Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions - 2000

Section E (1999)

Investigator’s Name(s): James S. Buckner¹, Thomas P. Freeman², & Dennis R. Nelson¹.

Affiliation & Location: ¹USDA--ARS, Biosciences Research Lab., Fargo, ND; ²Electron Microscopy Center, Plant Pathology Department, North Dakota State University, Fargo, ND.

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions.

Dates Covered by the Report: 1999

The Physiology of the Whitefly Egg Pedicel

Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) eggs oviposited on artificial membranes of stretched Parafilm M^® were used to study the function of the egg pedicel as an essential interface between the host plant and subsequent nymphal development and hatch. Feeding females oviposited eggs on membranes covered with a media consisting of 20% sucrose. After oviposition, the media was removed, the membrane surface with exposed egg pedicels was rinsed three times with sterile deionized water, and residual water removed by blotting with a dry tissue. For experimentation, the pedicels of oviposited eggs were submersed in test solutions, placed in a desiccator cabinet and held at 25 ° C and a relative humidity (rh) of 95-100%.

To demonstrate media uptake by B. argentifolii eggs, the pedicels of the eggs oviposited on membranes were exposed to [¹⁴C]-inulin (12.6 dpm/nl) diluted in 7.5% sucrose. After exposure to label for 48 hr, portions of membranes containing eggs and no eggs (controls) were suspended in 0.5 ml water. The suspension was frozen by submersion in a bath of dry ice/ethanol, thawed, sonicated, diluted to 7 ml with scintillation fluid and assayed for radioactivity using a scintillation counter. In two experiments with 69 and 127 B. argentifolii eggs, label uptake per egg was 7.6 dpm or approximately 0.6 nl. Since inulin is a polysaccharide that cannot cross a biological membrane, it was assumed that the labeled inulin resided within the pedicel of the egg and not within the developing embryo. Radiolabeled sodium acetate was used to demonstrate egg pedicel uptake of a chemical into the egg. B. argentifolii eggs were exposed to radiolabeled acetate in 7.5% sucrose for 48 hr and then at 95-100% rh until nymphal hatch. The resulting hatched nymphs were collected, digested in tissue solubilizer and assayed for ¹⁴C. In three experiments with 198, 290 and 503 eggs on membranes, egg hatch ranged from 91-100% and uptakes of ¹⁴C in dpm/nymph were 339, 349 and 672, respectively. Factoring in the concentration of [¹⁴C]-acetate in the media, the uptake volumes in nl/nymph were 1.6, 0.8 and 1.8, respectively. Thus, our experiments demonstrated that B. argentifolii eggs use their pedicels to transport water and solutes from the media into developing nymphs.

To demonstrate that B. argentifolii eggs require water for normal embryonic development and nymphal hatch, the pedicels of eggs on membranes were exposed to environments at various levels of relative humidity. Egg laden membranes (65-256 eggs per membrane) were incubated at 25 ° C for 7-8 days and percentage values for egg hatch were determined. At humidity ranges of 0-20% rh, 55-65% rh and 75-85% rh, none of the eggs hatched. At 95-100% rh, the mean percentage hatch value for five membranes with 109-242 eggs/membrane was 95.0 ± 5.2. Two membranes were used as controls (where the pedicels were in contact with sterile water) and the percentage hatch values for 152 and 184 eggs were 86.8 and 98.3, respectively. These results using humidity and radiolabeled materials strongly suggest that whitefly egg hatch is dependent upon water uptake from the host plant either in the form of free water or very humid air.

Transmission electron microscopic (TEM) and scanning electron microscopic (SEM) techniques were used to examine the structure of the B. argentifolii egg pedicle. SEM analyses of mature eggs removed from ovaries of females revealed that the surfaces of the distal portion of the pedicel consisted of a tangled array of fibers. TEM analyses of cross-sectional and longitudinal views of pedicels from mature eggs indicated that about 20-25% of the outer diameter of the distal portion of the pedicel was made up of the fibrous material. The arrangement of the fibers and the nature of their connection to the core of the pedicle were suggestive that the fibers function as the collector and conduit for water (vapor), and perhaps solute, movement into the egg.

Investigator’s Name(s): Y.X. Jiang^1,2, G. Nombela² and M. Muñiz².

Affiliation & Location: ¹Department of Entomology, University of California, Riverside, CA; ²Departamento de Protección Vegetal, Centro de Ciencias Mediombientales (CSIC), Madrid, Spain.

