrips is because thrips have been debilitated by cumulative cold stresses that significantly reduces the fitness of survivors. The ability of bean thrips to form overwintering aggregations inside navels may be mediated by a male-produced aggregation pheromone. Elucidation of a male-produced aggregation pheromone could provide novel detection and control tools. The severity of bean thrips infestations of citrus fruit fluctuates from year to year, and annual variation may be driven, in part, by weed abundance which is affected by winter rainfalls and irrigation. Good weed abatement may reduce infestation risks to California citrus growers and subsequently lessen risks of shipment rejections at ports of entry.

REFERENCES: [Additional references may be found at: MELVYL Library ]

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Bailey, S. F. 1937. The Bean Thrips. Monograph Bulletin 609. University of California Experiment Station, Berkeley, USA. 

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Harmon, J. A., Mao, C. X. & Morse, J. G. 2007. Selection of colour sticky trap for monitoring adult bean thrips, Caliothrips fasciatus (Thysanoptera: Thripidae). Pest Management Science 63: 201-216.

Hoddle, M. S. 2020. North American bean thrips: A perennial export problem for California citrus growers. Citrograph 11(2): 40-43. 

Hoddle, M. S.; Stosic, C .D. & Mound, L .A. 2006. Populations of North American bean thrips, Caliothrips fasciatus (Pergande) (Thysanoptera: Thripidae: Panchaetothripinae) not detected in Australia. Australian Journal of Entomology 45: 122-129.

Morse, J. G. 2006. Management of bean thrips. Citrus Research Board Annual Report. 3 pp. (last accessed 10 December 2019)

Mound, L. A.; Zhang, H.-R & Bei, Y.-W. 2011. Caliothrips tongi sp. n. (Thysanoptera: Thripidae) from China and a dubious record of North American bean thrips. Zootaxa 2736: 57-62.

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Mound L. A.; Hoddle M. S. & Hastings A. 2019. Thysanoptera Californica - Thrips of California. Lucidcentral.org, Identic Pty Ltd, Queensland, Australia. Last accessed 22 October 2019.

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