Scirtothrips perseae
(avocado thrips)

CLIMEX modelling indicates that this pest has the potential to invade all major avocado-growing regions in the world. CLIMEX software is available from CSIRO, Australia (http://www.csiro.au/).

In 1971, a quarantine interception at the Port of San Diego, USA resulted in the collection of a single female specimen of an undescribed species of Scirtothrips on avocados (Persea americana) from Oaxaca in southern Mexico. This single specimen was very similar in appearance to the avocado thrips found in California, USA. Avocado thrips has only been found feeding and reproducing on avocados in California. Adult S. perseae may occasionally be found resting on non-avocado plants in heavily infested avocado orchards. Avocados are native to Central America, and humans have moved plants out of this region into South America, the Caribbean and elsewhere. Foreign exploration efforts for avocado thrips and its natural enemies, in the native range of avocados, have shown that S. perseae has a narrow geographic distribution and is found throughout the avocado-growing region between Michoacan in Mexico and central Guatemala. Avocado thrips has not been found on avocados in Costa Rica, the Dominican Republic, Trinidad, Brazil or Chile (Hoddle et al., 2002b).

Avocado thrips is most likely to be moved on plant material. Mature avocado fruit are an unsuitable food source for the adults and larvae. Immature fruit and leaves are the most preferred feeding and oviposition substrates.

In the home range of S. perseae, this pest is found feeding on avocados at elevations exceeding 1500 m and it has not been recovered below this elevation (Hoddle et al., 2002b). This insect thrives in commercial avocado orchards and back yard plants.

S. perseae has only been found feeding, reproducing and causing damage to avocados (Persea americana) in California, USA. The host plant range in the area of origin of this pest is unknown (Hoddle et al., 2002b).

Genetics

S. perseae is haplodiploid; unfertilized eggs produce males and fertilized eggs result in female progeny. Molecular work on S. perseae is currently underway in California, USA to examine the genetic constitution of this pest for comparison with conspecifics in Latin America.

Phenology

S. perseae is multivoltine in California and there appears to be no overwintering stage or diapause. Population levels typically peak in late spring/early summer and hot summer temperatures can cause abrupt population crashes (Hoddle et al., 2002a).

Reproductive Biology

S. perseae has six distinct life stages. The females lay eggs inside young leaves or fruit (Hoddle, 2002a). Two larval stages subsequently develop and feed on young leaves or fruit. The two pupal stages are non-feeding and pupation occurs either in cracks or crevices on branches, or in leaf duff (litter) below the trees. The adults that emerge from the pupal stage feed on leaves and fruit. Because they can fly, they can disperse to adjacent trees to search for young, tender leaves in which to lay eggs.

Laboratory studies have indicated that the avocado thrips larvae and adults exhibit the highest survivorship, longevity and fecundity at moderately cool temperatures (20-25°C). Moderately hot temperatures (30°C), cause high larval mortality and reduced adult longevity when the thrips are confined in small cages under constant laboratory conditions (Hoddle, 2002b).

Preferences for low temperatures may optimally co-ordinate avocado thrips development and reproduction with avocado phenology over the spring, when the plants are producing young leaves and fruit, which are ideally suitable for thrips feeding and oviposition.

Environmental Requirements

The cool temperatures in late winter to early summer, which favour avocado leaf flush and fruit initiation, are most preferred by S. perseae. The pest populations typically build rapidly under these climatic conditions and this pest is most severe in orchards that are in close proximity to the coast. The marine influence has a moderating effect on hot summer temperatures (Hoddle et al., 2002a).

Several species of insect predators eat avocado thrips in Californian avocado orchards. These natural enemies include green lacewing larvae (Chrysoperla spp.), two predatory thrips; Franklinothrips orizabensis and Aeolothrips kuwani, and predatory mites, in particular, Euseius hibisci. Minute pirate bugs (Orius sp.) have not been recorded attacking avocado thrips. In a small-scale field trial in Fallbrook, California, USA releasing green lacewing larvae onto top-worked trees (10 ft high) with 12 avocado thrips larvae per leaf at low (325 lacewing larvae per tree) and high release rates (5045 lacewing larvae per tree) did not provide significant control of avocado thrips (Silvers, 2000). The lacewing larvae were used augmentatively in this trial and adequate thrips control over a 6-week period may have been an unrealistic expectation. An alternative approach could be to use lacewing larvae inoculatively. With this approach, the natural enemies might be released early in the season and allowed to increase in density in response to increasing avocado thrips numbers. However, the efficacy of inoculative lacewing releases when avocado thrips densities are low has yet to be experimentally verified.

