Rick J. Hodges

Identification of major component of male-produced aggregation pheromone of larger grain borer, Prostephanus truncatus (horn) (coleoptera : bostrichidae)

Male beetles of the larger grain borer,Prostephanus truncatus Horn (Coleoptera: Bostrichidae) were shown to produce material causing an electroantennographic (EAG) response from both male and female beetles. Volatiles from mixed sex cultures were collected on Porapak Q and fractionated by liquid chromatography (LC) with gradient elution. The total volatiles and the fractions were analyzed by gas chromatography (GC) linked to EAG, and two compounds eluting in the same LC fraction elicited EAG responses from male and female beetles. These were assumed to be components of the male-produced aggregation pheromone. Amounts of these compounds obtained were very low and less than 10−3 times the amounts of the aggregation pheromone components produced by the related species,Rhyzopertha dominica, under similar conditions. The most abundant of these EAG-active compounds was identified as 1-methyiethyl (2E)-2-methyl-2-pentenoate by comparison of its GC retention times and mass spectrum with those of synthetic analogs. The synthetic compound elicited electrophysiological and behavioral responses in both male and female beetles significantly greater than those to structural analogs. This compound is attractive toP. truncatus beetles in the field, and has been given the trivial name of “trunc-call 1”.

Field monitoring Sitophilus zeamais and Sitophilus oryzae (Coleoptera: Curculionidae) using refuge and flight traps baited with synthetic pheromone and cracked wheat

Flight traps with open vertical baffles and refuge traps in the form of rectangular cardboard boxes were used to catch Sitophilus zeamais and S. oryzae in Kenya. Traps baited with the aggregation pheromone (Sitophilure), and for the first time traps baited with pheromone and food bait (cracked wheat), were field tested. Both traps types were effective in capturing Sitophilus spp and gave higher weevil catch when baited with pheromone and cracked wheat combined than with pheromone or cracked wheat alone. The combination of pheromone and cracked wheat had an additive effect on trap catch and not a synergistic effect as reported in earlier laboratory trials. Different pheromone loadings of 0.5, 1 and 2 mg were tested. The catch of S. zeamais in flight traps rose significantly with increasing dosage. For refuge traps, the same effect was observed although the difference between 1 mg and 2 mg was not statistically significant. The trend was for increasing S. oryzae catch with dosage, although total catches were very much lower than those for S. zeamais, and differences did not prove statistically significant. Traps placed on the floor near infested maize cribs captured significantly more Sitophilus spp than those actually placed in the cribs. Adult Sitophilus captured were sexed and in both flight and refuge traps the sex ratio was biased in favour of females even though the sex ratio of weevils from infested maize cobs, which were the source of the captured weevils, was approximately even.

Quinone contamination of dehusked rice by Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae).

Abstract The accumulation of quinone secretions, from the abdominal and thoracic defence glands of adult Tribolium castaneum , was investigated in dehusked rice of four different milling degrees. Three quinones were detected in measurable quantities, 2-methyl-1,4-benzoquinone, 2-ethyl-1,4-benzoquinone and 2-ethylhydroquinone, but these were present at very low concentrations. Even after substantial exposure to adults, equivalent to an adult-week product of 1345 on 80 g of rice, the combined total contamination with these three compounds was only about 1.0 ppm. The accumulation rate of quinones in the samples was estimated to range from 0.0470 to 0.0631μg/adult/week and seemed unaffected by the milling degree of the rice. However, the distribution of quinone contamination in the samples between the rice grain and insect frass was influenced by milling degree. In unpolished rice, about 80% of quinone contamination was in the rice itself rather than in the frass, while in rice of 95% milling degree the contamination was more or less evenly distributed. The low quinone contamination rates observed in rice suggest that the accumulation of these chemicals is unlikely to pose a serious health threat when rice is exposed to ‘normal’ rates of infestation by T. castaneum for typical storage periods, e.g. 6–12 months. Even the highest observed degree of contamination, 1.0 ppm, would in theory take about 32 y to accumulate if adult beetles remained at a relatively high average density of 10 adult T. castaneum /kg. These conclusions require confirmation by direct observation of rice after long-term warehouse storage in the tropics.

