Structural refuges in two stem-boring weevils on Rumex crispus

Abstract.

• 1 Differences in feeding positions, parasitoid complexes and parasitization rates of the two dominant insect species in stems of Rumex crispus were investigated. Apion miniatum (inhabiting the lower 10% of the stem) and Apion violaceum (occurring all along the stem) exploited distinctly different feeding positions, partly due to differences in time of oviposition.
• 2 A.miniatum suffered less parasitization thanA.violaceum, measured both by number of parasitoid species and parasitization rates. Its feeding position constitutes a refuge.
• 3 Parasitization of A.violaceum was least in individuals feeding in basal, thick parts of the stem. There is a trend that the developed refuge increases with increasing stem diameter.
• 4 The refuge of A.violaceum in basal positions only affected idiobiont ectoparasitoids, because koinobiont endoparasitoids attack the weevils before they enter the stem. Moreover, the koinobiont did benefit from the refuge itself by avoiding hyperparasitism.
• 5 Densities of A.violaceum increased with plant size, whereas percentage parasitism did not differ. Its three most abundant parasitoid species showed small-scale density-independent patterns of attack over all plants. Therefore a constant proportion of weevil larvae mine within the refuge independent of its potential size.
• 6 Synchronization between weevils and plants, as well as between parasitoids and hosts, appears to be the dominant factor for the spatial community structure of the stem borers of Rumex crispus and their enemies.

     

The impact of a stem-boring insect on the tissues, physiology and reproduction of Russian thistle

A stem-boring moth, Coleophora parthenica was introduced into California from Pakistan, in 1973, in a biological control attempt against the weed, Russian thistle (Salsola australis). A population of Russian thistle plants growing near Coalinga, California, which were infested and damaged by varying numbers of C. parthenica larvae, was studied to better understand the moth's impact.The xylem sap pressures were measured during the period of seed set. No significant correlations were obtained between the measured levels of water stress and the amounts of C. parthenica infestations and damage in these plants, except late in the season, but these are not considered to be important. Translocation of carbon-14 was studied during seed set. The levels of translocation and C. parthenica infestations and damage in the plants were not correlated. Measured levels of various reproductive indices did not correlate significantly with the levels of larval infestation and damage. Histological and x-ray examinations of the damaged stems showed that once C. parthenica larvae enter the stems of Russian thistle they feed only in the pith and do not damage the critical xylem and phloem. This explains why the plant's translocation, water balance, and reproduction were unaffected by C. parthenica. Coleophora parthenica appears to have very limited value as a biological control agent of Russian thistle, except perhaps as part of a complex of natural enemies attacking the plant.

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Résumé Coleophera parthenica Meyrick (Lep., Coleophoridae), mineuse de tiges, a été introduite en 1973 du Pakistan en Californie, pour lutter contre Salsola australis. Pour mieux appréhender l'effet de C. parthenica, une population de S. australis attaquée par la mineuse a été suivie près de Coalinga (Californie).La pression de sève du xylème a été mesurée pendant la période de production des graines. Aucune corrélation n'est apparue entre les niveaux de stress hydrique et les taux d'infestation et les dégâts de C. parthenica, sauf en fin de saison; mais ils n'ont pas été alors estimés importants.Le transfert de C¹⁴ a été étudié pendant la période de production de graines; aucune corrélation n'est apparue avec les infestations et les dégâts de C. parthenica. Les différents indices de la reproduction n'étaient pas significativement liés aux niveaux d'infestation et de dégâts larvaires.Les examens histologiques et par rayons X des tiges attaquées ont montré qu'après la pénétration des chenilles de C. parthenica dans les tiges de S. australis, celles-ci ne s'alimentent que dans la moelle sans atteindre gravement le xylème et le phloème.Ceci explique pourquoi, les transferts dans la plante, la circulation de l'eau et la reproduction, ne sont pas modifiés par C. parthenica; cet insecte semble n'offrir que très peu d'intérêt pour la lutte biologique contre S. australis, sauf peut-être comme élément du complexe des ennemis naturels de cette plante.

     

Selection of plant parts by Depressaria multifidae (Lep., Oecophoridae) on its seasonally-restricted hostplant, Lomatium grayi (Umbelliferae)

ABSTRACT.

• 1 Depressaria multifidae Clarke feeds on a broader variety of Umbelliferae plant parts than other Depressaria species.
• 2 Early instar larvae feed in the sheaths surrounding floral buds and leaves. Later instar larvae feed in the sheaths and floral stems and on flowers and leaves.
• 3 Floral stems bored by larvae had significantly larger basal stem diameters than floral stems that were not bored. Smaller stems usually have umbels with only male flowers, and wither after flowering, too soon for larvae to complete development. In contrast, larger stems often have umbels with some hermaphroditic flowers, which remain green and erect long enough for larvae to complete development. Hence, selection may favour larvae that bore only in relatively large stems.
• 4 In the laboratory, larvae fed sheaths with enclosed floral buds, flowers, or leaves all pupated at the same weight, but larvae fed floral stems pupated at a significantly lower weight. Larval and pupal development time was the same on all plant parts.
• 5 In the field, larvae restricted to a single umbel throughout development pupated at the same weight as those restricted to a single leaf.
• 6 Unlike in other Depressaria species, nitrogen levels only partly correspond to the pattern of use of plant parts in D.multifidae. Nitrogen values varied as follows: floral buds > immature leaves ≥ flowers > floral stalks > sheaths excluding floral buds or leaves.
• 7 The broad variety of plant parts used by D.multifidae may result partly from the problem of feeding on a small, seasonally restricted hostplant; the greater use of sheaths and floral stems than in other Depressaria species may result from selection for safety from parasites or predators.
• 8 The results for D.multifidae indicate that the way in which an insect feeds on a plant species can vary broadly even at a single site.

     

Avoidance of super-parasitism of stem-boring larvae byChasmodon apterus [Hym.: Braconidae]

The major parasitoid attackingOscinella spp. larvaeChasmodon apterus, demonstrates an ability to avoid superparasitism. However, avoidance is imperfect and in the present work 13% of parasitized larvae were superparasitized. When superparasitism occurred the parasitoid larvae competed, apparently by physiological methods, although physical fighting was observed on one occasion. Multiple parasitism was rare.