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host Plant Interactions.

Dates Covered by the Report: 1999

Analysis by DC Electrical Penetration Graphs of the resistance to Bemisia tabaci (Homoptera: Aleyrodidae) on two near-isogenic tomato lines

The tomato Mi gene confers resistance to nematodes, Meloidogyne spp., and to the potato aphid, Macrosiphum euphorbiae (Thomas). Previous greenhouse choice assays with Bemisia tabaci (Gennadius) showed that tomato varieties carrying this gene had significantly (P<0.05) lower values of host suitability and whitefly reproduction than varieties lacking Mi. This indicated that Mi, or another gene in its region, could regulate partial resistance. In order to characterize this resistance, the probing and feeding behavior of Bemisia tabaci B-biotype were studied with a DC Electrical Penetration Graph (EPG) on the near-isogenic tomato lines Moneymaker (without Mi) and Motelle (carrying Mi). Significant differences (P<0.05) between tomato lines were found in EPG parameters related to plant surface/epidermis and/or mesophyll tissues: number of probes made before attaining the first phloem phase (i.e., salivation and/or ingestion in a sieve element); the ratio: (number of probes made before first phloem phase)/(total number of probes); total duration of non-probing time (i.e., stylets not in plant); time it took for the whitefly to make its first intracellular puncture (stylet penetration to the phloem is mostly intercellular); time it took to reach the first phloem phase; and percentage of whiteflies that reached phloem phase. On Motelle, a lower percentage of whiteflies achieved phloem phase and they made more probes, had a higher ratio (number of probes before first phloem phase)/(total number of probes), had a longer total duration of non-probing time, and a longer time before making the first intracellular puncture and the first phloem phase. In contrast, most of the parameters related to phloem phase (for example, the total duration of phloem phase, the total duration of salivation in a sieve element, the total duration of ingestion from a sieve element) were not found to significantly differ between these near-isogenic lines. These data suggest that some resistance factors might be present in the plant surface/epidermis and/or mesophyll layers of Motelle plants, which may be regulated by the Mi gene or another gene in its region. Further studies are necessary to provide a better understanding of these mechanisms of resistance to whiteflies in tomatoes.

Investigator’s Name(s): Tong-Xian Liu.

Affiliation & Location: Texas Agricultural Experiment Station, Texas A & M University System, 2415 E. Highway 83, Weslaco, TX 78596-8399.

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions.

Dates Covered by the Report: 1999

Screening Cantaloupe Varieties for Whitefly Resistance

Silverleaf whitefly, Bemisia argentifolii Bellows & Perring, continues to be the most important pest insect for cucurbits, particularly on cantaloupe, in south Texas. The objectives of this study were to develop a rapid method to screen varieties, lines and PIs of cantaloupe for whitefly resistance; to determine the resistant mechanism of some whitefly resistant and susceptible varieties under laboratory and greenhouse conditions by studying the oviposition, development, survivorship and reproduction on each variety. A tuning table was used. The table can be easily turned around after the insects on the plants are counted. Six varieties and lines were used in this study: Hymark, Tam Sun x gl, Explorer, Tam Sun, Primo, and Perlita. Whitefly adults used in this study were cultured on cantaloupe and other crops in a greenhouse. When the plants for each variety or line grew up to 7-8 leaves, 2 fully expanded leaves (the 3rd and the 6th leaf from the terminal) were used for each plant, and the terminal and other leaves were removed. A turn-table, 2 ft in diameter, made of polywood, were used to hold 6 pots of melon plants, each representing a variety or line. The table with the 2-leaf plants were confined inside a cage. Whitefly adults were introduced into the cage at a rate of 50 adults per plant (leaf). The number of whitefly adults on each leaf were counted in 4 and 24 h after the whitefly introduction. Because the whiteflies are aggregated between and within plants, the location of the plant inside the cage may influence the number of whiteflies on it. To avoid this bias, the leaves with the whiteflies were disturbed after the number of adults was counted to allow the whiteflies to relocate their feeding or ovipositing site. The table were turned around, and the plants were relocated randomly. The number of whitefly eggs on each leaf were counted 24 h after the adult introduction. The leaf area was measured, and the number of whitefly eggs and adults on each leaf was computed. Trichome density of the selected leaves (2 per plant) were measured by laying 2 1-cm² templates over the adaxial surface of the leaves, 1 on each side of the main vein, and the number of trichomes were counted with the perimeter of the template. On a precision balance, all sample leaves were weighed individually. After counting the whitefly eggs and trichomes on the leaves, the leaf thickness was measured. Leaf area was then measured using a portable area meter. Ten to twenty cantaloupe plants form each variety, one per pot, were maintained on a bench in a greenhouse. The plants were used for experiment when the leaves are 30-40 cm². A leaf clip on cage was placed on a fully expanded leaf, and 20 whitefly females (<24 h old) were introduced inside the cage. Adults were removed 4 h after the introduction. Eggs on the leaf were marked and coded. The eggs were monitored daily for hatching. After the first instar crawler hatches, the development of each nymphal stage was monitored daily until the adult emerges. To determine the effects of variety to whitefly reproduction, the newly emerged adults were collected, and the adults were sexed, and the mated females were caged on the leaves of the same variety and another variety with different resistant characteristics. Oviposition were checked in 2-3 days.