Investigation of the effect of female larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae), and their residues on the production of aggregation pheromone by males

The rate of production of aggregation pheromone by male larger grain borer beetles, Prostephanus truncatus (Horn), is greatly reduced in the presence of live, conspecific female beetles. A similar effect is observed in the absence of live female beetles when males are placed on maize that was previously infested with males and females, suggesting that it is due to some chemical residue left by the females. This was shown to persist for at least 15 days, to be involatile and not perceived by receptors on the antennae of the males. Attempts to extract the chemical(s) responsible and transfer it to a new substrate gave variable results. Extraction of previously-infested maize with hexane, ether, dichloromethane, butanone or water under different conditions probably did not remove the suppression effect, but Soxhlet extraction with methanol did. However, when the resulting extract was recoated onto various substrates, no reduction in pheromone production by males could be demonstrated, suggesting that the methanol is extracting and/or destroying the chemical(s) responsible.

Phenotypic Plasticity of Rhyzopertha dominica Pheromone Signaling: The Effects of Different Hosts and Presence of Conspecific Females on Male Produced Aggregation Pheromone

Male Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) release an aggregation pheromone consisting of a blend of two components, dominicalure 1 (D1) and Dominicalure 2 (D2). Pheromone from single insects, in different contexts, was collected and measured to determine if this signal is phenotypically plastic. Release rates were lowered when males were moved from maize grains to groundnut kernels or when moved from solitary occupation of maize grain to grain occupied by seven females. The pheromone release was increased again once these moves were reversed. The release of D1 was more affected than D2: thus, on groundnuts or in the presence of females, less pheromone was released and the proportion of D1 in the blend was lowered. Possible reasons for the modifications of the signal are discussed.

Laboratory studies on behavioural interactions of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) with conspecifics, synthetic pheromone and the predator Teretriosoma nigrescens (Lewes)(Coleoptera: Histeridae)

Abstract A bioassay method was devised to test the responses of adult Prostephanus truncatus (Horn) to conspecific adults or synthetic pheromone and to observe the response of the predator Teretriosoma nigrescens (Lewes) to adult or larval P. truncatus or the synthetic pheromone of this species. The results confirmed earlier observations that male P. truncatus are the source of a pheromone attractive to both males and females, P. truncatus adults (in this case virgin females) are disinclined to leave food in response to the pheromone and maize itself is relatively unattractive to P. truncatus . For the first time it has been demonstrated that males are not attracted to females, females repel or avoid each other and males are less attracted to the naturally produced pheromone secretion than are females. In view of the fact that unmated females will not leave food to locate a calling male it was concluded that the female response to the pheromone is primarily a means of locating a food source, rather than a sexual partner. In contrast, males release their secretion to attract females. When adult P. truncatus were tested against grains treated with the synthetic pheromone the response of males and females was apparently identical, in contrast to the lower response of males when tested against conspecific males. It seems probable that the amounts of pheromone used when testing the synthetic mixture are greater than would be produced naturally by a calling male and that this may mask the apparently lower sensitivity of males to the natural pheromone. The pheromone of P. truncatus is known to be a kairomone for T. nigrescens , but in the bioassay this species was not attracted to synthetic pheromone and responded more strongly to empty maize grains than to adult or larval P. truncatus . It would appear that when walking, adult T. nigrescens are unwilling to approach closely to P. truncatus . The possible significance of this is discussed.

Intermale Variation in Aggregation Pheromone Release in Prostephanus truncatus

Intermale variation in pheromone signaling has been confirmed and quantified by measurements of pheromone produced by single adult male Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). Males varied in both the amounts of the two components of their aggregation pheromone and the ratio of one component to the other. The mean rates of production of the pheromone components T1 and T2 were 1.9 and 0.5 μg/day, respectively. There were repeatable differences among males in the amounts of T2 produced and the proportion of T1 in the pheromone blend over two weeks. Of the 15 males studied, one released a large burst of pheromone in a short period, while the remainder, if they did release, did so over an extended period. This suggested that there may be two alternative release strategies and the significance of this is discussed.