Two releases of Franklinothrips at a rate of 218 adults per tree failed to control avocado thrips at densities of 12 larvae per leaf. In this study, a major problem was the poor quality of the predator after shipping from Europe to California. After transit over 50% of the adults had died and the survivors were probably of marginal health. In two later studies, weekly releases of Franklinothrips at a rate of approximately 50 adults per tree failed to control avocado thrips at initially low (1 thrips per leaf) and high densities (25 thrips per leaf) at sites in Escondido and Ventura, USA respectively. In these studies, a major problem was a lack of consistent thrips populations at densities that could support predator population growth. The low-density populations did not develop and the high-density populations crashed soon after the trial started.

Laboratory work on Franklinothrips has identified the optimal temperatures (Hoddle et al., 2000), diets (Hoddle et al., 2001a), harvesting of pupae (Hoddle et al., 2001b) and automated sorting of pupae; techniques that are applicable to the cost-effective mass rearing of this predator. Adult Franklinothrips females can eat approximately 14 to 20 second-instar avocado thrips larvae in a 24-hour period (Hoddle, 2003a). This predator does not show a feeding preference for first- or second-instar avocado thrips larvae. Both life stages are equally likely to be attacked after the predator encounters them. On average, Franklinothrips spends 7 to 13 seconds probing avocado leaves with its mouthparts and such activity occupies around 2 to 5% of its time (Hoddle, 2003b). This observation may explain, in part, why Franklinothrips populations decline following applications of insecticides that exhibit translaminar activity [Agri-Mek (abamectin) and Success (spinosad)]. Leaf feeding in this manner may expose Franklinothrips to insecticides that have moved into the leaf material, thereby killing them (Hoddle, 2003b).

Avocado thrips larvae and adults are primarily found on the undersides of immature leaves and fruit. First-instar larvae are pale white-yellow, whereas second-instar larvae are much larger and bright-yellow. Adult avocado thrips are straw-yellow and the abdomens may appear greenish because of the chlorophyll extracted from plant material during feeding. The adults also have three bright red dots, or ocelli, between their eyes, which are light-sensitive organs. Avocado thrips are readily distinguishable from another pest thrips in avocados, the greenhouse thrips (Heliothrips haemorrhoidalis), which are much larger and black. Western flower thrips (Frankliniella occidentalis), a common native Californian thrips species, is also yellowish-brown, about 33% larger than avocado thrips and is never found feeding on immature leaves and fruit, but can be found in avocado flowers feeding on the pollen. Adult western flower thrips can be distinguished from adult avocado thrips based on their size, the obvious bristles on the tip of the abdomen, which avocado thrips lack, and their association with avocado flowers.

The most effective way to sample for avocado thrips larvae and adults is to use magnifying equipment to examine the undersides of leaves for the presence of thrips. Small fruit can be examined in a similar manner. Avocado thrips are highly attracted to yellow sticky cards (Hoddle et al., 2002c). Sticky cards can be a very useful tool for monitoring adult densities in orchards over time, gauging directionality of flights within orchards, and for monitoring natural enemy species and their relative densities. Sticky cards should only be used for avocado thrips monitoring if the person examining the cards is expert enough to separate avocado thrips from other yellow-coloured thrips species that are also likely to be caught on the cards.

Scirtothrips citri is readily distinguished from the major pest species of the genus Scirtothrips in California, by the brown markings on the body. S. perseae is similar in colour to the Californian species Scirtothrips aceri, but that has shorter median setae on the abdominal tergites that are much further apart than their length. Another species that has also been found on avocado (Persea americana) in Central America is Scirtothrips astrictus, which has the ocellar setae arising between the posterior ocelli. The genus Scirtothrips currently includes 90 species, although the identity of some of these is doubtful. Additionally, there is an undescribed congener from Costa Rica that has been found feeding on avocados (Hoddle et al., 2002b).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

A novel method of controlling avocado thrips pupating in the soil beneath host trees (approximately 78% of second-instar larvae drop from the trees to pupate in the soil) is the use of coarse composted organic yard waste. Composted material that is prepared correctly is devoid of pathogenic diseases and pestiferous arthropods. Small field trials have demonstrated that 50% fewer thrips adults emerge from mulch laid under avocado trees in comparison to non-mulched trees that have leaf duff (litter) and branches under them. It is probable that the mulch suppresses the pupation of avocado thrips because it harbours a more diverse fauna of natural enemies (including entompathogenic fungi, nematodes and generalist arthropod predators) that opportunistically feed on microarthropods living in the mulch. This diverse fauna is lacking in avocado leaf duff in California, USA. Consequently, these generalist natural enemies attack avocado thrips larvae that fall from the trees to pupate. Interestingly, about 98% of Franklinothrips orizabensis (a common predatory thrips in avocado orchards) larvae drop from the trees to pupate in the soil. The organisms living in the mulch do not adversely affect F. orizabensis, probably because they pupate within protective silk cocoons, something avocado thrips does not do. It is unknown if the mulch can provide orchard-wide suppression of avocado thrips; if the reduction of thrips emergence is great enough to prevent economic damage to fruit; if the level of suppression seen is consistent year to year; or how regularly the mulch needs to be reapplied as it decomposes (Hoddle et al., 2002a).