Number of silverleaf whitefly adults on different varieties in 2, 4 and 24 hours varied greatly, but did not show significant differences. Significantly fewer eggs were found on Tam Sun and Tam Sun x gl. than on any other varieties. Silverleaf whitefly developed significantly different on different varieties. Whitefly eggs developed longer on Hymark than on all other varieties. Whitefly nymphs developed significantly longer on Tam Sun and Hymark, followed on Tam Sun x gl. (glabrous). In contrast, whitefly pupae on Tam Sun x Glabrous developed significantly slower than on Tam Sun and Primo, but not slower than on Hymark, Explorer, and Perlita. The overall developmental durations of all immature stages were significantly longer on Tam Sun, Hymark and Tam Sun x gl. than on other three varieties. Natural percentage mortalities of silverleaf whitefly from egg to adult emergence varied greatly among the varieties with 60% mortality on Hymark; whereas those on other varieties were not significantly different.

Investigator’s Name(s): C. L. McKenzie, Sang Dae Lee, Hamed Doostdar, N. Kokalis-Burelle, & R. T. Mayer.

Affiliation & Location: USDA--ARS, Horticultural Research Lab, Ft. Pierce, FL.

Research & Implementation Area: Section E: Host-Plant Resistance, Physiological Disorders, and Host-Plant Interactions.

Dates Covered by the Report: 1999

Effect of Plant Growth Promoting Rhizobacteria on Pathogenesis-Related Protein Induction in Cucurbits Challenged with Different Levels of SLWF Infestation

Two lab trials were conducted sequentially to evaluate SLWF preference for two different cucurbit hosts (watermelon and cantaloupe) treated with eight different plant growth-promoting rhizobacteria (PGPR) and the subsequent effect on Pathogenesis-Related (PR) protein induction.

Watermelon seeds cv. ‘Star gazer’ were direct seeded into speedling trays and allowed to germinate. PGPR were applied to the potting mix as a gram positive dry spore formulation. When seedlings reached the 2-3 true leaf stage they were individually transplanted into 12 oz solo cups with Metro Mix 500, fertilized with a dilute Peter’s solution 20-10-20 at 5 ml per plant and allowed to acclimate for 72 hr. Five plants per replicate were arranged in a RCB design with each replication housed in a separate 24"x 24"x 24" Plexiglas cage. The first two fully expanded leaves from 3 plants per treatment were sampled for PR protein analysis prior to SLWF infestation. Five SLWF adults per plant were released in each cage and the total number of SLWF eggs, nymphs and adults were counted on the third fully expanded leaf from the terminal 7 days after infestation. The first two fully expanded leaves from the same plant used for SLWF counts were sampled for PR protein analysis immediately prior to insect counts taken that same day.

The second lab trial was similar to the first with the following exceptions. Cantaloupe cv. ‘Athena’ were at the 3-4 true leaf stage before being individually transplanted. Infestation level and infestation period were doubled to 10 SLWF adults per plant and 14 days, respectively. Leaf area was calculated for the leaf used for insect counts. PR protein leaf samples were sampled prior to infestation and immediately before insect counts at 14 days.

SLWF preferred to oviposit on PGPR treated watermelon and cantaloupe compared to the untreated control by at least two to one depending on treatment. SLWF nymph counts followed the same trend, but were more variable. Adult whiteflies preferred PGPR treated plants 4- and 8-fold more to the untreated for watermelon and cantaloupe, respectively. However, there was a positive correlation between leaf area and SLWF numbers detected for cantaloupe indicating preference may be due to leaf size as a result of the PGPR treatment.

