The effects of simulated rainfall on cochineal insects (Homoptera: Dactylopiidae): colony composition and survival on cactus cladodes

ABSTRACT. 1. A rainfall simulator is described that was used to investigate the effects of rainfall on the cochineal insect Dactylopius opuntiae (Cockerell). This species occurs in discrete colonies that are protected by a copious covering of opaque'woolly'wax, and is the most important biological control agent of the prickly pear cactus, Opuntia ficus-indica (L.) Miller, in South Africa.
2. We record the composition of cochineal colonies (stage and number of individuals) in relation to colony size to allow accurate predictions of colony composition from size measurements of the intact, wax-covered colonies before their exposure to rainfall.
3. Even short exposures (15 min) to simulated rainfall (delivered at a rate of about 50 mm/h) resulted in wax erosion, and the effect was most marked in small, recently established colonies. Coverings of compacted wax protected the mature colonies from erosion.
4. The number of colonies that were washed off the plant was a function of colony size and the duration of exposure to simulated rainfall. Most of the small, recently established colonies were washed from the plants after 120 min. All of the large, mature colonies, albeit badly mutilated and containing far fewer individuals, remained on the plants even after 240 min exposure.
5. Survival of individuals, in colonies of all sizes, was less than about 40% after short bouts of rainfall (30 min). First- and second-instar individuals accounted for most of the observed mortalities, and mortality of all stages increased with increasing rainfall duration.
6. These observations provide a partial explanation for the poor performance of D. opuntiae as a biological control agent of cacti in higher-rainfall areas.     

Natural physical disturbance and predation: their importance in structuring a marine sessile community

Settling panels were used to determine the distribution of sessile organisms. Experimental and control cages were located at two study sites that were of different physical exposures. Twenty-nine species of sessile organisms were recorded for the two sites. Data on the presence or absence of each species, for each sample, were classified. The resulting hierarchy showed natural physical disturbance to be the major structuring factor in the community. Predation was of less importance in the physically stressed study site and had no effect on species distribution in the protected site.     

An assessment of host resistance to ticks on cross-bred cattle in Burundi

Abstract. Groups of cattle of four different cross-breeds (Ankole × Friesian, Ankole × Brown-Swiss, Ankole × Guernsey, Ankole × Sahiwal) plus a group of pure Ankole cattle were immunized against tick-borne diseases: East Coast Fever, using a Theileria parva trivalent vaccine, and anaplasmosis, babesiosis and, subsequently, heartwater. With the exception of two small subgroups of Ankole and Ankole × Sahiwal which were treated by weekly spraying using Delnav, all the groups were exposed to natural field challenge of ticks to quantify the development of host resistance to ticks. The results indicate resistance to the most abundant species, Rhipicephalus appendiculatus , as follows: Ankole > Ankole × Sahiwal > Ankole × Brown Swiss > Ankole × Friesian > Ankole × Guernsey. In both Amblyomma variegatum and Rhipicephalus evertsi evertsi , the manifestation of different levels of resistance between breeds is inapparent. Correlations between total ticks and standard female ticks were good for R. appendiculatus (r = 0.73) and R. evertsi (r - 0.51) but poor for A. variegatum (r = 0.31). Correlations between species were reasonably consistent (r = 0.43-0.59). There was no significant correlation between tick burdens and daily liveweight gain (DLWG) over the. whole period of the study. Of the cross-bred groups, Ankole × Sahiwal, which had the highest level of resistance, had the highest DLWG. In contrast, the Ankole × Guernsey which developed the lowest level of resistance had only marginally lower DLWG, the difference being only 8 g/day. During the first period of exposure to ticks, however, the untreated Ankole × Sahiwal group had markedly lower DLWG, which may indicate that the development and manifestation of resistance is at the expense of productivity.     

Reproductive traits of the freshwater oyster Etheria elliptica (Bivalvia: Etheriidae) in the Pendjari River,Benin: implications for conservation

The African freshwater oyster Etheria elliptica, which is of great economic importance throughout the continent, is facing overharvesting in many fisheries in West Africa. Its reproductive traits (sex ratio, size at sexual maturity, oocyte diameter and fecundity) were studied at four stations located along the Pendjari River, northern Benin, in April 2013. Histological techniques were implemented to identify sex and gonad development stages. Oocyte sizes were measured based on the histological images and mean oocyte diameter was 38 µm. Fecundity, estimated by counting the developing eggs, averaged 106 724 eggs and increased significantly with shell size. The average sex ratio was approximately 1:1. Hermaphrodites were rare. Males reached sexual maturity at 57.6 mm dorsoventral height, earlier than females at 64.7 mm. The majority of the oyster specimens were in ripe-spawning stages, indicating that reproductive activities partly took place towards the end of the dry season. Both mature and immature individuals were subjected to harvesting at all the investigated fishing sites. The mean size of exploited oysters was 60.6 mm, lower than the size of females at maturity. Management strategies must enforce the minimum size of 65 mm required for harvesting, particularly in open access sites where juveniles are mostly harvested.