Influence of water-stressed cassava on Phenacoccus herreni and three associated parasitoids

The influence of cassava Manihot esculenta Crantz grown under condition of water-stress on development and reproduction of the cassava mealybug, Phenacoccus herreni Cox & Williams, and levels of parasitism of three encyrtid parasitoids, Apoanagyrus diversicornis Howard, Aenasius vexans Kerrich, and Acerophagus coccois Smith, were studied in the laboratory. Two cassava cultivars were used: CM 507-37 (drought-tolerant) and CMC 40. A 30 day period of water stress, imposed by reducing the irrigation volume, led to a reduction in shoot development and stomatal conductance of leaves of both cassava genotypes. Phenacoccus herreni development and reproduction were favoured by cassava under water shortage. Parasitism decreased and water stress appeared to enhance the encapsulation of parasitoid eggs or larvae by the mealybug. In the case of the parasitoid A. diversicornis, there was a decrease in size of female progeny, suggesting a lower fitness in this species on cassava plants under water stress. All results indicated that cassava grown under low water availability favoured P. herreni development and reproduction, and affected the success of parasitism and, depending on the species, parasitoid development. The drought-tolerance characteristic of cassava genotypes and parasitoid species most suitable for controlling P. herreni in drought-stricken areas are discussed.     

班氏跳小蜂对扶桑绵粉蚧的寄生功能反应

为评估班氏跳小蜂Aenasius bambawalei Hayat对其寄主扶桑绵粉蚧Phenacoccus solenopsis Tinsley的控制作用, 在实验室条件下（温度25±1℃, 相对湿度70%±5%, 光周期14L∶10D）, 研究了班氏跳小蜂对扶桑绵粉蚧3龄若虫、 雌成虫的寄生功能反应及其自身密度效应。结果表明: 寄生功能反应均符合Holling Ⅱ型方程, 且受寄主密度和寄生物密度的影响。当扶桑绵粉蚧 3龄若虫和雌成虫的密度分别大于15头/容器和10头/容器时, 班氏跳小蜂的寄生量增加幅度开始减小; 以瞬间攻击率/处理时间（α/Th)为评价指标, 班氏跳小蜂雌蜂寄生扶桑绵粉蚧3龄若虫时, α/Th为21.1307, 且大于雌蜂寄生雌成虫的情况（α/Th为6.2506）; 班氏跳小蜂寄生功能反应有较强的种内干扰作用, 随着自身密度的增加, 寄生效能逐渐降低; 通过方程E=QP^-m能很好地模拟班氏跳小蜂的寻找效应（E）与其自身密度（P）之间的关系, 对于3龄若虫和雌成虫的模拟结果分别为E=0.2931P^-0.6240和E=0.0944P^-0.4840。本研究为利用班氏跳小蜂开展扶桑绵粉蚧生物防治提供了基础数据和方法。     

班氏跳小蜂对寄主龄期选择的研究

班氏跳小蜂Aenasiusbam bawalei Hayat是扶桑绵粉蚧Phenacoccus solenopsis Tinsley的优势种寄生蜂。本文研究了在限制选择和自由选择的条件下,扶桑绵粉蚧的2龄若虫、3龄若虫和雌成虫对该蜂的寄生率、发育历期、子代性比和个体大小的影响。结果表明:在两种选择条件下,班氏跳小蜂对扶桑绵粉蚧3龄若虫的寄生率显著高于对雌成虫和2龄若虫的寄生率,但寄生粉蚧雌成虫的子代雌雄性比显著高于寄生3龄若虫或2龄若虫的性比;在两种选择条件下,雌蜂在寄主三种虫龄(态)上的发育历期差异不显著,但是雄蜂在粉蚧雌成虫中的发育历期显著长于寄生2龄若虫或3龄若虫的发育历期;在两种选择条件下,寄生于粉蚧雌成虫的子代雌蜂和雄蜂的个体都是最大,其后足胫节长度显著长于寄生于2龄若虫或3龄若虫的子代胫节的长度。     

不同生殖方式对班氏跳小蜂寄生率和子代性比的影响

为了明确班氏跳小蜂不同生殖方式及雌蜂密度对种群的影响,研究了班氏跳小蜂在不同雌雄比例的两性生殖和孤雌生殖条件下以及不同雌蜂密度情况时,其寄生率及子代的雌性比例。结果表明:班氏跳小蜂可进行两性生殖和产雄孤雌生殖,不同雌雄比例的两性生殖及孤雌生殖的寄生率随着雌蜂数量的增加不断升高,雌雄比为20∶1时的寄生率最高,为49.13%;在两性生殖雌雄比为15∶1的情况下,其后代雌性最高;不同粉蚧密度条件下,2头雌蜂同时存在时,其寄生率和子代性比均最高。说明两性生殖(雌雄比为15∶1)和2头雌蜂同时存在条件下,更利于班氏跳小蜂种群发展。     

扶桑绵粉蚧虫态对班氏跳小蜂寄生适合性的影响

为明确寄主虫态对班氏跳小蜂适合度的影响,在实验室条件下\[温度(27±1) ℃,相对湿度(70±5)%,光周期14L∶10D\]测定了寄生于不同发育阶段(3龄若虫和雌成虫)扶桑绵粉蚧的班氏跳小蜂子代性比和个体大小.结果表明: 寄生扶桑绵粉蚧雌成虫的班氏跳小蜂的子代雌雄性比、个体体长和后足胫节的长度均明显高于寄生3龄若虫,且个体体长与后足胫节长度之间有显著的相关性.扶桑绵粉蚧3龄若虫个体大小显著影响班氏跳小蜂子代个体大小,而雌成虫的个体大小对该蜂子代大小无显著影响.
     

