Rami Kfir

Increase in cereal stem borer populations through partial elimination of natural enemies

An insecticide exclusion method was used to evaluate the effect of parasitoids on level of infestation by the stem borers, Busseola fusca (Fuller) and Chilo partellus (Swinhoe), in grain sorghum. In field trials conducted at Brits and at Delmas, South Africa, a selective organophosphate insecticide, dimethoate, was applied twice weekly at each site to three subplots whereas three other identical subplots served as controls. Twelve plants were randomly selected from each subplot at weekly intervals and removed from the field. In the laboratory all plants were dissected to record borer infestation. In order to determine parasitism levels egg batches were kept in Petri dishes and all borer larvae and pupae were kept individually in vials until either parasitoids or moths emerged. At Brits ca. 97% of borers were C. partellus and 3% B. fusca, whereas at Delmas 37.5% were C. partellus and 62.5% B. fusca. The most abundant parasitoids of B. fusca were Cotesia sesamiae (Cameron) and Bracon sesamiae Cameron. The dominant parasitoids of C. partellus at both sites were C. sesamiae, Stenobracon spec., Dentichasmias busseolae Henrich and Pediobius furvus (Gahan). No egg parasitoids were found. At both sites, infestation levels in the sprayed plots were significantly higher than in the untreated plots. On the other hand, parasitism levels of borers in the unsprayed plots were significantly higher than in the treated plots. It was concluded that the higher infestation level of sorghum by stem borers in the sprayed plots was because of partial elimination of parasitoids and possibly other natural enemies by the pesticide.

The area of discovery and searching strategy of a primary parasite and two hyperparasites

1 Two hyperparasites, Cheiloneurus paralia (Walker) and Marietta exitiosa Compere, of Microterys flavus (Howard), a primary parasite of the brown soft scale Coccus hesperidum L. have been studied. 2 The area of discovery of the hyperparasites for secondary hosts (scale insects) is greater than that of the primary parasite, indicating higher searching efficiency of the secondary parasites. 3 Cheiloneurus has a higher searching efficiency measured as its area of discovery for discovering both the secondary (scale insect) and the primary (Microterys) hosts, as compared with Marietta. 4 The searching efficiency of Cheiloneurus increased in the presence of its own males. 5 In each species there is a non‐linear relationship between the searching efficiency and female density. 6 Cheiloneurus and Marietta present two differing searching strategies. Cheiloneurus tends to exploit as many primary hosts as possible in each secondary host encountered, whereas Marietta, tends to disperse its progeny more regularly by avoiding, to a certain extent, the exploitation of more than one host in each scale insect encountered.

Duration of diapause in the stem borers, Busseola fusca and Chilo partellus

The duration of diapause in the stem borers Busseola fusca (Fuller) and Chilo partellus (Swinhoe) was studied in South Africa by collecting diapausing larvae from the field throughout winter (April–August). B. fusca larvae emerged as moth around the middle of October regardless of the date of collection and the length of time they were kept in the laboratory under constant 21 °C. C. partellus larvae collected in April‐June emerged in November, those collected in July emerged in October, and those collected in August emerged in September. Regardless of the collection date C. partellus started to emerge from diapause earlier and moth emergence lasted up to twice as long as in B. fusca. Under laboratory conditions at 60% RH both borer larvae lost about 50% of their body mass during diapause. When provided with water B. fusca larvae lost about 30% of their body mass and adults emerged 20 days earlier than when kept dry. C. partellus, on the other hand, lost only 13% of their body weight and emerged 34 days earlier. The differences between the two species are discussed in light of different types of diapause; i.e., obligatory diapause in B. fusca and facultative diapause in C. partellus.

Hibernation by the lepidopteran stalk borers, Busseola fusca and Chilo partellus on grain sorghum

The lepidopteran stalk borers, the indigenous Busseola fusca (Fuller) (Noctuidae), and the exotic Chilo partellus (Swinhoe) (Pyralidae), were studied in grain sorghum fields, Sorghum bicolor (L.) Moench (cultivar SSK‐52), in the dry season of 1986 (April‐October) at Delmas and Brits, Transvaal, South Africa. More than 90% of plants at both sites were infested, but as winter progressed, the proportion of plants infested and the level of infestation dropped gradually. The borer larvae hibernated inside the dry stalks, but the location differed: 65% of B. fusca were in the lower third and 30% in the middle third, whereas for C. partellus it was 45 and 50% respectively. Both borers overwintered as larvae in either of the last three instars, but the proportions of larval instars were different. About 82% of B. fusca hibernated as 6th and 16% as 5th larval instars whereas with C. partellus it was 40 and 45% respectively. C. partellus started to emerge from diapause in the second part of August and it lasted until the first week of November, a period of 12 weeks, but with B. fusca on the other hand pupation lasted only 3 weeks during October‐November. Parasitism was low on both species, but higher on B. fusca. Three parasites, Chelonus curvimaculatus Cameron, Chelonus sp. and Pristomerus sp. hibernated in the borers, one parasite Iphiaulax sp., hibernated in its cocoon inside the stalks, and two parasites Apanteles sesamiae Cameron and Bracon sp. were active in winter. Two ants, Pheidole megacephala Febricius, and Dorylus helvolus (L.), preyed on the borer larvae in winter.

