Research and IPM

Survey California tomato production areas to determine the annual distribution of the psyllid, and document whether the populations found in California can be traced to endemic populations in Mexico or Southern Texas using molecular techniques.

Document the numbers of nymphs or adults needed to cause economic damage on the primary cultivars of tomatoes in California at different stages of crop growth in order to establish an economic threshold for treatments.

Evaluate sampling techniques to determine the best way to document when adult populations immigrate to a field, and provide reliable estimates of nymphal populations.

Investigate biorational and biological control strategies for psyllids, as well as chemical controls, that can be integrated into an IPM program for fresh market tomatoes in California.

Substantial populations of psyllids appeared in 2001 in Western North America and caused losses in tomato production exceeding 80%; losses in 2004 reached 50%. To determine if these new outbreaks were the result of a simple range expansion or the evolution of a new B. cockerelli biotype, the ISSR markers, COI, ITS2, and wsp sequence data were used to characterize populations of the psyllid. Western populations from Baja, Orange County, and Ventura County were compared with populations from the Central U.S. (Colorado and Nebraska) and Eastern Mexico (Coahuila, Mexico). Based on the ISSR markers, the psyllid populations clustered into two groups, with one group including populations from Western North America and the other group including populations from the Central U.S. and Eastern Mexico. For COI comparisons, there was one base pair difference found in the 544 bp-long COI fragments, but the populations again segregated along the same geographic lines. Two strains of Wolbachia were identified, the maximal differences between wsp clones from all populations was 5 bp for strain Bac1 and 23 bp for strain Bac2 out of a 555 bp fragment. The ISSR data therefore were consistent in indicating development of a new psyllid population that has adapted to Western North America, but the other genetic data sets were less conclusive. The paper describing this research is now published in Entomologia Experimentalis et Applicata.

Two genetically distinct potato psyllid populations [Bactericera cockerelli (Sulc) (Homoptera: Psyllidae)] were identified in our previous study: native and invasive. The invasive population, ranging from Baja, Mexico to central California was the result of a recent invasion, while the native population is endemic to Texas. The native (Texas) and invasive (California) populations were collected from tomato and pepper, respectively, and were examined on both hosts to test the comparative fitness of invasive populations. Our results indicated that on both plant hosts, psyllids from the native range demonstrated higher survivorship, a higher growth index, and shorter development times than the psyllids from invasive populations. The fecundity of the native-range psyllids also was significantly higher than that of invasive psyllids on tomato, but not on pepper. For the native population, host plant differences for all fitness measurements were not significant. However, within the invasive population, psyllids feeding on tomatoes showed consistently better survivorship and a higher growth index than those feeding on pepper, despite the decreased developmental time required on peppers. The LC₅₀ values (concentrations causing 50% mortality) of both populations were determined for three pesticides. Resistance to two of these pesticides was found in the invasive population. Thus, the invasive quality of the California populations may be related to increased pesticide resistance. However, it is impossible to determine if the California population was preadapted to pesticide resistance, or if the resistance developed after the range expansion and is simply a contributing factor to maintaining the expansion. This work is in press in Entomologica Experimentalis et applicata.

Plant lines tested included the commonly-planted commercial cultivars 'Shady Lady' and 'QualiT 21,' an older, previously commercial cultivar '7718 VFN,' a common cultivar planted by consumers 'Yellow Pear,' and a wild type plant accession, PI 134417. When given a choice, psyllids significantly preferred 'Yellow Pear' and avoided PI 134417 for oviposition. Under no-choice conditions, psyllids laid significantly fewer eggs on PI 134417, but all the other plant lines were equally good substrates for laying eggs. Thus, oviposition preference will not likely provide a functional management strategy in large plantings. On 'Shady Lady,' psyllids preferred to oviposit on plants already infested with adults. On 'Shady Lady' and '7718 VFN' oviposition was significantly greater on plants previously infested by nymphs as compared to uninfested control plants.

This suggests that, at least for some cultivars, there is a physiological change in plant attractiveness following psyllid feeding.'Yellow Pear' and 'QualiT 21' were relatively tolerant of psyllids, requiring 18 nymphs per plant to produce the disease symptoms; only eight nymphs per plant were needed on 'Shady Lady' and '7718 VFN.' For all cultivars, the pest density showed strong correlations with measurements such as the number of yellowing leaves and leaflets and distorted leaves that were as good as or better than the first factor extracted from principal component analysis. Therefore, such measurements have the potential to simplify field surveys. The paper describing this work is now in press in the Bulletin of Entomological Research.

Although tomato psyllid annually causes significant losses in potato and tomato crops in Eastern Mexico and the Central United States, infestations in Western North America have been historically rare. However, substantial populations appeared in 2001 in Western North America and caused losses in tomato production exceeding 80%; losses in 2004 reached 50%.

To determine if these new outbreaks were the result of a simple range expansion or the evolution of a new B. cockerelli biotype, the ISSR markers, COI, ITS2 and wsp sequence data were used to characterize populations of the psyllid.

Western populations from Baja, Orange County, and Ventura County were compared with populations from the Central U.S. (Colorado and Nebraska) and Eastern Mexico (Coahuila, Mexico). Based on the ISSR markers, the psyllid populations clustered into two groups, with one group including populations from Western North America and the other group including populations from the Central U.S. and Eastern Mexico. For COI comparisons, there was one base pair difference found in the 544 bp-long COI fragments, but the populations again segregated along the same geographic lines. Two strains of Wolbachia were identified, the maximal differences between wsp clones from all populations was 5 bp for strain Bac1 and 23 bp for strain Bac2 out of a 555 bp fragment. The ISSR data, therefore, were consistent in indicating development of a new psyllid population that has adapted to Western North America, but the other genetic data sets were less conclusive. The paper describing this research is now in press in Entomologia Experimentalis et Applicata.

Insecticides tested against the commercial cultivars included a kaolin-based particle film, pymetrozine, pyriproxyfen, spinosad, and imidacloprid. Although all chemicals significantly reduced egg-adult survivorship, the effectiveness of some insecticides varied between plant lines as measured by survivorship, development time, and growth index data, which indicated significant interactions between plant lines and insecticides. For example, survivorship from egg to adult varied significantly between cultivars under pymetrozine treatment. For kaolin-based particle film applications, the number of days required to reach the adult stage were significantly different between cultivars. Growth index values were also variable between cultivars for pymetrozine and spinosad.

Although all chemicals tested had potential for psyllid control within an integrated pest management program, imidacloprid and pyriproxyfen worked consistently well on all cultivars tested. For the other chemicals, cultivar selection could influence pesticide efficacy. A second manuscript from this project has now been published in a peer-reviewed journal.

In other experiments, we collected adults from two locations in California, and four locations outside of California for molecular studies designed to evaluate similarity of DNA and Wolbacia genetic profiles. We have evaluated numerous primer systems for potential usefulness in separating possible strains. Analyses are in progress.

In other experiments we collected adults from two locations in California, and two locations outside of California for molecular studies designed to evaluate similarity of DNA and Wolbacia genetic profiles. We have evaluated numerous primer systems for potential usefulness in separating possible strains. Analyses are in progress.