Why net­ting won’t de­feat all bras­sica pests

Amateur Gardening - - This Week In Gardening -

WHO said be­ing green was easy? Cer­tainly not me. There’s no magic wand in­volved in eco-gar­den­ing, but I am a keen ob­server. I spend a lot of time look­ing and, if I see a prob­lem, I nip it in the bud. That means ei­ther rub­bing pests off with my fin­gers or ‘big boot­ing’ them un­der­foot, and look­ing for any dis­eased leaves that need pluck­ing off.

My bras­si­cas are all grown un­der net­ting to stop large and small cab­bage whites lay­ing eggs. The but­ter­fly net­ting is a dou­ble-edged sword how­ever. It lets smaller in­sects through, and that can be good, but re­stricts birds from pick­ing off pests – and they will eat small, newly hatched bras­sica-eat­ing cater­pil­lars. The net doesn’t pre­vent march­ing cater­pil­lars gain­ing ac­cess, as I dis­cov­ered on the al­lot­ment one year when I fol­lowed a trail from a nearby plot to mine.

While peer­ing the other day, I spot­ted a hun­dred or so newly hatched cab­bage white cater­pil­lars on a corner plant of the kale patch in the gar­den. I cut the leaves off, al­though the slight­est trem­ble sent sev­eral drop­ping to the ground to hide away. I gath­ered them all up and gave them the big boot treat­ment and so far none seem to have mi­grated to the rest of my kale plants.

As I touched my cater­pil­lar-rid­den kale plant a lit­tle white cloud puffed up. This is the cab­bage white­fly (Aley­rodes pro­letella) which is a dif­fer­ent species from the green­house white­fly (Tri­aleu­rodes va­po­rar­i­o­rum). Both cause enor­mous prob­lems in com­mer­cial crops be­cause white­fly suck a lot of sap for their size, spread­ing viruses as they go. They also pro­duce masses of hon­ey­dew re­sult­ing in black sooty mould.

Most are re­sis­tant to pes­ti­cides so com­mer­cial green­house grow­ers pro­duc­ing a mono­cul­ture (i.e. one crop like toma­toes) strug­gle once they have an in­fes­ta­tion.

How­ever white­fly are pre­dated by spe­cialised wasps, la­dy­birds and some bugs and I sus­pect hov­er­fly lar­vae de­vour them too. These preda­tors can get through my net­ting and into my open green­houses. Also on the plus side, white­fly are sub­ject to fun­gal at­tack in warm, damp con­di­tions.

And I’m wary about cut­ting tomato fo­liage off low down, be­cause this is where white­fly preda­tors such as adult la­dy­birds, their lar­vae and eggs tend to lurk. A cold win­ter does kill white­fly: most of the 2,500 species of them in the world need warmish temperatures to sur­vive.

“But­ter­fly net­ting is a dou­ble-edged sword”

Net bras­si­cas and grow them in the gar­den – rather than the al­lot­ment – where you can keep a very close eye on them These cab­bage white cater­pil­lars have slipped through the net

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