Jeffrey Granett

Jasmonic acid induced resistance in grapevines to a root and leaf feeder.

Abstract We investigated the effects of induced resistance to the folivore Pacific spider mite, Tetranychus pacificus McGregor (Acari: Tetranychidae), as well as the root-feeding grape phylloxera Daktulosphaira vitifoliae (Fitch) (Homoptera: Phylloxeridae) in grapevines using exogenous applications of the natural plant inducer, jasmonic acid. Foliar jasmonic acid application at concentrations that caused no phytotoxicity significantly reduced the performance of both herbivores. There were less than half as many eggs produced by spider mites feeding on the induced leaves compared with control grapevine leaves. Induction reduced the numbers of phylloxera eggs and nymphal instars by approximately threefold and twofold, respectively, on induced compared with control grapevine roots. The negative demographic effects of jasmonic acid application appeared to be caused by changes in fecundity for the Pacific spider mite, and possibly changes in development rate and fecundity for grape phylloxera.

Chemically-induced resistance against multiple pests in cotton.

Plants can be treated with natural elicitors to induce resistance to herbivores. The effects of induced resistance to cotton aphids Aphis gossypii Glover (Homoptera: Aphididae), two-spotted spider mites Tetranychus urticae Koch (Acari: Tetranychidae), and western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in cotton plants were investigated using applications of the natural plant inducer, jasmonic acid (JA). Preference was reduced by more than 60% for aphids and spider mites, and more than 90% for thrips on JA-induced leaves compared with control leaves. Survival of aphid nymphs was reduced by two-fold and percentage of nymphs attaining the adult stage was reduced by about eight-fold on induced leaves compared with controls. Induction reduced survival and reproduction of adult aphids by about 40% and 75%, respectively. Induction did not affect survival in spider mites or thrips. However, induction reduced egg production in spider mites by more than 75% and feeding in thrips by up to 80% compared with controls. Induction reduced leaf area in cotton seedlings by about 28%. The implications of our results for cotton pest management are discussed.

Habitat diversification tactic for improving biological control: parasitism of the western grape leafhopper

In a previous study we demonstrated greater abundance of the parasitoid Anagrus epos (Girault) in grape vineyards located downwind of prune trees that function as overwintering habitats. This study examines whether these higher A. epos numbers translated into higher egg parasitism rates of the grape leafhopper, Erythroneura elegantula (Osborn). Paired commercial wine‐grape vineyard plots, one with and one without adjacent prune trees, were studied within a complete block design in northern and central California. A. epos was the key mortality factor affecting E. elegantula eggs. Point estimates of A. epos parasitism rates were significantly greater in vineyards associated with prune trees during the first E. elegantula generation in both 1991 and 1992. No consistent differences in parasitism rates were observed during the second or third generations. The results indicated that prune trees enhance early season parasitism rates. Cumulative estimates of egg parasitism across E. elegantula generations demonstrated that enhanced early‐season parasitism resulted in a net season‐long increase in the degree of mortality imposed by A. epos on E. elegantula eggs. Two factors were found to influence parasitism rates: the abundance of early‐season A. epos adults moving into vineyards and the density of E. elegantula eggs in vineyards. Our results indicate that diversification of vineyards using prune trees supports overwintering populations of a specialist parasitoid and thereby alters host‐parasitoid interactions to favor enhanced parasitism in vineyards.

GRAPES, GALLS, AND GEOGRAPHY: THE DISTRIBUTION OF NUCLEAR AND MITOCHONDRIAL DNA VARIATION ACROSS HOST‐PLANT SPECIES AND REGIONS IN A SPECIALIST HERBIVORE

