Pheromonal Control of Diamondback Moth in the Management of Crucifer Pests

The effect of delayed mating on the reproductive potential of the diamondback moth, P lutella xylostella (L.), population was evaluated in the laboratory. The insecticideresistant diamondback moth could be controlled by synthetic sex pheromones which are not harmful to beneficial species. However, the pheromones had no effect on other pests like aphids and common cabbageworm. Predators crawling on the ground, like lycosid spiders, played an important role as a biotic mortality agent of immature stages of diamondback moth. The chitin synthesis inhibitors that are selective insecticides were very effective on caterpillars like the common cabbageworm. Pheromone and chitin synthesis inhibitors which are harmless to beneficial arthropods are regarded as main chemicals acceptable in management of crucifer pests. Introduction The main drawbacks in insecticidal control of diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), are: (1) development of insecticide resistance; (2) resurgence of the insect after applications of insecticide; (3) nonselective killing of harmless and beneficial species (Nemoto 1986). Synthetic sex pheromones have been utilized to suppress insect pest populations through either disruption of communication between the sexes or mass trapping of adult males. The efficacy of the pheromone is frequently measured by making comparisons between the percentage of tethered virgin females that mate in treated and control plots (Nemoto et al. 1985). In this paper, we present a converted Kiritani and Kanoh’s equation (ER) and attempt to evaluate the effect of delayed mating on the reproductive potential of the DBM population by laboratory experiment. A disruption experiment was conducted in a commercial grower’s field to evaluate efficacy of synthetic sex pheromones in controlling DBM. Nemoto (1986) reported that according to immunological tests lycosid spiders are important as biotic mortality agents of DBM. The role of predator or parasite in this experiment was evaluated by artificially excluding them from the estimation of DBM mortality. The control of only one species of pest is meaningless for crucifers that require simultaneous protection from other pests. The effect of insecticides on these pests was evaluated. Evaluation of delayed mating DBM population The possible role of a pheromone-induced delay in mating on the reproduction of the oriental tea tortrix, Homona magnanima Diakonoff, has been examined by Kiritani and Kanoh (1984), who considered that the delayed mating or fertilization on the part of females might reduce their fecundity through shortening their reproductive period and aging. They proposed an equation for the expected reproduction (ER) of a t-day-old female. 91 92 Nemoto, Yano and Kiritani A high percentage of mating inhibition, for example 90% /night, however, does not result in the same degree of suppression of the target population if the mean longevity of the adults exceeds 1 day. If the virgin females lived for only 1 day, a 90% matin inhibition/night would result in 10% of the females being mated, while up to 65% (1 = 0.65) of the females would eventually be fertilized if they lived for 10 days. On the other hand, the delay in mating reduces total fecundity through shortening the .female reproductive period. Yamada (1979) observed that most of the newly emerged females mated on the first night of their emergence and began to lay eggs on the following night. Unmated females lay a few eggs. The effect of delayed mating on various reproductive traits is presented in Table 1. Fecundity, viability of eggs and oviposition period all decreased with an increase in number of days elapsing before pairing after emergence. Table I . Effect of delayed mating on the reproductive traits of DBM. Expected reproduction No. of fertilized of of pairs laid/ovipositing eggs /ovipopairing siting female No. No. (and %) No. of eggs pairs mated female Days after emergence and before (RER) 0 20 108. I 8.8a 102.9 28.9e 2 19 100.9 97. 1 10.9e 92( 100) 6 15 I 0(67) I I .8c 60.3 15.3c 51.1 15.6f 4 20 I 10.0bc 77.9 I O . I ef 4 8 14 13.6 24.3 d Unmated control 28 The expected reproduction (the total number of viable eggs that could be laid during the female lifespan) of a t-day-old female which mated first on the tth day after emergence can be expressed by the following equation (Kiritani and Kanoh 1984): Expected reproduction Percentage Survival Total no. of of a t-day-old = successful rate until viable eggs female (ER) mating tth-day deposited This may be converted to the following equation: Percentage the age-specific age-specific mating female adults ER = successful survival rate of fecundity Successful mating refers to mated females that lay fertilized eggs. All mated females laid fertilized eggs in the present experiment. The relative expected reproduction (RER) of the female mated t days after emergence is then calculated (Table 1). The effect of delayed mating on ER becomes highly significant when the delay exceeds 6 days. This effect would be intensified under natural conditions, where adult survival would be influenced by weather, predators and other factors. Utilization of synthetic sex pheromones for DBM control aims to reduce the number of fertilized females in a given area. The effect of pheromone on the pest population therefore depends on the extent to which the pheromone application inhibits mating of virgin females. The realized RER (see the 4th column in Table 2) of the treated population at various levels Pheromonal Control of DBM 93 of inhibition was calculated to evaluate the effect of this inhibition on the reproduction of the DBM population. The cumulative percentage of mated females and the realized RER of populations subjected to various levels of mating inhibition are shown in Table 3. If it is assumed that the life span of females is invariably 12 days, then mating inhibition as high as 72 % will have little influence on the population size of the following generation. Ninety percent inhibition is expected to reduce the target population to 41 % of the untreated population, assuming no immigration from outside of the plot. A similar conclusion has been reached by Kiritani and Kanoh (1984) for H. magnanima and Nakasuji and Fujita (1980) for Spodoptera litura (F.) populations by means of computer simulations. Comparison between the cumulative mating percentage and the realized RER shows that the initial reduction of RER occurs at a lower level of daily mating inhibition than the cumulative mating percentage (Table 3). This is because realized RER involves the effect of delayed mating on reproduction. Delayed mating also leads to a delay in oviposition or lengthening of the preoviposition period. Although this factor has not been considered in the present discussion, it may play an important role in slowing down the population growth of insects under certain conditions. From the practical point of view, the optimum level of mating inhibition will depend on the cost: benefit aspects of the relationship between the number of pheromone sources required and the necessary level of mating inhibition. The latter is also a function of pest density and the reproductive traits of the species. It can be concluded that the effect of sex pheromone on the target DBM population depends not only on the direct effect of mating inhibition, but also on the indirect effects of delayed mating which reduces ER. A high level of mating inhibition (more than 90%) and/or other mortality agents would be required for a substantial reduction of DBM populations. Table 2. Cumulative percentage of mating females and the realized relative expected reproduction (RER) for the population when mating of females is inhibited. Pivotal age RER Daily rate of Realized RER 0.5 95 0.10 9.5 I .5 98 0.09 8.8 2.5 100 0.08 8. 1 3.5 79 0.07 5.8 4.5 44 0.07 2.9 5.5 30 0.06 I .8 6.5 23 0.05 1 .2 7.5 18 0.05 0.9 8.5 16 0.04 0.7 9.5 16 0.04 0.6 10.5 15 0.03 0.5 11.5 15 0.03 0.5 of female (A) virgin females (B) (C) = (A).(B) Lonegevity of females and the level of daily mating inhibition are assumed to be I 2 days and 90%, respectively Table 3. Cumulative percentage of mated females and the realized RER in a hypothetical population where all females live for 12 days under various levels of mating inhibition. Daily level of