Host-Plant Resistance

This area has not been studied for S. perseae.

Chemical Control

In developing a strategy for managing avocado thrips, a grower should consider a large number of interrelated factors including tree size and vigour (which may affect the amount and timing of leaf flushes that avocado thrips prefer), the past history of avocado thrips population dynamics, and fruit scarring experienced in the grove and surrounding regions. Also the amount of leaf flush and/or small fruit present; whether these fruit are the major set or additional fruit sets are expected; avocado thrips levels; natural enemy levels; grove topography; spray equipment availability; the grower's tolerance for fruit scarring; and an interest in preserving avocado thrips pesticide susceptibility. One should also consider that the weather, and the timing and amount of leaf flushes and avocado fruit sets, can vary tremendously from year to year. Just because avocado thrips caused significant fruit scarring one year or in a particular grove, does not mean that this or other nearby groves will have economic avocado thrips levels in subsequent years. Many groves do not need to be treated for avocado thrips and as a general principle, if practical, it is best to withhold treatments as long as possible to maximize the negative impact of the weather and natural enemies on avocado thrips levels. Under ideal circumstances and if practical, treatments should not be applied until it is clear that economic levels of avocado thrips (approximately three to ten immature thrips per fruit depending on the threshold for treatment) are present on a significant fruit set and are likely to cause economically significant damage (i.e. withhold treatments on off-bloom fruit, if hot weather is imminent, if natural enemy levels are high, or if the grower has a high tolerance for fruit scarring).

Because making decisions on the need for and timing of avocado thrips treatments can be difficult, it is important that the grower becomes as familiar as possible with avocado thrips and natural enemy biology, possible control options, and/or employs a knowledgeable pest control advisor to assist with scouting and decision-making.

There are three major options for the chemical control of avocado thrips as listed below and the materials are listed from least to most persistent in the control of avocado thrips.

- Veratran D plus sugar - this material is a botanical pesticide made from the ground seeds of a lily-like plant, Schoenocaulon officinale. The spray tank should be acidified to pH 4.5 prior to adding the Veratran D to the tank because acidification helps to maximize treatment efficacy. Veratran D residues are not persistent on leaves and are reduced to 50% of the initial level approximately 4 days after treatment. This results in perhaps 1-3 weeks of control depending on weather, application method, and thrips levels. Additives should not be added to a Veratran D treatment unless experience has shown that efficacy is not compromised. Veratran D is a stomach poison, thus it is relatively innocuous to most natural enemies.

- Success 2 SC plus Narrow Range 415 Spray Oil - Success (spinosad) is in the macrocyclic lactone class of chemistry, is unstable in sunlight (most surface residues are degraded within a day), and exhibits translaminar activity (it moves into the upper cell layers of leaves or fruit where it is toxic to avocado thrips when they feed). Success is relatively innocuous to natural enemies and treatments normally last for 2-4 weeks.

- Agri-Mek 0.15 EC plus Narrow Range 415 Spray Oil - Agri-Mek (abamectin) is also a macrocylic lactone, is unstable in sunlight, exhibits translaminar activity and should be used with oil. Thrips poisoned by Agri-Mek take 3-5 days to die and thus, control can be somewhat slower than with faster acting insecticides. This material is quite persistent in leaves and treatments can last for 6-10 weeks or more. Agri-Mek is also fairly innocuous to natural enemies.

Resistance Potential

Six applications of Veratran D in a single orchard over 2 years, resulted in the development of an 11-fold resistance by avocado thrips to this material. The development of avocado thrips resistance is a real concern and unnecessary treatments should be avoided. If Veratran D is being used for avocado thrips control, a maximum of two to three applications per year should be applied and this material should be rotated with either Agri-Mek or Success to reduce the rate at which insecticide resistance can develop. Avocado thrips have the potential to develop a resistance to Agri-Mek and Success. Because Agri-Mek and Success are of similar chemistry, there is a concern that cross-resistance might appear (i.e. if thrips develop resistance to one material, they might become resistant to the other). For these reasons, growers should only apply a single treatment of Agri-Mek per year, up to a maximum of two applications of Success per year (it is less persistent in leaves and fruit than Agri-Mek is), or up to a total of one Agri-Mek and one Success treatment per year (Yee et al., 1999, 2001a, b).

Early Warning Systems

None are used for S. perseae.

Field Monitoring/Economic Threshold Levels

At present the economic injury level used for avocado thrips ranges from three to five larvae and adults per three-quarters of expanded avocado leaf during bloom and when the fruit are still less than 3 cm long.

IPM Programmes

These are still under development for S. perseae.