Abstract.— Studies of patterns of molecular variation in natural populations can provide important insights into a number of evolutionary problems. Among these, the question of whether geographic factors are more important than ecological factors in promoting population differentiation and ultimately speciation has been an important and contentious area in evolutionary biology. Systems involving herbivorous insects have played a leading role in this discussion. This study examined the distribution of molecular variation in a highly specialized gall‐forming insect, grape phylloxera (Daktulosphaira vitifoliae Fitch), that is found on both sympatric and allopatric host‐plant species of the genus Vitis. In addition, the relationship of insects in the introduced range in the United States to ancestral populations in the native range was examined. Evidence for differentiation along host‐plant lines from both nuclear (RAPD) and mitochondrial (COI) DNA was confounded with the effect of geography. Differentiation was found where hosts were allopatric or parapatric, but no evidence was found for such differentiation on two hosts, V. vulpina and V. aestivalis, that are broadly sympatric. The question of population differentiation onto these sympatric hosts can be considered to be resolved—it has not occurred in spite of a long history of association. Evidence was equivocal, but suggestive of a period of divergence in allopatry prior to reestablishment of contact, for insects associated with another host plant species, V. cinerea, found in both sympatric and parapatric populations. A low level of diversity and placement of samples collected from the grape species V. riparia at the tip of a phylogenetic tree supports the hypothesis that this host has been recently colonized from populations from the Mississippi Valley. A polyphyletic origin for biotype B grape phylloxera was supported: Although most samples collected from vineyards in the introduced range in California had similar haplotypes, they were closely related to natives on V. vulpina from the Atlantic Coast‐Piedmont region. All samples collected from vineyards in Oregon and Washington were closely related to natives on V. riparia in the northern United States.

LIFE TABLES OF PHYLLOXERA ON RESISTANT AND SUSCEPTIBLE GRAPE ROOTSTOCKS

Grape phylloxera, Viteus vitifolii Shimer, were reared on excised grape roots maintained in petri‐dish chambers. Life tables were constructed using 2 susceptible rootstocks, Vitis vinifera L. Cabernet Sauvignon and Muscat of Alexandria, and 2 resistant rootstocks V. rupestris St. George and Ganzin I. Phylloxera on the resistant varieties had lower rates of 1st‐instar establishment (24% and 9.5%) than on the susceptible varieties (57% and 36%). Similarly, gross reproductive rates (GRR) and net reproductive rates (Ro) on the resistant varieties were lower than on the susceptible varieties. Mean generation times (T) were variable (19–49 days) and appeared not to be related to rootstock susceptibility. Finite rates of increase (λ) were less than 1.0 on the resistant varieties, and 1.06 and 1.10 on the susceptible varieties. Phylloxera on Cabernet Sauvignon tuberosities had higher GRR, Ro and T‐values than on the nodosities. However, λ‐values were nearly equal, 1.05 and 1.07 on the tuberosities and nodosities, respectively.

Jasmonic Acid: A Vaccine Against Leafminers (Diptera: Agromyzidae) in Celery

Abstract Plant resistance to herbivores can be induced by application of elicitors such as jasmonic acid, which is a signal molecule that increases naturally in plants that have been damaged. The use of elicitors to induce resistance of crop plants against herbivores has been successful in small scale experiments but has not been developed in commercial agriculture. We tested jasmonic acid sprays as elicitors of resistance against leafminers and other pests on commercial celery fields in the central coast region of California. In two of three trials, significantly fewer leafminer adults emerged from the plants sprayed with jasmonic acid than the plants that were left untreated. In these two trials, aphid control was erratic, and thrips were not significantly reduced on the treated plants although thrips were not common during the trials. Leafminer parasitoid population densities tracked the number of available leafminer hosts regardless of treatment. The only effect found in the third trial was that conventionally grown celery had more leafminer oviposition stings than either the plants treated with jasmonic acid or untreated control plants. Treatment of plants with jasmonic acid seems to hold promise as an alternative pest management tool.

Influence of dispersal from almonds on the population dynamics and acaricide resistance frequencies of spider mites infesting neighboring cotton

Neighboring almond and cotton fields were sampled for spider mites in four locations in the San Joaquin Valley of California. The dominant species in the almonds wasTetranychus pacificus McGregor. In three cotton sites.T. pacificus was present in significantly higher densities near the almonds on at least one sampling date. In contrast.T. urticae Koch andT. turkestani Ugarov & Nikolski were equally abundant across the cotton fields. Almonds appeared to act as a continuous early-season source ofT. pacificus for cotton, with peaks in aerial dispersal from almonds occurring due to overcrowding, plant water stress, and applications of repellent acaricides. Cotton, which experienced little water stress, supported very high densities of spider mites and so acted primarily as a sink for spider-mite dispersal from almonds and other field crops throughout the growth-season. The frequencies of resistance expressed byT. pacificus andT. urticae were similar between neighboring crops, even if the acaricide had been registered for use only in almonds (cyhexatin) or cotton (dicofol). Thus, longterm acaricide selection and movement of spider mites between the two crops resulted in similar proportions of resistant individuals. In these study sites, large-scale dispersal ofT. pacificus from almonds rarely directly affected acaricide efficacy in cotton, because resistance frequencies were similar for spider mites from the two crops and because acaricide applications were usually made in cotton after dispersal from almonds was completed. In two cotton sites, field selection with dicofol was reversed by subsequent immigration of spider mites from neighboring field crops.

