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Predators, Parasitoids & Pathogens of Insects (click here for PDF reprints)

These studies have focused on the biology and behaviour of insect natural enemies and their efficacy as agents for the biological control of insect pests.  I have outlined three of these studies here (a predator + a parasitoid + a pathogen).

1. Predators

Kleptobiotic spiders steal prey items from the webs of web-building spiders.

The question posed in this study was "why do kleptobiotic spiders of the genus Argyrodes seem to be associated with spiders of the genus Nephila worldwide?"

Observations following the introduction of experimental insect prey items of different sizes and weights on to host webs in the south of Mexico revealed that:

(1) Small prey are more effectively retained on the web of Nephila clavipes than on the web of another common host, Leucauge venusta.

(2) N. clavipes did not consume small prey that accumulated on the web whereas larger, heavier prey were enveloped and stored.

(3) We observed clear partitioning of prey items between N. clavipes and Argyrodes spp.; diet selection by Argyrodes did not overlap with that of N. clavipes but closely overlapped with that of L. venusta.

(4) L. venusta responds very quickly to prey impact whereas N. clavipes is slower, offering a temporal window of opportunity for Argyrodes foraging.


(5) The ability of L. venusta to detect and respond to small items also means that it acts aggressively to Argyrodes spp., whereas N. clavipes does not.

As a result, food-acquisition behaviours of Argyrodes were clearly less risky with N. clavipes compared with L. venusta. The fact that Nephila species present such characteristics explains the worldwide association with Argyrodes kleptobiotic spiders (see Henaut et al., 2005).

Nephilia clavipes (photo Yann Henaut)

Nephilia clavipes in a coffee plantation in Chiapas, southern Mexico (photo Y. Henaut)

          

Parasitism of cabbage whitefly nymphs by Encarsia tricolor results in the production of female wasps

Encarsia tricolor females ovipositing on nymphs of Aleyrodes proletella (for production of females)

2. Parasitoids

Aphelinid parasitoids have an outstanding record of success in programmes of classical biocontrol against whiteflies and scale insects.

Heteronomous hyperparasitoids are aphelinids in which the sexes develop on or in different hosts.

The female always develops as a primary endoparasitoid of Homoptera.

The male develops as a secondary parasitoid (hyperparasitoid) of his own or another species of homopteran endoparasitoid.

Caged experiments were performed with the cabbage whitefly, Aleyrodes proletella, to examine the invasion of a population of a conventional parasitoid, Encarsia inaron (both sexes primary endoparasitoids) by a heteronomous hyperparasitoid, E. tricolor.

In all cages the heteronomous hyperparasitoid successfully invaded an established population of the conventional parasitoid and the conventional species population quickly declined to very low levels.

High levels of male production by E. tricolor were observed, indicating that hyperparasitism of the conventional species was probably an important factor in causing the decline in the E. inaron population.

In a reciprocal experiment in which E. inaron was introduced to an established population of E. tricolor the conventional species failed to invade or persist.

In the majority of cases, heteronomous hyperparasitoids are the most important species in complexes of parasitoids attacking whiteflies or scale insects.

However, high competitive ability against other parasitoids is not necessarily a good indicator of the ability of
a parasitoid to maintain high levels of pest control, especially when hyperparasitic behaviour is involved. (Williams, 1996)

Parasitism of developing endoparsitoids (including conspecifics) results in male offspring.

Pupae of E. tricolor (black scales) can be parasitized for the production of males.

          

3. Pathogens

The entomopathogenic fungus, Beauveria bassiana is one of the few natural enemies available for of the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae).


In the South of Mexico, this fungus is produced locally using rice as a substrate, although the efficacy of the resulting product tends to be rather variable.

In an attempt to enhance the efficacy of this pathogen, 22 substances was tested in simple laboratory tests using natural sunlight or a UV light source.

Unprotected B. bassiana spores were almost completely inactivated by exposure to 60 minutes of direct sunlight.

Of these substances, sucrose, yeast, yeast extract, uric acid, casein, and molasses had limited photoprotective properties.

However, known sunscreens such as methyl anthranilate, p-amino cinamic acid, titanium dioxide, ethyl p-amino benzoate and oxybenzone were detrimental to spore germination.

Several other substances provided little or no photoprotection including zinc oxide, starch, and the optical brightener Tinopal LPW (a known chitin synthesis inhibitor).

The most effective substances tested were egg albumen and skimmed milk powder which could extend the persistence of B. bassiana spores by a factor of almost three.

A mixture of 3% (w/v) albumen and 4% (w/v) milk powder gave the highest degree of spore protection per unit cost.

A field trial failed to demonstrate improved control of H. hampei following the application of spores formulated with albumen + milk, probably due to adverse weather conditions.

Study published by Edgington et al. (2001).

Tinopal LPW did not provide photoprotection to fungal spores - indeed, this compound inhibits chitin sysnthesis and may reduce germination and adversely influence the efficacy of the pathogen as a biological insecticde.

Fungal spores treated with optical brightener
 

Effect of formulation of ultraviolet (UV) protection of Beauveria bassiana spores.
 

Effect of a mixture of milk powder and egg albumen on B. bassiana spore inactivation in natural sunlight. Grey squares represent unprotected spores, white circles represent spores with egg and milk. Germination values have been arcsine transformed.

Sporulation of Beauveria bassiana in coffee berries infested by the coffee berry borer.

After spraying Beauveria bassiana on coffee berries, the sporulation of the fungus on the cadavers of coffee berry borers can been seen as white spots emerging from the tunnels that the insects make in the berries.
 

Application of Beauveria bassiana spores for coffee berry borer control in a coffee plantation in Chiapas, southern Mexico

Steven Edgington applies spore suspension to coffee plants in Chiapas, southern Mexico.

 


Click here for publications on insect natural enemies
 

 
          
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