In watermelon, total protein concentrations were higher in preinfested plants. The reverse was true for cantaloupe with infested leaves having 1.8-fold higher total protein concentrations even though the infestation time and level was doubled. PR proteins analyzed included chitinase, glucanase and peroxidase. No significant treatment interactions could be detected among the PGPR treatments. Only the date interaction was significant indicating that SLWF infestation was more effective in inducing PR proteins than PGPR treatments when proteins were induced. No differences between dates for total chitinase could be detected for watermelon or cantaloupe. Total chitinase per leaf was 1.7-fold higher in preinfested leaves for watermelon and in contrast infested cantaloupe leaves were 1.7-fold higher. Total glucanase was 1.6-fold higher in infested watermelon leaves than preinfested, but no differences between dates for total glucanase per leaf could be detected for watermelon or cantaloupe. In cantaloupe, total peroxidase and peroxidase per leaf was 6- and 3-fold higher in preinfested leaves compared to infested. However, the reverse was true for watermelon. Infested leaves were 14- and 10-fold higher for total peroxidase and peroxidase per leaf respectively in infested watermelon leaves. Studies are ongoing to elucidate the sharp contrasts found for these two cucurbits.

Investigator’s Name(s): E. T. Natwick¹, G. Walker², D. Johnson², C. C. Chu³, T. J. Henneberry³, D. Brushwood⁴, & G. Constable⁵.

Affiliation & Location: ¹University of California Cooperative Extension, University of California Desert Research and Extension Center, 1050 E. Holton Road, Holtville, CA 92250, ²University of California, Riverside, CA, ³USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ, ⁴USDA--ARS, Cotton Quality Research Station, Clemson, SC and ⁵CSIRO, Narrabri, NSW Australia.

Research & Implementation Area: Section E: Host-Plant Resistance, Physiological Disorders and Host Plant Resistance.

Dates Covered by the Report: March 1999 - December 1999

Normal Leaf and Okra-Leaf Upland Cotton Cultivars Susceptibility to Infestation by Silverleaf Whitefly

Sixteen upland cotton, Gossypium hirsutum L., cultivars and experimental breeding-lines were evaluated in the field for susceptibility to silverleaf whitefly, Bemisia argentifolii Bellows and Perring, sown at the UC Desert Research & Extension Center, Imperial Valley, CA, into plots of a randomized complete block design experiment replicated four times, and irrigated 26 March, 1999. The normal leaf cultivars were DP 20, DP 50, DP 90, DP 5415, DP 5432, DP 5461, DP 5557, HCR 9257, HCR 9240, HCR 7126, and, Stoneville 474 and the okra-leaf cultivars and experimental breeding-lines were Siokra L23, , FiberMax 832, CSIRO 91209-194, and CSIRO 89230-244-1028. Individual plots measured 14 m in length with 8-beds on 1 m centers or 8m wide. No insecticides were applied to the cotton plots. Silverleaf whitefly adults were sampled from ten plants at random in each plot via the leaf turn method using the 5th main stem leaf from the terminal on 27 May, 3, 25, 30 June, 7, 14, 20, 28 July, 4, 11, 18, & 25 August, 1999 Silverleaf whitefly nymphs were counted on 1.65 cm² leaf disks of from ten 5th position leaves down from the terminal extracted from randomly selected plants in each plot on 30 June, 7, 14, 20, 28 July, 4, 11, 18, & 25 August. Seed cotton was hand picked from 0.002 acre per plot and yield data were recorded on 10 September, 1999. The okra-leaf entries as a group had fewer silverleaf whitefly adults and nymphs than the normal leaf cotton entries. The okra-leaf experimental breeding-lines CSIRO 91209-194, and CSIRO 89230-244-1028 had the lowest numbers of silverleaf whitefly adults and nymphs among the okra-leaf entries. Stoneville 474, a hirsute-leafed cotton, had the greatest numbers of silverleaf whitefly adults and nymphs among the normal leaf cottons. There were no differences in seed cotton yield among the entries, P =< 0.05, SNK.

Investigator’s Name(s): E. T. Natwick¹, C. G. Cook², R. L. Gilbertson³, Young-Su Seo³, & T. Turini¹.

Affiliation & Location: ¹University of California Coop. Ext., Holtville, CA, ²United Agri Products, Santa Rosa, TX, and ³University of California Davis, Davis, CA.