Preference and performance responses of California grape phylloxera to different Vitis rootstocks

Laboratory experiments on host preference and performance were simultaneously conducted with Cabernet Sauvignon, AXR#1, and 5C Vitis rootstocks for the California biotypes A and B grape phylloxera. Preference bioassays recorded over a 3‐day period indicated that phylloxera exhibit differential host choice. AXR#1 is antixenotic to biotype A, and 5C is antixenotic to both phylloxera biotypes. Preferences observed with biotype B for Cabernet Sauvignon and AXR#1 were not statistically different, whereas biotype A had shown a significant preference for Cabernet Sauvignon. On 5C the preferences scored were significantly lower than on either of the other two rootstocks, and there was no significant difference beetween biotypes. Performance bioassays recorded over a 29‐day period indicated that survival, development, and reproduction of both biotypes were affected by Vitis rootstock. Both biotypes were unable to survive or develop on 5C suggesting the presence of antibiotic resistance in 5C against these phylloxera biotypes. The antibiotic effects observed with AXR#1 against biotype A were expressed as significant reductions in survival, development, and reproduction. Performance of biotype B on AXR#1 was similar to its performance on Cabernet Sauvignon. For both AXR#1 and 5C rootstocks and both phylloxera biotypes A and B the antibiotic mechanism was considerably stronger than the antixenotic mechanism. This research indicated that host preference and performance are positively correlated in grape phylloxera with the rootstocks and phylloxera biotypes tested.

METABOLIC BASIS OF DIFFERENTIAL SUSCEPTIBILITY OF TWO FOREST LEPIDOPTERANS TO DIFLUBENZURON

Sixth instar Choristoneura occidentalis Freeman and 5th instar Orgyia pseudotsugata (McDunnough) were fed ring‐labelled 14C‐diflubenzuron coated on Douglas‐fir needles. After ingestion the gut was purged of radioactive needle residues with artificial diet. Radioactivity in body and frass extracts and residues were measured. Diflubenzuron in extracts were separated from metabolites by thin layer chromatography.

Distribution and Abundance of Leaf Galling and Foliar Sexual Morphs of Grape Phylloxera (Hemiptera: Phylloxeridae) and Vitis Species in the Central and Eastern United States

Abstract To better understand the host range and factors acting on the population structure of native grape phylloxera, Daktulosphaira vitifoliae (Fitch), Vitis species in 22 central and eastern U.S. states were surveyed for grape phylloxera presence and abundance. Data for frequency of attack and mean number of galled leaves per vine were compared among Vitis species and among six geographic regions defined according to topographic, ecological, and heuristic criteria. Four of the seven Vitis species that were identified had been attacked by grape phylloxera and both the frequency of vines that were attacked and the mean number of galled leaves differed among species and regions. In general, V. riparia (Michaux) and V. vulpina (L.) were attacked more frequently than expected and V. cinerea (Englemann) less than expected. The distributions of three of these species, V. vulpina, V. aestivalis (Michaux), and V. cinerea, were largely overlapping, but the distribution of V. riparia was not. It was found, to the near exclusion of other species in the northern United States, in the region formerly occupied by the Laurentide Icesheet. An increasing south to north gradient in phylloxera abundance was observed. Fourteen per cent of the surveyed vines were attacked in the Gulf Coast, 34% in the Central region, and 66 and 64% in the Northcentral and Northeastern regions. The mean number of galled leaves per vine conformed to this trend. In addition, sexual morphs were produced by apterous gallicolae in galls on V. cinerea but not on other Vitis species. This life cycle variant has previously been described only in the southwest United States. These data will be useful for future molecular phylogeographic studies and in the understanding, evaluation, and deployment of phylloxera resistant germplasm.