Solvent extraction of cues in the dust and frass of Prostephanus truncatus and analysis of behavioural mechanisms leading to arrestment of the predator Teretrius nigrescens

Abstract.  Teretrius nigrescens is a predatory beetle released into Africa for classical biological control of Prostephanus truncatus, currently the most serious insect pest of stored maize on the continent. T. nigrescens is arrested by the dust and frass produced by P. truncatus boring into maize and it has been concluded that this effect is most likely to be caused by chemicals in the dust/frass. A bioassay is presented in which EthoVision software (Noldus Information Technology, the Netherlands) captures and analyses the movement of individual insects within arenas containing zones to which solvent extracts are applied. The results demonstrate that cues enabling adult T. nigrescens to discriminate between maize flour and dust/frass are extractable in hexane, methanol and chloroform, but not water. Such discrimination is manifested by an increase in residence time, distance covered and path sinuosity in the dust/frass zone. Analysis of behaviour demonstrates cues that trigger inverse orthokinetic mechanisms contributing to arrestment in adult T. nigrescens. An increase in angular velocity would also suggest a direct klinokinetic component, although this parameter could be affected by postulated tropotactic and/or transverse klinotactic and/or longitudinal klinotactic mechanisms occurring at the edge of the application zone. Development of this bioassay, for the first time, permits responses of larval T. nigrescens to prey cues to be assessed. The results show that larvae apparently respond to cues extracted from dust/frass in a stronger manner than adults, with significant increases in the same descriptive parameters of time, distance and path sinuosity. However, unlike in the adults, there is no orthokinesis. It is proposed that strong larval response is attributable to either direct klinokinesis and/or tropotaxis interacting with transverse klinotaxis. Observations of larval backtracking at the zone boundary may also indicate longitudinal klinotaxis. The reasons for the differences in the adult and larval response to prey cues are considered, and how the described behavioural mechanisms combine to improve prey foraging is discussed.

Flight behaviour of Prostephanus truncatus and Teretrius nigrescens demonstrated by a cheap and simple pheromone-baited trap designed to segregate catches with time

The storage pest Prostephanus truncatus (Horn) (Coleoptera: Histeridae) and its predator Teretrius nigrescens (Lewis) (Coleoptera: Histeridae) are both known to disperse by flight. The pattern of flight activity of the two beetles in Ghana, across 11 months of the year, was investigated using a novel flight trap that separates catch at 3-h intervals. Prostephanus truncatus showed most flight activity around dusk with a smaller peak around dawn. Teretrius nigrescens had a strong diurnal peak. There were considerable differences in catch of both species during the year and when catch was low the peaks in activity were also less distinct.

Responses of Teretrius nigrescens Toward the Dust and Frass of Its Prey, Prostephanus truncatus

Teretrius nigrescens is considered to be a specialized predator of Prostephanus truncatus, a serious pest of stored maize and dried cassava roots. Using a bait-bag behavioral bioassay, this investigation found T. nigrescens to be strongly arrested by dust and frass produced by P. truncatus on maize, whereas responses to maize flour were weak. Attempts to increase the arrestiveness of flour by altering its physical properties (coarseness or particle size range) were unsuccessful. The arrestive property of dust/frass did not degrade with long-term storage nor did it volatilize or degrade with oven baking up to 150°C. However, extraction with methanol and reapplication was successful in transferring the arrestive property from the dust/frass onto maize flour. The prey dust/frass also induced more oviposition than did maize flour, as judged by production of F1 offspring. The results provide strong evidence for the existence of a high boiling point compound(s) in P. truncatus dust/frass that acts as a contact kairomone for T. nigrescens. This, or another compound(s) also seems to act as an oviposition stimulant for female T. nigrescens.