Alternatives to insecticides: bio-inspired coatings and sprays to tackle insect pests

Samenvatting

In this thesis, we presented novel, eco-friendlier alternatives to insecticides inspired from plant surfaces for use in buildings and agriculture, namely, paints slippery to Atta cephalotes ants and spray emulsions that protect plants against Frankliniella occidentalis thrips.

In Chapter 2, we first reviewed the issues related to insect pests and to the use of pesticides to control pest populations. Coatings where insect attachment is minimised is thought to be a promising alternative to insecticides and insecticidal paints in the building industry. Understanding the locomotion behaviour of insects (both hairy and smooth pads) on both synthetic and natural surfaces is of importance to formulate such coatings and was hence reviewed. We then discussed some of the most promising state-of-the art coatings which could be used to this end, such as the SLIPS or particle film technologies.

We presented in Chapter 3 the fabrication of model paints slippery to insects (A. cephalotes ants). The paints’ slipperiness for A. cephalotes ants was evaluated in climbing tests on vertical paint panels by recording the percentage of fallen ants. These paints only contained water, an acrylic polymer binder, TiO2, CaCO3 and a coalescent. The Pigment Volume Concentration > Critical Pigment Volume Concentration (PVC > CPVC) condition was found to provide good slipperiness to ants as pigment and extender particles detach from the coating to adhere to the tacky attachment pads of insects due to lack of polymer binding the particles. Attachment was minimised when using CaCO3 rather than TiO2 particles, likely owing to their larger size and platelet shape.

In Chapter 4, we studied the particle detachment properties of the paints from Chapter 3. To this end, we assessed the feasibility of JKR-type experiments by using polydimethylsiloxane (PDMS) hemispheres to mimic ants’ smooth adhesive pads. We measured the adhesive stress of PDMS after repeated pull-offs of paints on PDMS and the number of contaminating particles present on PDMS resulting from the test. We found that the contamination level increased with load and that even paints formulated at PVC < CPVC presented detachable particles for high loads (> 40 mN). Although the results could not directly be compared to insect climbing tests, this experiment opens the field for future research as an alternative to mechanical analysis AFM for larger probes and could replace the hassle of performing climbing tests experiments.

Chapter 5 extends the work presented in Chapter 3. We studied the influence of the polymer binder size and pigment extender (CaCO3) diameter. Large polymer binders (> 250 nm) reduce the coatings’ CPVC and lead to more porous coatings. Very slippery surfaces were hence produced, due to a likely synergistic effect between particle detachment and ants’ adhesive fluid absorption by the pores. Extender pigments sized between 1 µm and 10 µm impede the insects’ self-cleaning mechanism and hence lead to more slippery surfaces in combination with increased porosity and reduced surface roughness. In particular, the slipperiness continuously decreased before the surface roughness average reached the threshold value of Ra ≈ 5 µm, corresponding to the claw tip diameter of A. cephalotes workers.

Chapter 6 describes the formulation of bio-degradable bio-compatible linseed oil-in-water emulsion sprays for use in agriculture. Emphasis was given on the protection of crops against F. occidentalis thrips by trapping thrips through adhesion at the surface of leaves. Sticky coatings and crosslinked droplets successfully reduced the damage to Chrysanthemum baltica leaves incurred by thrips. This work opens research for novel vegetable oil-based alternatives to pesticides. Further work is needed to improve the appearance and size of the oil droplets once sprayed.

Finally, the general discussion (Chapter 7) summarises our findings, preliminary studies and outlines what further research can be done to improve the results discussed in the other sections. Preliminary results discussed the impact on the slipperiness to A. cephalotes of binders with varying glass transition temperature and chemistry; as well as pigment extenders of different shapes and chemistries. Overall, the results discussed in this thesis and particularly in Chapter 7 can be used as a guide to formulators seeking to produce paints slippery to insects.