Parasitoid wasps are a member of the order hymenoptera and provide an ecosystem service by means of biological pest control. They can be either generalist in their host choice or specialist; (host specific; within other insects) and are detrimental to their host’s survival, subsequently allowing them to commandeer their hosts to create suitable environments to promote and support their own development, Beckage and Gelman (2004). Parasitoids are successful in commandeering their host due to morphological and behavioural adaptions. One important morphological adaptation is their ability to locate and hone in on a host through chemical cues in the olfactory receptor organs in the sensilla placodea of the antennae, Ochieng et al., (2000). Flight has helped enable wasps to travel to their host and prey species quickly and effectively. Once located the female wasp will inject venom into the host whilst ovipositing, (which subdues prey or hosts and/or provide defence against potential predators), Poirie et al., (2014). Developing larvae will derive nourishment from this host until it reaches a particular life stage and emerges, (usually killing the host once it has evacuated). Life history of the maximum number of mature eggs stored in the ovary is dictated by total number of stem line oocytes, these oocytes also control the rate and size of egg maturation which may be important to the compatibility between egg and host availability, Jervis et al., (2001).
To survey and count the amount of insects within an area first an appropriate location must be found where they are known to be readily available by considering their behaviour and life history strategies. For instance parasitoid hymenoptera are known to be attracted to host species therefore to find a particular species one would need to find the crop in which the host is likely to occur. One method of surveying used by Fraser et al., (2008), is the malaise trap which is a form of flight interception trap that can be left unattended for long periods of time which equates to being cost and time effective. These traps can be useful for understanding natural habitat and parasitoid abundance/diversity relationships. Due to the nature of these traps physical observation counts can be achieved as the target species is essentially trapped inside the netting although depending on the taxa targeted this may need to be adjusted due to small sizes which make it difficult to gain the correct number of sampled individuals although estimates can be inferred.
To measure the function that parasitoid wasps perform in the ecosystem the host species being parasitized must be detected to assess how many pests are being eliminated within a set time frame (parasitism rate). Effect of parasitism rate on consumer-resources can be examined using simplified and controlled models in a laboratory or in field cage environments in order to gain an estimate of the rate of which pests are controlled, Gamfeldt et al., (2005). This technique however does not assess outside effects, such as environmental changes on parasitism rate within real landscapes. Therefore Tylianakis et al., (2006), created a study whereby they assessed the stability of parasitism over time in differing habitat types of anthropogenic modification using trap nests where the parasitoid and host would usually occur. Tukeys pairwise comparisons were used in order to assess the differences between habitat types and the affect this may have on parasitoid numbers. Richness, abundance, and Simpson diversity were intercorrelated for both hosts and parasitoids, which were treated as a multivariate response variable in both models in order to generate and idea of the total quantities of pests to be controlled and parasitoids to control within a particular (fixed) environment.
Biological pest control is of great ecosystem service to the wealth of human agriculture as it naturally controls and supresses’ pest populations by honing in on a particular pest colony and decimating it. This provides great economic benefits in agricultural crops as it can reduce yield loss without the use of chemical pesticides which impose negative environmental consequences, Ostman et al., (2003). To quantify the value of natural enemies we can assess the annual savings due to biological control from the reduction in pesticides. For instance Gurr et al,. (2004) suggests properly utilised usage of biological controls on farmland crops can in consequence cause annual savings worth billions of dollars and this principle may be enhanced using ecological engineering. Sandhu et al., (2010) provide an example of successful biological insect pest control in vineyards of New Zealand whereby Dolichogenidea tasmanica (a parasitic wasp) reduced Epiphyas postvittana, (a leaf roller pest) up to 50% to a level where agrichemical sprays were no longer required. Subsequently it was estimated that the annual valued savings was $250 ha−1 yr−1 in New Zealand. Unfortunately it is hard to quantify the benefits to human health from the use of organic agriculture although the benefits are of obvious value.
Parasitic wasps function as a biological control agent can be reduced by the use of pesticide and human land management options. Low dose exposure of insecticide to parasitoids is to be expected as there is widespread uses of pesticides which can induce sublethal effects; (particularly with the use of neurotoxic insecticides), Haynes (1988). Olfaction plays an extremely important part in a parasitoids host searching behaviour, so depending on levels of exposure to neurotoxic insecticides they may interfere with olfaction as it is entirely dependent on nervous transmission which negatively impacts on all foraging behaviour, Desneux (2004). Land management effects natural enemies due to differing habitat compositions. Bianchi et al., (2006) constructed a study to see if landscape composition had an effect on biodiversity and natural pest control. They found that enhanced natural enemy activity was associated with herbaceous habitat (80%) and less often with 71% in wooded habitat and 70% if the landscape was fragmented. This study showed that landscape complexity enhanced natural enemy populations. This is probably due to the number of flowering plants in which they can acquire sustenance as adults and number of host species available for their larvae to develop.
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