Research & Implementation Area: Section E: Host-Plant Resistance, Physiological Disorders, and Host Plant Resistance.

Dates Covered by the Report: March 1999 - December 1999

Resistance To Cotton Leaf Crumple Geminivirus Disease In Upland Cotton

Eight upland cotton, Gossypium hirsutum L., cultivars or experimental breeding-lines were evaluated in the field for resistance to the silverleaf whitefly, Bemisia argentifolii Bellows and Perring, transmitted cotton leaf crumple disease caused by cotton leaf crumple geminivirus (CLCV) in Imperial Valley, CA in 1999. The cultivars were Texas 121, AP 4103, AP 6101 and Stoneville 474 and the breeding-lines were DG 2165 and DG 2108 and with Cedix parentage were DG 2383, and DG 2387. The following rating scale for CLCr disease symptom was used on 23, 27 and 30 August and on 6 September: 1 = leaf smooth, few if any bumps or blisters; 2 = some obvious blisters and crumpling, but less than 50% leaf with symptoms; 3 = obvious crumpling, blisters, vein clearing from more than 50% to close to 100%, leaf not rolled; 4 = severe crumpling, blisters, leaves noticeably rolled and distorted. Leaf and petioles from each plot were used to confirm the presence of CLCrV by squash blot hybridization with a general DNA probe, which detects the presence of whitefly-transmitted geminiviruses (Gilbertson et al. 1991). DNA sequencing of a polymerase chain reaction (PCR) amplified fragment from an infected plant was used to confirm that the geminivirus was CLCrV.

Cotton cultivars and breeding-lines were evaluated in 1999 in Imperial Valley, California for resistance to the silverleaf whitefly-transmitted cotton leaf crumple disease, caused by cotton leaf crumple geminivirus (CLCV). Results showed differences in whitefly infestation levels and virus disease symptoms among cotton entries. The variety Stoneville 474, with hirsute leaves, had more adult silverleaf whitefly for the seasonal mean than any of the other entries, (SNK; P# 0.05). Seasonal silverleaf whitefly nymphs per cm² mean separations were as follows: Stoneville 474 (15.1 A), DG 2108 ( 8.7 B), DG 2387 (5.4 C), Texas 121 (5.3 CD), AP 6101 (5.1 CDE), AP 4103 (4.9 CDE), DG 2383 (3.8 DEF), DG 2165 (3.5 F). (The breeding-line with Cedix parentage had a lower CLCV disease rating than other entries; DG 2165 (1.5), DG 2108 (1.4), DG 2383 (1.1), and DG 2387 (1.1). Varietal CLCV disease rating were as follows: Texas 121 (2.1) < Stoneville 474 (2.5) < AP 6101 < AP 4103 (3.6). Seed cotton yield as pounds per acre and mean separations were as follows; AP 6101 (2743 A), AP 4103 (2455 AB), DG 2383 (2301 AB), DG 2165 (2014 BC), Texas 121 (2001 BC), DG 2387 (1900 BC), DG 2108 (1616 CD), and Stoneville 474 (1250 D).

Investigator’s Name(s): Laura O. Petro & Richard A. Redak.

Affiliation & Location: Department of Entomology, University of California, Riverside, CA 92521, USA.

Research & Implementation Area: Section E: Host-Plant Resistance, Physiological Disorders, and Host Plant Resistance.

Dates Covered by the Report: July - December 1999

Host Plant Preference and Performance of Bemisia argentifolii (Homoptera: Aleyrodidae) on Poinsettia (Euphorbia pulchirrima) in Relation to Cultivar

We investigated the resistance qualities of seven economically important red cultivars of poinsettias against its major pest, the silverleaf whitefly, Bemisia argentifolii, Bellows and Perring. Additionally, two cultivars selected on the basis of previous adult preference studies (representing preferred and non-preferred ) were monitored for nymphal development and survivorship. After 6 d of exposure, the cultivars "Red Velvet", "Supjibi", and "Pepride" were less preferred as oviposition sites than the other 4 cultivars evaluated. "Peterstar" was the most preferred host for oviposition. After 21 days of exposure, there were significantly fewer eggs and nymphs observed on "Pepride" and "Red Velvet" and "Peterstar" again had significantly more eggs than the other preferred cultivars. "Success" and "Petoy" had a significantly greater numbers of surviving nymphs than the other cultivars evaluated. Total numbers of all live stages observed were significantly lower on "Freedom Red", "Red Velvet" and "Pepride". These latter cultivars demonstrated the greatest potential for resistance against the silverleaf whitefly. Observed plant morphology in addition to plant chemistry may explain the differences in suitability among the poinsettia cultivars. The population dynamics of silverleaf whiteflies on the most preferred and non-preferred poinsettia hosts are represented as life tables. The wider implications of the mechanisms of poinsettia resistance to whiteflies and future uses in integrated pest management for greenhouse cropping systems will be discussed.

