B.A. Dover

Influence of temperature on the functional response of Anisopteromalus calandrae (Hymenoptera: Pteromalidae), a parasitoid of Rhyzopertha dominica (Coleoptera: Bostrichidae)

The functional response of Anisopteromalus calandrae (Howard) parasitizing 4th-instar Rhyzopertha dominica (F.) on wheat was estimated over a range of temperatures and host densities. A functional response equation was used in which a quadratic component that included temperature was substituted for handling time. The instantaneous search rate increased with increasing temperatures. The maximum rate of parasitization was 13 larvae/24 h at 30°C and 35°C. Handling time was lowest at 30°C and highest at 20°C. The ability of A. calandrae to find and parasitize R. dominica over a broad range of temperatures makes it a good candidate for natural control of stored grain pests.

Biochemical mechanisms of organophosphate resistance in Rhyzopertha dominica (Coleoptera: Bostrichidae) populations from the United States and Brazil

Four possible biochemical mechanisms of organophosphate resistance were examined in populations of Rhyzopertha dominica (Fabricius) collected from eight locations in the states of Minas Gerais and Sao Paulo in Brazil and from seven locations in northeast Kansas, United States. Results from insecticide bioassays with synergists (triphenyl phosphate, diethylmaleate, and piperonyl butoxide) suggested that metabolic detoxification mechanisms might be involved in organophosphate resistance in some of the populations of R. dominica . However, overall low synergism, non-synergism or even antagonism presented by these synergists in all 15 resistant populations suggested that carboxylesterases, cytochrome P450 monooxygenases and glutathione S -transferases were not likely to play a major role conferring organophosphate resistance in R. dominica . This hypothesis was strongly supported by our biochemical studies showing no significant difference in these enzyme activities, as well as cytochrome b 5 , P420, and P450 levels between the susceptible and resistant populations. In contrast, all resistant populations of R. dominica showed higher acetylcholinesterase activity than the susceptible population both in the presence and absence of malaoxon. Furthermore, acetylcholinesterase from the resistant populations was less sensitive to inhibition by malaoxon than that from the susceptible. These results suggested that quantitative and qualitative changes in acetylcholinesterase might contribute to organophosphate resistance in these populations of R. dominica .

Suppression of juvenile hormone esterase in Heliothis virescens by Microplitis demolitor calyx fluid

Abstract Microplitis demolitor is a polydnavirus-carrying braconid wasp that parasitizes the larval stage of Heliothis virescens . Previous studies indicated that parasitized hosts exhibit developmental arrest whereas larvae injected with either M. demolitor calyx fluid or polydnavirus (MdPDV) form larva-pupal intermediates. Here we investigated the role of teratocytes in altering host development and the effects of calyx fluid on juvenile hormone esterase activity in H. virescens . Injection of superphysiological doses of teratocytes or coinjection of a physiological dose of teratocytes and calyx fluid affected fourth-stadium host development similarly to parasitism, but parallel injections had lesser effects in fifth-stadium hosts. Juvenile hormone (JH) titers in parasitized, fifth-stadium H. virescens were 12-fold higher by 60 h post-parasitism than those in controls. Unparasitized H. virescens exhibited two peaks of JH metabolic activity in the last instar, one at 60 h after larval ecdysis and the other just before pupation (132 h post-ecdysis), whereas JH metabolism was completely suppressed in parasitized and calyx fluid-injected hosts. The effect of calyx fluid on JH metabolism was independent of the presence of teratocytes and, based on general esterase assays, was specific for JH esterase. Experiments using hemolymph from parasitized larvae suggested that the reduction in host JH esterase activity was not due to a factor in the hemolymph.

STADIUM-SPECIFIC DEGENERATION OF HOST PROTHORACIC GLANDS BY CAMPOLETIS SONORENSIS CALYX FLUID AND ITS ASSOCIATION WITH HOST ECDYSTEROID TITERS

Abstract Campoletis sonorensis is a polydnavirus-carrying wasp that parasitizes Heliothis virescens larvae. Previous studies indicated that C. sonorensis polydnavirus (CsPDV) induces degeneration of host prothoracic glands when injected into last-stadium hosts. Degeneration also occurs when last instars are naturally parasitized, but it is not observed in hosts parasitized as penultimate instars. This study investigated the effect of CsPDV on the development of penultimate-stadium hosts and the temporal sensitivity of H. virescens prothoracic glands to virally-induced degeneration. We found that calyx fluid (containing CsPDV) caused degeneration of host prothoracic glands when injected into newly-ecdysed, penultimate-stadium hosts, but degeneration was delayed until injected larvae reached the critical head capsule size for the last stadium, even when injected larvae exhibited supernumerary molts. Host prothoracic glands became sensitive to virally-induced gland degeneration just prior to apolysis to the last larval larval molt , simultaneous with an ecdysteroid peak. Prothoracic gland degeneration in response to calyx fluid was also noted in two other hosts of C. sonorensis , Helicoverpa zea and Spodoptera exiguae .

