Biological Control Of Pests

One of the most exciting developments in pest management is using biological methods rather than chemical ones.

By Kashif Hussain^a_, Maria Kausar^a, Muhammad Sohail Sajid^a, Talha Khan^b

The rising cost of new chemical pesticides, the growing problem of arthropod resistance to pesticides, and the goal to reduce the number of chemical residues on Earth motivate research into potential alternatives. In general, pest biocontrol agents are safer for the environment than chemical pesticides, and they usually continue to be effective even after being supplemented with an inundation. The serious downside is that chemical pesticides may be used to kill such a wide variety of arthropods. Although practical, broad-spectrum insecticides can have unintended consequences, such as eliminating beneficial insects along with pests and even increasing the pest population or the metamorphosis of non-pest arthropods into big pests.

Fig.1. Depicting various kind of pests of public health significance

In contrast to chemical pesticides, biocontrol agents take more time to have an effect, but their biotic suppression of a given pest population lasts much longer. The user needs much more information about each pest and how it should be utilized effectively as a biocontrol agent because most biocontrol agents have a shorter shelf life. There are three main ways to achieve classic pest biocontrol: (1) introducing new, beneficial bioagents, (2) manipulatively enhancing existing biocontrol agents (through inoculative or inundate augmentation), or (3) conserving to induce environmental changes that favor the existing biocontrol agent population. In the last 15 years, augmentation has become increasingly popular in biocontrol initiatives aimed at plant pests. Commercial distributors of biocontrol products in North America expanded from 53 in 1985 to 106 in 1994.

Additionally, these businesses have been progressively increasing their output. Introducing a single bioagent has nearly eliminated pest populations in some successful plant pest biocontrol programs. At times, many bioagents of varying types have been raised repeatedly to control a small number of pest species in a single crop, with or without the strategic use of chemical pesticides at key points.

Despite the extensive research and development into biocontrol of plant pests, cattle and poultry biocontrol have been mostly ignored. Even though ticks inflict significant harm to animals and humans, tick biocontrol is less prioritized than that of most other considerable animal pests. Their biocontrol is several decades behind that of plant pests. Approximately 300 of the more than 700 studies on tick enemies or pathogens were published between 1900 and 1970. Since 1970, the subject’s popularity has steadily increased linearly, with an average of 14 publications appearing annually.

Fig.1. Depicting the biocontrol of pests

Tick Biocontrol

Literature reports about 100 different pathogens, seven parasitic wasps, and almost 150 predators that feed on ticks. A sizable collection of species from which to choose those most likely to be effective biocontrol agents.

Bacteria

Brazilian researchers discovered that up to 40% of pregnant Boophilus microplus females were infected with Cedecea lapagei (Enterobacteriaceae).

The tick’s vaginal epithelial is infected and eventually destroyed by these bacteria. When engorged females are injected with 1 mL of a bacterial suspension containing 2 x 109 bacteria or submerged in the solution, 100% of the females die. Finding more virulent isolates or using selection and bioengineering to create a more virulent species, along with an optimum formulation, could make these compounds useful as anti-tick drugs. Bacillus thurengiensis var. kurstaki toxin (250 g/ml) killed all engorged female Argas persicus. Females and eggs of this tick and Hyalomma dromedari that haven’t been fed appeared to be more resistant to infection.

Fungi

Ticks have been linked to the presence of several entomopathogenic fungi, some of which are particularly virulent to ticks in laboratory settings. Metarhizium and Beauveria are the two entomopathogenic fungi that have been studied the most extensively (Deuteromycotina). They can enter the body through the cuticle and often exhibit virulence toward all stages of ticks. The findings of several outdoor studies were encouraging. However, the fungi can only germinate in conditions of rather high humidity, and it takes a few days before the ticks are completely eradicated. When looking to introduce fungi as effective anti-tick control agents, these two constraints may offer important themes for inquiry.

Nematodes

Thirteen hard tick species and two argasid ticks were susceptible to entomopathogenic nematodes from the Steinernematidae and Heterorhabditidae families, whereas just one species was resistant to the nematode strains studied. The preimaginal stages of several nematodes appear to be more lethal to adult ticks. Commercial nematodes are already used against several plant pests, although they may be limited in their application because of their vulnerability to low humidity and cold temperatures. The release of novel nematode strains and formulations can potentially expand the range of ecological niches that can support them.