Investigator’s Name(s): W. T. G. van de Ven¹, C. S. LeVesque², T. M. Perring², & L. L. Walling¹.

Affiliation & Location: Departments of ¹Botany and Plant Sciences and ²Entomology, University of California, Riverside, CA 92521. >

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interaction.

Dates Covered by the Report: January 1999 - January 2000

Characterization of Two Squash Genes Induced by Silverleaf Whitefly Infestation

Squash genes induced in apical, silvered leaves after silver-leaf whitefly (Bemisia argentifolii) feeding were isolated. SLW1 and SLW3 were differentially expressed by B. argentifolii and the sweetpotato whitefly (B. tabaci). SLW1 and SLW3 RNAs accumulated locally and systemically after nymph feeding. SLW1 RNAs were detected in infested leaves, distal, apical leaves and the shoot apex in B. argentifolii-infested plants but was not induced after B. tabaci feeding. SLW3 RNAs accumulated in infested leaves and in proximal non-infested leaves from B. argentifolii and B. tabaci-infested squash. SLW3 RNAs accumulated in more distal leaves only in response to B. argentifolii. SLW1 RNAs were detected in flowers and fruit, while SLW3 RNAs were not detected in any organ other than leaves. Whitefly feeding did not alter this developmental programming. SLW1 (a M20b peptidase-like gene) and SLW3 (a b -glucosidase-like gene) are modulated by different signal transduction pathways. SLW1 RNAs and proteins were abundant in response to exogenous methyl jasmonate (MeJA) and water-deficit stress. SLW1 RNAs were detected at low levels after wounding, Pseudomonas syringea pv syringea infection and ABA or ethylene treatments. Like SLW1, SLW3 RNAs accumulated to high levels in response to water-deficit stress. However, SLW3 RNA levels were not influenced by pathogen infection, wounding, MeJA, ethylene, salicylic acid, or ABA treatments. Possible roles for SLW1 and SLW3 are discussed. (Supported by USDA 95-37301-2081 to TMP and LLL and USDA 99-35301-8077 to LLW)

Investigator’s Name(s): G. P. Walker, D. D. Johnson, & H. Costa.

Affiliation & Location: Department of Entomology, University of California, Riverside, CA

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host Plant Interactions

Behavioral Response of Silverleaf Whitefly Adult Females to Plant Species Varying in Host Suitability

The behavior of adult female silverleaf whiteflies on four plant species, lima bean, broccoli, corn, and sugar beet, was studied by making visual observations of individual whiteflies on these plants. The plants were chosen to represent a range of host suitability for silverleaf whitefly from excellent to poor host. This study is part of a larger project whose objective is to determine how whiteflies distinguish among plant species of different host suitability. In relation to host plant resistance, there are few cases of a single crop species having different varieties whose host suitability to silverleaf whitefly ranges from highly susceptible to highly resistant. Consequently, to study a range of plants that include these extremes, we used four different plant species.

First, we verified that these plant species represent a range from very suitable to very poor hosts. Adult female whiteflies were confined singly in clip-on cages to leaves of young plants (< 1 month old) for 2 days on each of the four plant species. Whiteflies used in the experiments were reared on lima bean, and had not been exposed to the other plant species prior to the experiment. Mortality over the 2 day period increased from lima bean (8% mortality), to broccoli (16% mortality), to corn (36% mortality), to sugar beet (62% mortality). Mortality on sugar beet was significantly higher than on lima bean and broccoli (P < 0.05, chi-squared test), whereas mortality on corn was significantly higher than on lima bean but not significantly higher than on broccoli. Whitefly fecundity over the 2 day period (calculated only for females that survived the 2 day period) was significantly higher on lima bean and broccoli (17-20 eggs per female, respectively) than on sugar beet and corn (6-10 eggs per female, respectively). We conclude that lima bean and broccoli are good hosts for silverleaf whitefly; sugar beet is a very poor host, and corn is a moderately poor host. These results were expected because 1) the whiteflies were reared on lima bean; 2) silverleaf whitefly is well documented to build large populations on broccoli; and 3) silverleaf whitefly is rarely considered pests of sugar beet and corn even though both crops are grown in areas with high populations of silverleaf whitefly.

