Joanne K. Itami

A strong relationship between oviposition preference and larval performance in a shoot-galling sawfly

To investigate the relationship between oviposition preference and offspring performance we tested three hypotheses about the oviposition behavior of Euura lasiolepis: (1) the preference hierarchy hypothesis that Euura had an oviposition preference based on shoot lengths; (2) the larval survival hypothesis that the preference hierarchy corresponded to the suitability of shoots for larval survival; and (3) the flexibility hypothesis that the preference hierarchy changed as resource availability changed. The oviposition preference hierarchy of Euura lasiolepis, a shoot—galling sawfly, for plant shoots of different lengths strongly corresponded with the rank of shoot lengths for larval survival. The oviposition preference hierarchy was related to shoot growth; the more rapidly a shoot was growing the higher the probability that an oviposition site on a shoot would be attacked. Larval survival was highest on long shoots on young ramets. The probability of shoot abscission was negatively related to shoot length, and Euura in galls on abscised shoots invariably died. Intraspecific interactions and willow clone identity, independent of shoot length and ramet age, had relatively small influences on larval survival. The oviposition hierarchy was flexible; if long shoots were not available shorter shoots were accepted for oviposition. Shoots were always attacked in order of length, so that a strong relationship between preference and performance was always maintained.

Resource Regulation by a Stem‐Galling Sawfly on the Arroyo Willow

We studied the impact of the stem—galling tenthredinid sawfly, Euura lasiolepis, on the growth and branch age structure of the arroyo willow, Salix lasiolepis. Normally, as willows age they become less susceptible to galling, but heavy Euura galling maintains willows at a young, relatively susceptible juvenile stage. The Euura form more galls and long shoots. Long shoots are found on clones with young branches. Heavy galling stunts or kills growth distal to the gall, stimulating sprouting by indefinitely dormant buds located near branch bases. The resulting young branches keep the clone susceptible to further galling. In contrast, increasing branch age of lightly galled clones confers resistance to galling. We term this model of gall—induced resource maintenance the resource regulation hypothesis. Resource regulation is the maintenance or increase of high—quality resources by an herbivore species that impacts immediately subsequent generations of the same herbivore species on the same plant. See full-text...

The Window of Vulnerability of a Shoot-Galling Sawfly to Attack by a Parasitoid

We studied the susceptibility of a shoot-galling sawfly, Euura lasiolepis, to attack by the parasitoid Lathrostizus euurae. Each Euura had a window of vulnerability: a limited time period when the parasitoid attacked it. Lathrostizus attacked a limited range of gall sizes and ages during each sample period. Gall age was related to its position on a shoot, and Lathrostizus shifted the gall position it attacked as it tracked vulnerable-aged galls. The parasitoid emergence period matched the window of vulnerability of its host. Lathrostizus attacked and successfully parasitized a low proportion of hosts encountered. A low interval-parasitism rate uniformly distributed among Eulura galls formed at different times is consistent with the pattern expected if the window of vulnerability influenced attack. The window of vulnerability was formed by the interaction of gall and sawfly devel- opment rates. Two hypotheses that defined the window were supported: (1) the host de- velopment hypothesis, that Lathrostizus is a larval parasite and does not attack galls containing eggs and (2) the gall toughness hypothesis, that large, old galls that are tough are not attacked by Lathrostizus. Our data did not support (3) the ovipositor limitation hypothesis, that some galls were too large for the larva inside to be reached by the Lath- rostizus ovipositor. We observed that if these galls were soft, they were attacked. Lathrosti- zus parasitism rates on Euura varied widely among willow clones. Willows differed in the rate of gall toughening, which produced different-sized windows and thus contributed to variation in parasitism rates among Euura populations on different willow clones.

Impact of Shoot Galler Attack on Sexual Reproduction in the Arroyo Willow

The assumption that herbivores cause reduction of plant fitness is central to plant herbivore theory, but empirical support from field studies is scant. We tested the hypothesis that the shoot-galling sawfly Euura lasiolepis significantly influences the fitness of its obligate host, the arroyo willow (Salix lasiolepis). Surveys of reproductive attributes on field willow clones at different elevations indicated that the presence of shoot galls led to a significant reduction in reproductive buds, and thereby inflorescences, on individual shoots on both male and female plants. Shoots with galls suffered a loss of 43% of their reproductive buds relative to shoots without galls. Inflorescence size, measured as flowers per inflorescence, seed mass, germination success, and pollen germination success were not significantly affected by gall presence. Two sawfly exclosure-enclosure experiments on plants that had been established from cuttings from clones showed reduced reproductive bud production on plants with galls at both the whole-plant and individual-shoot levels; whole-plant losses of 34-56% and per- shoot losses of 17-55% of reproductive buds on different clones were coupled with densities that varied from 0.56 to 2.70 galls per shoot. Most plants suffered relatively small repro- ductive losses because of low sawfly densities. However, 20% of all plants suffered losses of 10%, while 7.5% experienced losses of 20% or more of their reproductive potential. A small willow clone attacked over the range of 0.5-2.0 galls per shoot would lose 381 000- 1 500 000 seeds of a potential 3 760 000 seeds in the year of attack. Genetic variation among plants in susceptibility to sawfly attack and subsequent herbivore-related repro- ductive loss was suggested by a common garden study; some clones had 12 times as many galls as others.

Dispersal of a galling sawfly: Implications for studies of insect population dynamics

1. Utilizing population censuses and field experiments we investigated the dispersal behaviour and the role that dispersal plays in the population dynamics of the shoot-galling sawfly Euura lasiolepis. 2. Active, non-directional dispersal occurred out to 8 m, was strongly female biased (88% female) and relatively common (92% of dispersing sawflies). Passive, wind-directed dispersal occurred out to 128 m, was not female biased (56% female, the same sex ratio as emerging sawflies) and relatively rare (8 % of dispersing sawflies). 3. Dispersal was density-independent but increasing density decreased the number of galls per individual initiated by non-dispersers. 4. Life-table analysis was used, including field survivorship and sex ratio data, to model the role dispersal can play in the population dynamics of this sawfly

Plant wound compounds from oviposition scars used in host discrimination by a stem-galling sawfly

The stem-galling sawfly Euura lasiolepisuses one or more plant wound compounds resulting from oviposition scars as cues in host discrimination (avoiding sites occupied by conspecifics). Four experiments were conducted to test hypotheses about how Euurapartitions resources. Experiment 1 demonstrated that Euuraavoids ovipositing on nodes with scars from previous ovipositions. Experiment 2 showed no evidence that the sawfly uses oviposition-deterring pheromones and indicated there is a time lag following oviposition before the oviposition scar becomes a deterrent. Experiment 3 showed that sawflies avoid artificially formed scars, demonstrating that a plant cue alone can lead to host discrimination. Experiment 4 showed that visual or tactile cues are not necessary for host discrimination and indicated that a plant wound compound functions as an oviposition deterrent. Both experimental results and field surveys showed that Euuraoviposition scars were more uniformly distributed than expected if sawflies were ignoring previous ovipositions.