REPRODUCTION CHARACTERISTICS OF TRICHOGRAMMA BRASILIENSIS AND T. LUTEA, PARASITISING EGGS OF HELIOTHIS ARMIGER

The search rate, the number of ♀ progeny per ♀ parent and the sex ratio of the progeny of Trichogrammatoidea lutea Girault were severely affected by increasing the parasite density from 1/150 hosts to 8/150. Trichogramma brasiliensis Ashmead was very little affected. Females of T. lutea were more susceptible than ♀ ♀ of T. brasiliensis to interference from other searching ♀ ♀ or by encountering hosts that had been previously parasitised. T. lutea ♀ ♀ tended to leave the area they searched following interference. T. lutea was found to be arrhenotokous and T. brasiliensis deuterotokous. T. lutea clustered its progeny in the hosts more than T. brasiliensis. The average number of T. lutea progeny that developed in a host was 2.02 as compared with 1.28 for T. brasiliensis. The clustering of ♀ progeny in the parasitised hosts by the 2 species did not differ. The average number of T. lutea ♀ ♀ per host was 1.32. When more than one progeny of a mated T. lutea ♀ developed in a host, usually only one ♀ emerged, the rest being ♀ ♀. T. lutea produced more ♀ progeny per ♀ parent than T. brasiliensis and its developmental time was shorter.

Diadegma mollipla parasitizing Plutella xylostella: host instar preference and suitability

Oviposition decisions (i.e., host selection and sex allocation) of female parasitoids are expected to correspond with host quality, as their offspring fitness is dependent on the amount and quality of resources provided by a single host. The host size model assumes that host quality is a linear function of host size, with larger hosts believed to contain a greater quantity of resources, and thus be more profitable than smaller hosts. We tested this assertion in the laboratory on a solitary larval–pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) developing on three instars (second–fourth) of one of its hosts, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). In a no‐choice test, parasitism levels and sex ratio (i.e., proportion of female progeny) were significantly high in hosts attacked in the second instar followed by third then fourth instars. However, the few parasitoids that completed a generation from the fourth instars did so significantly faster than conspecifics that started development in the other two instars. In direct observations, however, the parasitoids (i) randomly attacked the various host instars, (ii) spent a similar period examining the various host instars with their ovipositors, (iii) subdued all three host instars with about the same effort, and (iv) no statistical differences were observed in the attack rates on the three host instars. In a choice test, the females parasitized significantly more third instars followed by second then fourth instars. However, total parasitism in this experiment was 43% lower compared to parasitism of only second instars in the no‐choice test. No significant differences were detected in progeny sex ratios. In both choice and no‐choice tests, significantly more fourth instars died during the course of the experiments than second instars, while third instars were intermediate. The higher parasitism of third than second instars in the choice test indicates that the females perceived larger hosts as higher quality than smaller hosts, despite their lower suitability for larval development.

Effect of mannose‐binding lectin from peanut and pea on the stem borer Chilo partellus

A mannose‐binding lectin found in vegetative tissues of peanut, Arachis hypogaea, was compared with mannose‐binding lectin from pea, Pisum sativum, for toxic effects on larvae of the stem borer Chilo partellus (Swinhoe). After 10 days, the mortality of larvae fed on artificial diet containing 0.5% (m/m) peanut lectin was 46.2%. The mortality of larvae fed on 1.0% peanut lectin was similar (48.1%) but insects were significantly smaller than those of the 0.5% treatment. Larvae of both lectin treatments stopped feeding within three days. Larval size and mortality was not significantly reduced by 0.1% peanut lectin and 1% heat‐treated lectin did not show toxic effects. The mannose‐binding lectin from pea was not toxic to C. partellus at concentrations up to 1%. Peanut lectin bound to the apical membranes of columnar epithelial cells in the mid‐gut of C. partellus. This suggests that peanut lectin has an antinutritive action and that it may protect vegetative tissues of peanut against insect pests.

LABORATORY STUDIES OF COMPETITION AMONG THREE SPECIES OF HYMENOPTEROUS HYPERPARASITES

Competition tests were carried out in the laboratory among the hyperparasitic wasps Cheiloneurus paralia, Marietta javensis and Pachyneuron concolor, developing upon the primary parasite Microterys flavus in the brown soft scale. In single‐species experiments Marietta was the most efficient; it eliminated the population of the host whereas Cheiloneurus and Pachyneuron were not able to do so. In 2‐species experiments Marietta completely replaced Cheiloneurus within 6 weeks and Pachyneuron within 8. Cheiloneurus replaced Pachyneuron within 20 weeks. In 3‐species experiments Cheiloneurus was eliminated within 4 weeks and Pachyneuron within 9. During the course of all experiments the mortality rate of immature parasites and hyperparasites increased, and progressively more cells failed to give rise to either an adult Microterys or a hyperparasite. This was due to host feeding and superparasitism. Pachyneuron was able to use Cheiloneurus as a host but not vice versa. The superiority of Marietta may be partly due to the fact that it is capable of utilizing its competitors as hosts, whereas they cannot develop upon it.

An improved method for mass rearing Paratheresia claripalpis for biological control of lepidopteran stalk borers

An improved method for mass rearing Paratheresia claripalpis Van der Wulp (Diptera: Tachinidae), for biological control of lepidopteran stalk borers is described and compared to the commonly used method. The improved method was found to be superior by being less labour intensive and by producing 2.25 times more parasites from the same number of hosts.