Using a stereo-microscope focused on the underside of a leaf held at a 45 degree angle, continuous behavioral observations were made during the first 15 minutes of whitefly contact with the leaf. The sequence and duration of the following behaviors were recorded: labial dabbing (rubbing the apex of the labium over the leaf surface), probing (stylet insertion, indicated by the labium being held motionless with its apex contacting the leaf surface at a right angle), and oviposition. If the whitefly was probing at the end of the standard 15 minute observation period, observations were continued until either the whitefly terminated the probe or until the probe exceeded 15 min in duration, whichever occurred first. Thus, the maximum probe duration that could be recorded was 15 min. Labial dabbing was selected as a relevant host selection behavior because previous work in our lab indicated that the whitefly labium has mechanoreceptors and chemoreceptors at its apex.

Two hundred and ten probes were observed among the 4 plant species, and all 210 were preceded by labial dabbing. The duration of labial dabbing preceding the first probe differed little among the 4 plant species. The first bout of labial dabbing on lima bean was slightly, but significantly, shorter than on corn (2.7 versus 3.6 sec) while the duration of the first bout of labial dabbing was intermediate on broccoli and sugar beet. It is questionable whether this small difference is biologically significant or only statistically significant. The time it took to initiate the first probe did not differ significantly among plant species (range: 6.2 - 8.1 sec). The duration of the first probe also did not differ significantly among the 4 plant species (range: 580 - 391 sec); however when all probes were taken into consideration, average probe duration was significantly longer on lima bean than on the other 3 plants. Oviposition occurred for 8/23 whiteflies on lima bean, 13/23 whiteflies on broccoli, 2/24 whiteflies on corn and 9/21 whiteflies on sugar beet. For those whiteflies that oviposited, the average time to oviposition did not differ among the 4 plant species and ranged from 454 to 657 seconds from the beginning of observations. Calculated from the beginning of the probe in which oviposition occurred, the time to oviposition again did not differ significantly among plant species and ranged from 163 to 463 seconds. It is noteworthy that this is not enough time for the whitefly to reach the phloem. Thus, these results agree with our previous study on electronically monitored whiteflies that after first encounter with a new plant species, the great majority of the first eggs laid by silverleaf whiteflies are oviposited before the whitefly can test the phloem for suitability.

Investigator’s Name(s): Linda L. Walling¹, David P. Puthoff¹, & Thomas M. Perring².

Affiliation & Location: ¹Dept. of Botany and Plant Sciences. ²Dept. of Entomology. University of California, Riverside.

Research & Implementation Area: Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions.

Dates Covered by the Report: January 1999 - January 2000

Whitefly Feeding Induces Local and Systemic Changes in Tomato Gene Expression

Whitefly infestations of crops across the Southern United States and throughout the world results in millions of dollars in damage each year. Not only do whiteflies remove photo-assimilates and vector and array of devastating viruses, but they also cause developmental disorders in plants such as squash, broccoli, and tomato. Many preceding studies have focused on the plant defense responses to chewing insects. In this report, the tomato response to phloem-feeding whiteflies (B. argentifolii and T. vaporariorum) is characterized. RNA blot analysis using defense-regulated gene probes have shown that whitefly feeding induces the accumulation of pathogenesis-related protein (PR) gene transcripts. The basic PR gene RNAs, such as basic glucanase and basic chitinase, accumulate to high levels nine days after whitefly feeding. In contrast, low levels of acidic PR gene RNAs were detected. These data indicate that the salicylic acid-independent pathway is strongly induced, while whitefly feeding induces the SA-dependent pathway weakly. The levels of SA were monitored for 9 days following whitefly feeding; SA concentrations did not increase significantly following whitefly feeding. Wound-response genes regulated by the octadecanoid pathway, such as LapA and pin2 genes, were not expressed following whitefly feeding at the level of RNA blot analysis orbe analysis of transgenic LapA:GUS tomato plants. These studies indicate that tomato plants perceive phloem-feeding silverleaf and greenhouse whiteflies in a manner distinct from that of chewing insects.