What colors of the grubs you found? White grubs are immature beetles of European chafers or Japanese beetles. They feed on the roots of lawn grasses, and cause damage to your lawn. Severe grub damage in a lawn causes large sections of brown and dead. When they become beetles, they eat leaves of many plants, especially fruit trees, ornamental trees, shrubs, vines, and vegetable crops. Severe infection may defoliate whole tree. They are highly destructive plant pest. You need prevent losses by early treatment before they cause damage. I prefer to natual treatment without using chemicals. Japanese beetle traps are very effective and I use traps every year. It uses a combination of a sex pheromone to lure female adult and a floral lure to attract both male and female. It lasts the whole season (from June to Spetempber) and each trap can catch hundreds and thousands of beetles. Adult beetles can fly long distances, so those caught in your yard may have come from up to a mile away. For this reason, it is better to take action for the whole community. It is hard to estimate how many are trapped from my own garden,, but at least, my trees and vegetables get a good protection from those traps. Do not hung traps directly under plants the bettles eat. In soil, some microscopic worms known as nematodes infect and kill grubs. The commercial product is called Scanmask, which contains Steinernema Feltiae (SF), insect "eating" nematodes. Grubs, larvae, caterpillars, and maggots, are most susceptible. It is safe for people, pets, plants, and birds. Earthworms and benefical insects like bees, ladybugs, and lacewings are resistant to SF. The only concern is the cost. But SF works for a long time and they can reproduce by themselves.
We have a big problem with grubs where I work. There are big patches brown patches on the lawn. I'm not sure which does more damage...the grubs themselves or the skunks and other pests that dig up the grass overnight (every night!!). We now have a ritual every morning of repairing the damage and top dressing and re-seeding the lawn. We recently had a company come in and apply nematodes and I have not noticed a difference. How long does it take for them to work? Another question: I have never noticed any Japanese Beetles anywhere on the property. What could all these grubs be? I'm in zone 5, southern Ontario if that helps.
A heavy infestation of grubs usually indicates an imbalance in ecosystem, and many times, such imbalance is the result of the use of harsh pesticides and chemicals which can kill other beneficial predators. Grubs can thrive by reproducing themselves more quickly than their predators. Nematodes enter through the skin and lay their eggs in the body of the grub. The bacteria carried by nematodes break down the internal structure of the host through enzymes. Once nematode eggs hatch, the young nematode larvae feed on the broken down interior of the insect. And it go through four stages of growth and exit the host body during their third stage of maturation to begin the cycle again. I don't know how long it will take, but as it happens in nature, it takes time for nematodes to establish and to reach to a balance point. Nematodes are most successful when applied to a warm and moist lawn without recent application of any pesticides or chemicals. We maintain a chemical free lawn and we still have grubs in our lawn but never see any damage. When grubs emerges to beetles, they will probally fly away if there is nothing for them to eat in your property. They eat leaves of certain trees, especially fruit trees. I have two cheery trees which are the most vulnerable to beetles, but they ignore nearby brad pear and crab apple trees.
Another way is to use Milky Spore, a natural bacteria that infects grubs with a disease that paralyzes and kills the host grub. When the dead grub breaks down, it releases billions of new spores into the soil, which in turn infect and kill other grubs. An infected grub will die within one to three weeks.
How gross! Wow how complicated. My grubs are big fat ringed white blobs with backish-brown heads. Don't have a healthy lawn because most of it is planted flower sections. When I am turning mulch, etc. to plant, I find fat grubs. I don't really have any major deaths of plants, accept those that I cause by planting in too much sun or do not water (so I have believed. I may have been falsely accusing myself of murder!) I do put ashes from my beloved pot belly stove heater in the winter and may be creating an inbalance in my garden. I accidentally murdered two perinniel red hibiscus' with the ashes. Those nemetodes (what-have-you with the spelling) sound scarrier than the grubs. Someone asked me how could I garden being so squimish and I said, "Now you understand the depth of my love for my garden. I conquer fear when absolutely necessary for those that I love." Enjoy nature and enjoy life
Nan has given such good advice and onto of creating an balanced eco-system as mentioned I'd suggest attracting more beneficial insects and birds that pray on garden pests.