Parasitic Wasps

Only two of the lab’s seven parasitoid species in the Ixodiphagus hookeri and Ixodiphagus texanus have been extensively researched. Adult wasps emerge from nymphs 1-2 months after they have abandoned their host, where they had been laying eggs. Attempts to introduce I. hookeri to new locations in the later part of the 20th century were met with failure or, in cases when the wasp successfully established in a new place, had little effect on the local tick population. Given these realities, it’s easy to see why tick biocontrol researchers might be hesitant to pursue this avenue of research any further. To what extent other introduced wasp species might be more effective and why these particular wasps were so ineffective remain open questions. Several impressive successes with these parasitoids in plant pest control suggest that these lines of inquiry are of specific interest. The on-host nymph population of Amblyomma variegatum was reduced by nearly 95% after four months of the first release, and the tick population was maintained at a low level for at least two years, according to a single experiment conducted in Kenya.

Birds

The yellow-billed oxpecker (YBO) (Buphagus africanus) and the red-billed oxpecker (RBO) (Buphagus erythrorhinchus) are two of the most important of the roughly 50 species of birds known to feast on ticks. Oxpeckers play a crucial role in the biocontrol of ticks due to their diet, which consists virtually entirely of arthropods, pests of mammals. Daily food intake for the larger oxpecker (YBO) can reach up to 13,600 nymphs or 109 female ticks (Boophilus decoloratus), while the smaller oxpecker (RBO) can reach up to 12,500 nymphs or 98 females of the same tick species. But oxpeckers can also pry open mammalian scars and use open wounds to eat bits of skin and blood. They prolong the wounds’ openness, raising the risk of secondary infections such as myiasis. Whether or when this behavior also leads to the mechanical transmission of mammalian diseases remains to be seen.

Regarding feeding on wounds, the YBO appears to be roughly ten times more voracious than the RBO. Mass use of chemical pesticides intended to kill ticks has led to the extinction of these birds in wide swaths of Zimbabwe and South Africa. The oxpecker population in Kruger Park, South Africa has recovered without human intervention since tick control activities have been curtailed, and the acaricides have been replaced with substances less damaging to birds and the environment. The oxpeckers of Zimbabwe were relocated to eleven parks after having vanished from those areas. The birds have successfully repopulated five different parks. Those looking to bring oxpeckers to other parts of Africa or continents can benefit from the lessons learned from previous translocation efforts.

1. Purple-labeled acaricides should not be used within a 10-km radius of the area designated for the birds’ release.
2. To support an oxpecker population, there should be about 500 host mammals and an area of at least 3,000 ha.
3. Oxpeckers are social animals; therefore, at least 20 birds should be introduced to each new site.
4. RBOs are less aggressive than YBOs, and tend to feed less on wounds; therefore, RBOs seem to be a better choice for introduction projects.
5. A large tick population may reduce wound feeding.
6. Using a good tick population monitoring system can help evaluate the success of such an introduction.

When the alimentary canal of a small species of falcon called the Milvago chimango from Brazil was examined, researchers discovered an average of 122 Boophilus microplus ticks and a maximum of 289 ticks per bird. The falcons removed ticks from either themselves or their host. It was determined that a single bird could cut the tick population on a cow badly afflicted by approximately 13% in a single day. Improving the birds’ habitat to encourage more of their offspring to survive and breed, or relocating them to new territories, should not be done until after the birds’ potential impact on other species has been evaluated. After a period spent adapting to their environment, it was discovered that domestic chickens eagerly consume ticks. When hens were allowed to forage at will in a cow yard, they consumed an average of 75, 79, and 48 ticks per bird per hour after the first, second, and third hours of foraging, respectively. The birds collected the ticks on and off the livestock. Because of this, it will be desired and cost-effective for small mixed farms to let hens scavenge the cow yards in the morning before letting the cattle out to graze to lower the cattle ticks’ population. It will take place before the cattle are let out to graze.

Authors:

Kashif Hussain^a_, Maria Kausar^a, Muhammad Sohail Sajid^a, Talha Khan^b

a: Department of Parasitology, University of Agriculture Faisalabad, Pakistan.

b: Department of Applied Chemistry, Government College University Faisalabad, Pakistan.