Effect of mating combinations on the host parasitisation and sex allocation in solitary endoparasitoid,Aenasius arizonensis (Hymenoptera: Encyrtidae)

Aenasius arizonensis (Girault) is an important solitary endoparasitoid of Phenacoccus solenopsis Tinsley. To optimise the mass production of high-quality females, it is important to assess the influence of mating regimes on the progeny fitness and sex allocation. We, therefore, hypothesise that mating combinations in A. arizonensis adults emerged from different host instars may influence parasitism and sex allocation in the subsequent generation. Therefore, we compared three nymphal instars (1st, 2nd and 3rd) and adults host stages of P. solenopsis for parasitism and sex allocation by A. arizonensis. Further, F₁ female progeny of the parasitoid emerged from different host instars was henceforth evaluated for its fitness in six mating combinations. A. arizonensis females parasitised all the host stages except the 1st instar nymphs. The parasitised 2nd instar nymphs yielded only males, while the sex ratio in the later host instars was strongly female-biased. The parasitoid females preferred 3rd instar nymphs with respect to higher parasitism (74.0–84.0%) and produced more females in the F₁ progeny as compared to other host stages. F₁ females that emerged from 3rd instar nymphs produced significantly higher parasitism (74.0–79.0%). These mating combinations also yielded more female progeny in the F₂ generation. However, parasitism by F₁ females was significantly lower (9.0–12.0%) when mated with males that emerged from 2nd instar P. solenopsis nymphs. Moreover, latter combinations yielded only male progeny in F₂ generation. These findings can be used in laboratory mass rearing of this parasitoid vis-à-vis biological control of P. solenopsis.     

Report of a parasitic wasp (Hymenoptera: Encyrtidae) parasitizing cotton mealybug (Hemiptera: Pseudococcidae) in Pakistan and use of PCR for estimating parasitism levels

A parasitic wasp, Aenasius bambawalei, was studied for its biological parameters and parasitism levels in the cotton mealybug (Phenacoccus solenopsis). Biological parameters including parasitism efficiency, time to pupation, time to eclosion and adult sex ratio were studied under lab conditions. Parasitism levels in field collected mealybug were determined using PCR. Results showed an increase in parasitism over the study period, with higher parasitism levels in 2009 compared to the preceding 2 years.     

Ant–hemipteran association decreases parasitism of Phenacoccus solenopsis by endoparasitoid Aenasius bambawalei

1. Mutualism between ants and honeydew-excreting hemipterans is ubiquitous in the ecosystem. It is widely accepted that ant tending facilitates the colony growth of hemipterans by protecting them from predators and parasitoids. However, few studies have explored how ant tending helps defend against natural enemies. 2. Ghost ant Tapinoma melanocephalum and the invasive mealybug Phenacoccus solenopsis have a close mutual relationship. Previous studies have shown that ghost ant tending can definitely reduce parasitism and visit frequency of Aenasius bambawalei, the dominant endoparasitoid of P. solenopsis. However, the ghost ant workers seldom attack the parasitoids. It is still unclear how the ghost ant adversely affects parasitoids. This study explored the mechanism underlying the impacts of ants on natural enemies of the mealybugs. 3. Honeydew produced by P. solenopsis was an attractant to A. bambawalei. Parasitoids exhibited less searching activity, shorter longevity and lower parasitism when supplied with less honeydew. Aenasius bambawalei showed significant avoidance of pygidial gland secretions and visual cues of ghost ants. Parasitism in plants treated with 6-methyl-5-hepten-2-one, actinidine, and gland extracts was significantly lower than that in plants treated only with solvents (paraffin oil or double-distilled water). 4. It is concluded that honeydew consumption by ghost ants could negatively influence the performance of parasitoids. The pygidial gland secretions and visual cues of ghost ants also significantly inhibit the parasitism. These results may contribute to a better understanding of the regulation mechanism in ant–hemipteran–enemy interactions.     

Host specificity and comparative foraging behaviour of Aenasius vexans and Acerophagus coccois, two endo-parasitoids of the cassava mealybug

Two encyrtid parasitoids, Aenasius vexans Kerrich (Hymenoptera: Encyrtidae) and Acerophagus coccois Smith (Hymenoptera: Encyrtidae), were compared for their degree of dietary specialisation and the impact this has on their foraging strategies. Both parasitoid species are significant for biological control of the cassava mealybug, Phenacoccus herreni, Cox & Williams (Homoptera: Sternorrhyncha) a major Latin American pest of cassava, Manihot esculenta Crantz, an important root crop. Host acceptance and parasitism were analysed in seven mealybug species (with different levels of polyphagy) occurring in and around cassava fields. Results demonstrate that, in this ecosystem, An. vexans is a specialist for P. herreni while Ac. coccois is a generalist on the first and second trophic level. Of the seven mealybug species, P. herreni and P. madeirensis Green were the most acceptable hosts for Ac. coccois, followed by Ferrisia virgata Cockerell. Ac. coccois did not accept the other four mealybug species.The foraging and oviposition behaviour of individual parasitoids was observed in bioassays with cassava leaves infested by P. herreni. The two species used different strategies to locate their host. Aenasius vexans spent significantly more time walking and standing on an infested leaf and examined a host longer than did Ac. coccois. Acerophagus coccois, in contrast, spent more time for oviposition. As a consequence An. vexans parasitised more hosts in a given time than did Ac. coccois. Because the rate of offspring production of the two species did not differ, we conclude that the gregarious Ac. coccois's strategy to deposit several eggs at once might compensate for its relatively low number of ovipostitions, compared with the solitary An. vexans. These findings suggest that, given the advantages and limitations of each species, a multi-species approach to biological control of P. herreni may yield best results.