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Homalodisca coagulata (Say) is a xylem-feeding leafhopper that is the principal vector of many economically important diseases resulting from infection by Xylella fastidiosa (i.e., plum leaf scald, phony peach disease). We have previously established that high abundances and high consumption rates of H. coagulata occur on host species with high amide concentrations in the xylem fluid. Several lines of research suggest that selection of “marginal hosts” (those that typically have low abundances of leafhoppers) may be influenced by ovipositional, as well as feeding, preferences. In northern Florida, Euonymus japonica consistently has the highest densities of eggs and young nymphs, but is only a marginal host for adults. Adults caged on this host feed little and have a short longevity. In contrast, young insects (second instar) caged on the host have high survivorship rates and assimilate dietary nutrients with high efficiency. H. coagulata are abundant on Prunus germplasm in northern Florida during the month of June, but only occasionally visit Prunus after this period. In a study of 10 Prunus scion/rootstock combinations, we established that abundances of H. coagulata on Prunus during the peak period were correlated to leafhopper consumption rates. During summer, when Prunus serves as a marginal host, leafhopper abundances are tightly coupled to fecundity rates. Understanding of ovipositional preference may be central to our understanding of Xylella acquisition. These preliminary experiments suggest that leafhoppers may sample xylem fluid during ovipositional selection, as they preferentially select ovipositional sites that have proper nutrient profiles for development of young nymphs (“mother knows best”). Although consumption rates are low for marginal hosts, repeated probing for ovipositional preference may contribute to the spread of diseases caused by X. fastidiosa.

Diurnal variations in the chemical composition of xylem fluid have been established for many plant species exhibiting positive root pressure; similar patterns have not been well documented in transpiring plants. Diurnal changes in plant water status and xylem fluid chemistry were investigated for `Flordaking' peach [Prunus persica (L.) Batsch], `Suwannee' grape (Vitis hybrid), and `Flordahome' pear (Pyrus communis L.). Xylem tension was maximum at 1200 or 1600 hr and declined to <0.5 MPa before dawn. Xylem fluid osmolarity ranged from 10 to 27 mm and was not correlated with diurnal patterns of xylem tension. The combined concentration of amino acids and organic acids accounted for up to 70%, 45%, 55%, and 23% of total osmolarity for irrigated P. persica, nonirrigated P. persica, Vitis, and P. communis, respectively. The concentration of total organic compounds in xylem fluid was numerically greatest at 0800 or 0900 hr. For irrigated P. persica the osmolarity of xylem fluid was reduced by 45% from 0800 to 1200 hr, 1 h after irrigation, compared to only a 12% reduction from 0800 to 1200 hr for nonirrigated trees. Asparagine, aspartic acid, glutamine, and glutamic acid were mainly responsible for diurnal changes in the concentration of total amino acids and organic N for P. persica; the diurnal variation in organic N for Vitis was due to glutamine. Arginine, rather than the amides, was the primary source of organic N in xylem fluid of P. communis, and there was no consistent diurnal change in the concentration of amino acids or organic N. The predominant organic acids in all species examined were citric and malic acids. No consistent diurnal trend occurred in the concentration of organic acids or sugars in xylem fluid.

A diurnal increase in the concentration of amino acids (and N/C ratio) in xylem fluid of Lagerstroemia indica occurred from ca. 1230 to 2030 HR. Diurnal trends were similar for irrigated or non irrigated plants. Since the concentration of total organic nitrogen, total amino acids and most individual amino acids (but not organic acids or sugars) were also proportional to xylem tension (Xt) two experiments were performed to discern whether variations in chemistry were due to diurnal changes in moisture stress. First, L. indica when exposed to variable levels of moisture stress during midday manifested an increase in organic acids and a reduction in the N/C ratio. Second, chemical profiles of xylem fluid were collected at noon and midnight and were compared for plants exposed to a natural photoperiod, constant darkness, or continuous light. After 1 day the midday increase in concentrations of amino acids persisted for all treatments; the variation was greatest (10-fold) for plants in constant darkness where Xt varied from 0.20 to 0.25 MPa. Only plants exposed to continuous light lost this tendency after 3 days. Thus, the circadian rhythm was endogenous, terminated in continuous light and not mediated by changes in moisture stress. Glutamine accounted for most of the diurnal variation in total amino acids, organic nitrogen or N/C ratio in xylem fluid.

The xylophagous leafhopper Homalodisca coagulata Say is an important vector of diseases caused by the bacterium, Xylella fastidiosa (e.g., Pierce's disease, phony peach disease, plum leaf scald, etc.). Neither leafhoppers or X. fastidiosa can be controlled by chemical sprays. For many plant species there is no resistant germplasm. H. coagulata is highly polyphagous, and within Prunus spp. host preference ranges from moderate (plum) to low (peach). The abundance, feeding and performance of H. coagulata on many unrelated plant species have been previously correlated with the amino acid profile, and particularly the amides in xylem fluid. We tested Prunus scion/rootstock combinations, which provided for a range of xylem fluid chemistry, on the behavior (abundance, feeding) and performance (survivorship, fecundity, body weight and body composition) of H. coagulata. Leafhopper abundance on various rootstock/scion combinations was determined seasonally. During the period of peak abundance (June 14 to 30) feeding rates and performance indices were determined. Leafhopper abundance and feeding increased with an increased concentration of amino acids. Abundance and feeding rate were most highly correlated with the amides in xylem fluid; performance indices were influenced by the amides and certain essential amino acids. Lower consumption rates decreased survivorship, reduced body dry weight and the carbon concentration of surviving insects.

Homalodisca coagulata (Say) is a xylem feeding leafhopper that is the principal vector of many economically-important diseases resulting from infection by Xylella fastidiosa (i.e., plum leaf scald, phony peach disease, Pierce's disease). Xylem fluid consists primarily of dilute concentrations of amino acids, organic acids and inorganic ions, and thus provides less nitrogen and carbon for herbivorous insects than any other plant tissue. Despite these nutritional constraints, H. coagulata is highly polyphagous. To assess how H. coagulata subsists on this dilute food source we examined host utilization by different instars on Lagerstroemia indica L. (preferred adult food source) and Euonymus japonica Thumb. (preferred ovipositional site).

Different instars survived and utilized nutrients at varying rates on the two hosts. Second instar nymphs survived at higher rates on E. japonica and utilized nitrogen more efficiently than on L. indica, yet assimilated nitrogen was less as a result of lower feeding rates. Adults on L. indica were more successful than those on E. japonica, used carbon more efficiently, and assimilated higher quantities of both carbon and nitrogen. Efficiencies of nutrient utilization were high for E. coagulata compared to other types of insects with assimilation efficiencies of specific compounds often exceeding 90%.