Allozyme variation among Brazilian and U.S. populations of Rhyzopertha dominica resistant to insecticides

The lesser grain borer (Rhyzopertha dominica (F.)) is an important pest of stored grain in many parts of the world (Paleartic, Ethiopian, Oriental, Australian, Neotropical, and Neartic regions) with the ability to fly long distances. These insects have been shown to be resistant to organophosphorus insecticides in several studies. Polyacrylamide gel electrophoresis was used to assess the genetic variability within and among eight Brazilian and seven United States populations of R. dominica and to determine how insecticide resistance may be spreading within both countries. Significant variation in allele frequency among populations was observed at all six polymorphic enzyme loci that were examined. The Brazilian and U.S. populations were genetically differentiated from one another; populations within the U.S. and those within Brazil were also differentiated from one another. The mean genetic similarity among the seven U.S. populations collected in a small region in northeast Kansas was smaller than that among eight Brazilian populations collected in a relatively large geographical area. These results are consistent with the resistance ratios to chlorpyriphos‐methyl in R. dominica populations from Brazil and the U.S. and the information available concerning patterns of flight activity in this insect.

Flight response, body weight, and lipid content of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) as influenced by strain, season and phenotype

Effects of geographical origin of strain, season of field collection, genetic background, and number of generations of laboratory rearing on flight initiation, body weight, and lipid content of lesser grain borer, Rhyzopertha dominica, were studied. Significant differences in flight initiation, body weight, and total lipid content occurred among the four field strains collected from Kansas, Oklahoma, Texas, and Mexico. Flight activity tended to be higher for strains with higher average body weight. Oleic, palmitic, and linoleic acids accounted for about 95% of all fatty acids present in the strains tested. However, the percentage of lipids per fresh body weight was not significantly different among these four field strains. Beetles collected in summer tended to fly more and have a higher percentage of lipid content than beetles collected in spring and autumn. Results from crosses between strains with high and low flight responses suggested that the female contributes a higher proportion of additive genetic variance for flight initiation than the male. Rearing for 17 generations in laboratory conditions had no discernible effect on flight initiation of a strain of R. dominica collected from the field. Thus, flight behavior can persist for many generations of laboratory rearing.

Effect of crowding, food deprivation, and diet on flight initiation and lipid reserves of the lesser grain borer, Rhyzopertha dominica

Effects of crowding, food deprivation, and type of cereal diet upon flight initiation, development, body weight, lipid content, and fatty acid composition of the lesser grain borer, Rhyzopertha dominica (F.), were studied in two field strains and one laboratory strain. Beetles of all strains reared under crowded conditions had significantly higher flight initiation than beetles reared on isolated kernels (uncrowded). Regardless of degree of crowding, flight initiation increased with the period of food deprivation up to a maximum at 24 h, after which flight initiation declined. Body weight and lipid content decreased as the food deprivation period increased, whereas fatty acid composition was not significantly affected by food deprivation. Beetles from a field strain collected in 1995 had higher flight initiation and increased lipid content compared with beetles from the laboratory strain. However, beetles from the laboratory strain were larger, developed faster, and were more fecund than beetles from this field strain. The cereal diet on which beetles were reared also had a significant effect on flight initiation, lipid content, and fatty acid composition. Beetles reared on whole rice and wheat produced adults with higher flight initiation, higher lipid content, and higher oleic acid concentration than beetles reared on whole corn and sorghum.

CHARACTERIZATION OF ACETYLCHOLINESTERASE PURIFIED FROM THE LESSER GRAIN BORER, RHYZOPERTHA DOMINICA (COLEOPTERA: BOSTRICHIDAE)

Acetylcholinesterase (AChE, EC 3.1.1.7) purified from the lesser grain borer (Rhyzopertha dominica) was significantly inhibited by higher concentrations of the substrates acetylthiocholine (ATC), acetyl-(beta-methyl) thiocholine (A beta MTC) and propionylthiocholine (PTC). 2. The efficiency of AChE for hydrolyzing different substrates was ATC > A beta MTC > PTC > S-butyrylthiocholine. The enzyme activity was completely inhibited by 10(-5) M eserine or BW284C51, but was only partially inhibited by ethopropazine at the same concentration. These results confirmed that the purified enzyme was an typical insect AChE. 3. Non-denaturing and SDS polyacrylamide gel electrophoresis (PAGE) showed only one major molecular form in the purified AChE with a molecular weight of about 107,000 prior to reduction and about 56,000 after reduction, suggesting the homodimer of AChE linked with disulfide bonds.

Effect of host ligation and starvation on the development and emergence of the parasitoid Campoletis sonorensis

The development of some parasitoids is coordinated by their hosts, via endocrine head or thoracic factors. Ligation of these hosts disrupts parasitoid development. To determine if host ligation affects the solitary, larval endoparasitoid Campoletis sonorensis (Cameron) (Ichneumonidae: Hymenoptera), Heliothis virescens (F.) (Noctuidae: Lepidoptera) larvae were parasitized by C. sonorensis and ligated or starved at various times after parasitization. Ligation and starvation increased parasitoid developmental time and the time of ligation or starvation affected parasitoid emergence. However, ligation and starvation had similar effects on parasitoid development and emergence. Based on our data, C. sonorensis development does not appear to be closely coordinated by hormonal factors produced by the host head and/or thorax.