stages are typically hidden in cracks or crevices in the tree or the soil, the winged adults are difficult for most predators to capture, and the egg stage is fairly protected inside plant tissue. Thus, any predator species trying to “make a living” off citrus thrips has sporadic availability of the larvae and cannot respond well numerically to increased thrips levels <strong>with</strong>in a particular year. Triapitsyn & <strong>Morse</strong> (1999) searched for wasp parasitoids attacking citrus thrips and, although they found low levels of two parasitoid species on laurel sumac (a common native host, see below), these insects were not found associated <strong>with</strong> citrus. The citrus thrips has adapted over time to citrus, but perhaps the parasitoids have not, or they are not present at high enough levels to be detected easily. Jones & <strong>Morse</strong> (1995) used isoelectric gel electrophoresis to study to what degree the predaceous mite Euseius tularensis (Figure 2) feeds on citrus thrips, as this predator has been proposed as one of the more common natural enemies of citrus thrips on California citrus. Only 7 of 556 (1.3%) adult female E. tularensis tested positive for citrus thrips in their gut. Given this, we wonder if E. tularensis perhaps reduces citrus thrips levels only when the pest first starts to build from low levels but not when both species are present at moderate to high levels. Euseius spp. are generalist predators that feed on pollen, mites, insects and leaf sap, so they are not specifically tracking citrus thrips populations. Grafton-Cardwell demonstrated that pruning and fertilizing trees generated higher densities of Euseius than augmentative releases by providing the environment Euseius prefers. It is generally accepted that densities of >0.5 Euseius per leaf are associated <strong>with</strong> good citrus thrips control, but it is possible that the presence of this level of Euseius is indicative of good biological control in general because a suite of natural enemies provide citrus thrips suppression rather than Euseius specifically. Monitoring for citrus thrips Strategies used by pest control advisors and growers for managing citrus thrips vary. PCAs typically monitor citrus thrips levels on young, developing fruit immediately after petal fall to decide if treatments are needed. Post-petal 24 Citrograph March/April 2012 Fig. 3. We rate navel orange scarring caused by citrus thrips on a 0-4 scale where 0 = no scarring by citrus thrips (not shown), 1 and 2 are slight scarring (not sufficient to cause fruit to be downgraded from first to second grade) and 3 and 4 are severe (economic scarring). The threshold for fruit downgrading varies from year to year but is typically set at a level of 3 scar or worse. fall treatments are not needed every year. This is because thrips levels vary from year to year, and also the timing of when the second and third generation of immature thrips appears in relation to fruit size varies. In the San Joaquin Valley, wet weather during bloom typically results in lower thrips levels after petal fall, in part due to greater mortality of the pupae in the soil. Careful monitoring can reveal orchards that have low levels of immature thrips on young fruit, and treatments can be delayed or eliminated altogether. Reducing the number of treatments will reduce the selection pressure for resistance to insecticides. Problems controlling citrus thrips in a particular grove are more likely if one or more treatments are used each year in contrast to a treatment perhaps being used only 5 years out of 10 years based on sampling for thrips severity each year. Insecticides can aggravate thrips populations Over the period 1972 – 2003, we ran citrus pesticide screening trials in Field 12 (Atwood navel oranges) at the Lindcove Research and Extension Center (LREC). Untreated control plots were always included in order to assess how much fruit scarring would result if no treatment were applied. We used a 0-4 rating scale to assess the severity of citrus thrips-caused fruit scarring (see Figure 3), and scarring levels 3 and 4 were categorized as “economic scarring”. We set the threshold for economic scarring as the level that would typically lead to fruit being downgraded from first to second grade. Over the 20 years 1972-1991 (data prior to 1981 from O. L. Brawner and Dr. Bill Ewart), citrus thrips economic scarring on untreated trees ranged from 1.2% (1986) to a maximum of 69.0% (1988) on outside lower fruit <strong>with</strong> a mean of 30.2% economic scarring. In contrast, over the 12 years 1992-2003, economic scarring ranged from 0.1% (2000) to a maximum of 10.7% (1997) <strong>with</strong> a mean of 4.4%. Clearly, something changed dramatically between these two time periods. The maximum level of severe scarring over the latter time period was about 1/3 of the average level over the earlier period. We believe the reason for this is that citrus thrips is, to a considerable degree, a pesticide-induced pest.
March/April 2012 Citrograph 25
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